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The growth of soybean plants were examined when subjected to three contrasting irradiance levels and to various combinations of nutrient solution Fe and Mn concentrations. Two Rhizobium-nodulated soybean genotypes (PI 227557 and Biloxi), which had been previously found to differ in their growth response to various Fe and Mn solutions, were studied. Both genotypes displayed the poorest growth, nodulation and the lowest chlorophyll and nodule ureide concentration at high irradiance (HI), regardless of the solution Fe and Mn concentrations. However, the genotypes differed under HI in their accumulation of Fe. For solution concentrations greater than 13 microM, PI 227557 accumulated up to 1200 microg Feg(-1) leaf dry wt mainly in the form of ferritin crystals within chloroplasts. In contrast, leaf Fe concentrations in Biloxi only reached 300 microg Feg(-1) dry wt and there were no ferritin crystals. Also, in PI 227557 HI induced more severe distortions in leaf cells and nodule ultrastructure than in Biloxi. Based on its poor growth under HI, PI 227557 could be categorized as an Fe-inefficient genotype prone to undergo photoinhibition at HI, in spite of the ferritin crystals in the chloroplasts. Enhanced growth, nodulation, chlorophyll and ureide concentrations in nodules as well as leaf ureide catabolism occurred in both genotypes grown at moderate irradiance (MI) in Fe solutions from 13 to 60 microM supplied with 20 microM Mn. At low irradiance (LI), plant growth and nodulation were lower than at MI values, but higher than those of plants at HI. Irradiance and solution Fe concentration did not alter leaf Cu and Zn concentration in either genotype, with the higher concentrations of these two elements detected in Biloxi. Solutions with Fe concentrations greater than 100 microM were deleterious for both genotypes at all irradiances. Low Fe and high Mn concentrations in leaves was bound to result in the best growth at HI.

Adequate supplies of phosphorus (P) and iron (Fe) to legumes have been shown to be crucial in obtaining high nitrogen fixation rates and growth. These responses are anticipated as a result of the high requirement for P in energy transfer processes in the nodule and for Fe as a constituent of nitrogenase and leghemoglobin. However, little attention has been given to documenting the response of nitrogen fixation rates resulting from concentrations of P and Fe that actually exist in nodules. In particular, an open question is whether there is an interaction between nodule P and Fe concentrations that maximize nitrogen fixation activity. This study was designed to induce various concentrations of P and Fe in the nodule and to measure the resultant nitrogen accumulation and nitrogen fixation rates. Plant nitrogen accumulation was linearly correlated with both nodule P and Fe concentration, and with total plant nitrogen fixation rate as measured by acetylene reduction rate. Therefore, total nitrogen fixation rate was also correlated with nodule P and Fe concentrations, but a higher linear correlation was obtained for Fe as compared to P concentration. Surprisingly, nodule ureide concentration, which is generally assumed to be a positive index of nitrogen fixation rate, was negatively correlated with nodule P and Fe concentrations. These results indicated that higher concentrations of P and Fe in the nodules not only stimulated higher nitrogen fixation rates, but were associated with an enhanced ability to export ureides from the nodules. Since there was a linear response to both P and Fe over the range of nodule concentrations induced in these experiments, no evidence for optimum interactive concentrations of these two elements in the nodules was obtained.

Short days are thought to alter dry matter (DM) partitioning in rhizoma peanut (Arachis glabrata Benth.). Under controlled environmental conditions, six lines of rhizoma peanut germplasm (cultivars Florigraze and Arbrook; Florida lines Ecoturf, Arbrook Select, and Arblick; and PI 262826) were exposed to natural and extended photoperiod regimes (15 or 18 h). The study started 21 Sept. 2000 and ran until the following May; aboveground DM was harvested five times. The following year the same lines were transplanted into the field, where they received either natural light or 15 h extended photoperiod conditions between 20 Sept. 2001 and 19 Apr. 2002 and again during the winter of 2002-2003. In the field, cover and DM was determined on approximately 9 wk intervals. In the controlled environment study, both extended photoperiod treatments increased DM yield, but the 18 h treatment was most consistent across selections and dates. The February response was greatest when there was greater than fivefold increase across selections for the 18 h treatment above natural photoperiod (2.80 vs. 0.52 g/pot, respectively). In the field, percent cover was enhanced by extended photoperiod for all selections except Ecoturf by the second sampling date after the lights were turned on (avg. 90 vs. 38% cover, respectively, for extended and natural photoperiod). Cover differences persisted until June 2002. Dry matter yield also was increased by treatment during the fall of both years. These studies suggest that selection for late and early season DM production in rhizoma peanut will select for photoperiod insensitivity.

Freeze protection of bahiagrass in subtropical pastures might be accomplished through genetic improvement of existing diploid (sexual) cultivars, if clones with a leaf freeze tolerance trait (FT) could be confirmed. Clones originally identified as having FT under field conditions were compared under controlled temperatures to confirm those clones that specifically expressed a FT trait. An initial study was undertaken to determine whether damage induced by placing potted plants in freezing temperatures was confounded by low soil temperature, or leaf damage was directly a result of leaf freeze damage. This experiment confirmed that FT was a result of leaf tolerance to freeze injury, not root damage. Stress treatment of 10 h duration at successively colder temperatures (‐1°C, ‐3°C, ‐5°C, and ‐7°C) was evaluated on 26 clones grown in pots. Substantial variation in FT was observed among these clones, with ‘FL67’ being significantly more tolerant, even to temperatures as low as ‐7°C. An additional comp...

Plant gas-exchange response to drying soil in many instances tracks a common pattern when expressed as a function of fraction of transpirable soil water (FTSW). There is little decrease in gas exchange until FTSW decreases to a value in the range of about 0.3-0.45, then with further drying gas exchange declines approximately linearly. This unique pattern is hypothesised to reflect mainly changes in the water potential gradient between bulk soil and plant. The primary objective was to directly document the basis of this response by measuring the hydrostatic pressure gradient required in the soil to maintain leaf xylem at zero potential with decreasing FTSW. Pots in which soybean (Glycine max) plants were grown were placed in a pressure chamber and the pressure adjusted to maintain zero water potential in a leaf petiolule. These results showed a small, relatively constant hydrostatic pressure had to be applied to the soil to maintain zero leaf xylem water potential until FTSW decreased to approximately 0.3-0.45. Thereafter, the required hydrostatic pressure gradient increased as FTSW continued to decrease. Hydraulic conductance was calculated to be relatively stable early in the drying cycle, and then decrease as the soil dried to comparatively high FTSW of 0.5-0.7.

R.A. Fischer offered a spirited defense of the concept that grain number is a basic cause in determining wheat yield. While grain number is often well correlated with yield, we interpret grain number as a consequence of the ability of a crop to accumulate resources, particularly carbon and nitrogen. In this response paper, we clarify points of agreement and disagreement with Fischer. A basic point of agreement is that resource accumulation prior to anthesis is critical to determining grain number. On the other hand, an important point of disagreement is the hypothesis that after anthesis a key regulation of yield formation somehow passes to a dependence on grain number. Experimental evidence shows that yield is fundamentally driven by carbon and nitrogen resource accumulation essentially independent of grain number.

Slow-wilting has been observed in several soybean genotypes as a phenotypic response to drought stress. This trait has been proposed as useful in improving the yield of soybean under drought conditions, but the exact nature of the trait is unresolved. This research explored the hypothesis that slow-wilting is an expression of soil water conservation that results from a plant-imposed limitation on maximum transpiration rate. Therefore, gas exchange by slow-wilting and commercial genotypes was measured over a range of atmospheric vapor pressured deficit (VPD). Two experiments were undertaken to examine the response by whole plants and by plant canopies. The results showed that indeed the slow-wilting genotypes reached a maximum transpiration rate at a VPD of about 2.0 kPa with little or no further increase in transpiration rate above this value as VPD was increased. In contrast, the commercial cultivars showed continued increases in transpiration rate as VPD was increased above 2.0 kPa. These results indicated that the slow-wilting trait would be especially desirable in low humidity (high VPD) environments where water deficits commonly develop in the later part of the season. In these environments, restricted transpiration rate during the middle of the day with high vapor pressure deficit would result in water conservation allowing for both increased yield and water use efficiency.

Decreased growth of warm-season forage grasses during subtropical winters has been shown to be a response to short-daylength during those months. The objective of this research was to evaluate the influence of short-daylength on growth of forage grasses in low latitude locations. Three grasses (Argentine bahiagrass, Paspalum notatum Flugge; Common guineagrass, Panicum maximum Jacq.; Tifton-85 bermudagrass, Cynodon spp. Pers.) were grown at three locations (south Florida, Puerto Rico, and St. Croix) under extended (15 h) and natural daylength during 2 years. Plots were harvested at 5-week (October–February) and 4-week (March–April) intervals to measure dry matter (DM) yield and nutritive value of the herbage. Daylength effects were found on DM harvested for all grasses in south Florida ( P

Soya bean is often grown in regions prone to periodic flooding, thus selecting cultivars that maintain production under waterlogged conditions is desirable. An experiment involving flooded soya beans was planted in southern Florida to examine (1) stem and leaf growth; (2) morphological adaptations; and (3) the relationship between early-season and late-season flood tolerance in flooded soya beans. Eleven soya bean genotypes previously defined as tolerant or sensitive to flooding were subjected to three treatments at 21 days after sowing (DAS): (1) no flood, (2) 2-week flood and (3) 4-week flood. All plants were harvested 49 DAS. Flooded plants exhibited lower stem dry weights but greater partitioning to the stem. Non-flood treatments had greater leaf dry weight, leaf area and partitioning to leaves than flooded plants. There were positive correlations of genotype stem dry weight and leaf dry weight to early-season flood tolerance but stem partitioning was negatively correlated with early-season flood tolerance. Genotypic rankings of early-season flood tolerance in this study were not correlated with earlier studies basing flood tolerance on seed yield. Our study highlights the range of soya bean morphological adaptations in response to flood. However, our results indicate that early-season screening may not be an accurate predictor of soya bean genotypic response to late-season flood.

Soya bean is often grown in regions subject to periodic flooding, with the rooting zone most affected by flooding due to its proximity to the source of stress. Our objectives were to examine the effects of flooding soya bean on its primary roots, adventitious roots and root nodules, and to determine relationships between root morphological changes and early-season flood tolerance. The experiment was conducted in Belle Glade, FL with 11 soya bean genotypes subjected to (i) no flood, (ii) 2-week flood 21–35 days after sowing (DAS) or (iii) 4-week flood 21–49 DAS. All plants were harvested 49 DAS. Flooding reduced soya bean primary root weight, length and volume across genotypes. Adventitious root length and volume were greater in the 4-week than the 2-week flood. Soya bean nodule dry weight was threefold higher in the non-flooded treatments. Genotypic differences in root development and tolerance to flooding were noted, with early-season flood tolerance correlated with primary root dry weight, length and surface area, and adventitious root dry weight. However, there was no correlation between this study's early-season root development and late-season flood tolerance based on seed yield from previous studies. Our results indicate that full season trials may be necessary to identify flood-tolerant soya bean germplasm.

Recent changes in management of soybean production in Argentina may have large impacts on the soil water balance and on crop yield response. Changes in this system have included widespread adoption of a no-till management leaving crop residue on the soil surface, intensive cropping rotations (e.g. double cropping of wheat and soybean) so that the soil may not be fully recharged with water at the time of soybean sowing, and the occurrence of high water tables in a number of areas. The objective of this analysis was to assess the need to account for these factors in simulating soybean yields in Argentina. The influence of no-till management was simulated by simply decreasing the soil evaporation estimated for a bare soil by 70%. However, this alteration resulted in an over prediction in yield in many cases when it was assumed that the soil water content had been fully recharged at the initiation of the simulations. The difficulty with assuming a full soil water profile was confirmed when simulated yields were found to match well with observed yields when measured soil water content was used as an input to the model at the beginning of the soybean season. Finally, even with decreased soil evaporation there were still a few cases where simulated yield was less than observed yield. In these cases, a hypothetical water table, which relieved any drought stress once roots reached a depth of 1 m, resulted in yields that more closely matched observations. Overall, these results highlighted the need to estimate well both the influence of crop residue on soil evaporation and the soil water profile at sowing in simulating soybean yields in Argentina.

Water-deficit stress is a major constraint on plant productivity and consequently, is a major focus of much research. Stress is often imposed on plants in these experiments by withholding water from the artificial potting media on which the plants are grown. No attention has been given, however, to the possibility of differences in the dynamics of stress imposition between that resulting from dehydration of the artificial rooting media and that of drying of mineral soil. The objective of this research was to compare transpiration rates during drying of a mineral soil and of an artificial potting mixture for three test species: Arabidopsis thaliana, maize (Zea mays), and soybean (Glycine max). These results showed major differences in transpiration response between the two soil media. Drying of mineral soil confirmed previous observations that no decrease in transpiration rates occurred until 0.27–0.34 of the extractable water remained in the soil. Thereafter, there was essentially a linear decrease in transpiration with further soil drying. In contrast, transpiration rates of plants grown on the potting mixture began to decrease when about 0.6–0.7 of the extractable water still remained in the soil. Consequently, plants grown on the potting mixture as compared to the mineral soil were exposed to stress very early in the drying cycle and the stress was much more prolonged over a wide range of soil moistures. Caution is warranted in extrapolating to natural, mineral soils the results obtained from plants subjected to water-deficits using artificial potting mixtures.

Drought is by far the most important environmental factor contributing to crop yield loss, especially in soyabean [Glycine max (L.) Merr.] where symbiotic fixation of atmospheric nitrogen (N2) is sensitive to even modest soil water deficits. Decline of N2 fixation with soil drying causes yield reductions due to inadequate N for protein production, which is the critical seed product. In this paper, we present a combined physiological and breeding research effort to develop soyabean lines that have diminished sensitivity of N2 fixation to drought. A preliminary physiological screen was used to identify lines that potentially expressed N2 fixation drought tolerance. One hundred progeny lines derived from a cross between Jackson, a cultivar proven to have N2 fixation tolerance to drought, and KS4895, a high-yielding line, were tested in the screen. Seventeen lines were identified for subsequent yield trials in moderate- and low-yielding rainfed environments. Two lines, found to have higher yields than commercial checks in these environments were then tested in the greenhouse for their N2 fixation activity in drying soil. Nitrogen fixation activity was found to persist at lower soil water contents than exhibited by the sensitive parent. These two soyabean lines offer a genetic resource for increased yields under rainfed conditions as a result of decreased sensitivity of N2 fixation to water deficit

First observation that chemical spray can induce limited-transpiration rate under high vapor pressure deficit. It appears that acibenzolar may be key in inducing this water conservation trait. Irrigation and water use have become major issues in management of turfgrasses. Plant health products that have been introduced into the turfgrass market have been observed to improve plant performance in water stress conditions. In this study, we evaluated whether a selection of common plant health products alter the ability of creeping bentgrass (Agrostis stolonifera L.) to control transpiration under high vapor pressure deficit (VPD). The plant health treatments--Daconil Action, Insignia, and Signature--were applied to plots on golf course putting greens located in Raleigh NC and in Scottsdale, AZ. Using intact cores removed from the putting greens, transpiration rates were measured over a range of VPDs in controlled conditions. In all cases stretching over a 3-year period, bentgrass cores from field plots treated with Daconil-Action limited transpiration under high VPD conditions, while check treatments with water, and others treated with Insignia or Signature did not. Transpiration control became engaged when VPDs reached values ranging from 1.39 to 2.50 kPa, and was not strongly influenced by the field temperature at which the bentgrass was growing. Because all plots in NC had been treated with chlorothalonil-the key ingredient in Daconil Action to control diseases-it was concluded that the likely chemical ingredient in Daconil Action triggering the transpiration control response was acibenzolar. This is the first evidence that the limited-transpiration trait can be induced by a chemical application, and it implies significant potential for ameliorating drought vulnerability in cool-season turfgrasses, and likely other plant species.

A plant trait currently being exploited to decrease crop yield loss under water-deficit conditions is limited-transpiration rate (TRlim ) under high atmospheric vapor pressure deficit (VPD) conditions. Although limited genotype comparisons for the TRlim trait have been performed in peanut (Arachis hypogaea), no detailed study to describe the basis for this trait in peanut has been reported. Since it has been hypothesized that the TRlim trait may be a result of low leaf hydraulic conductance associated with aquaporins (AQPs), the first objective of this study was to examine a possible correlation of TRlim to leaf AQP transcriptional profiles in six peanut cultivars. Five of the studied cultivars were selected because they expressed TRlim while the cultivar York did not. Transcripts of six AQPs were measured. Under exposure to high vapor pressure deficit, cultivar C 76-16 had decreased AQP transcript abundance for four of the six AQPs but in York only one AQP had decreased abundance. The second objective was to explore the influence of AQP inhibitors mercury and silver on expression of TRlim and AQP transcription profiles. Quantitative RT-PCR data were compared in cultivars York and C 76-16, which had the extreme response in TR to VPD. Inhibitor treatment resulted in increased abundance of AQP transcripts in both. The results of these experiments indicate that AQP transcript abundance itself may not be useful in identifying genotypes expressing the TRlim trait under high VPD conditions.

Lentil (Lens culinaris Medik.) could possibly become a major crop in East Africa due to its many uses as a food and feed. Also, its ability to undertake symbiotic nitrogen fixation is an advantage over cereal crops. This study simulated lentil yield potential in order to determine the geographical areas in East Africa that offer potential for consistent lentil production. Results show that there is potential to further expand the geographical area in which lentil is currently grown in East Africa into Uganda, Kenya, Tanzania and even Somalia. Response to a change in management practices on potential yield of lentil as a result of different sowing dates was also examined. In addition, the effect of phenology on yield potential was examined by comparing a short-season type vs. a long-season type. Delaying sowing alone or in combination with a long-season genotype can result in a high probability of crop yield increase in East Africa. For the long-season genotype, an optimum sowing window was found between June and July (152–229 day of year) for areas to the north of the Rift Valley. Later sowing dates (229–243 day of year) were found to be optimal in southern areas of East Africa. These simulations indicated that selection and breeding for lentil accessions in East Africa should consider changes in plant phenology and/or sowing dates.

Future progress in crop breeding requires a new emphasis in plant physiological phenotyping for specific, welldefined traits. Success in physiological phenotyping to identify parents for use in breeding efforts for improved cultivars has been achieved by employing a multi-tier screening approach with different levels of sophistication and trait resolution. Subsequently, cultivar development required an integrated mix of classical breeding approaches and one or more tiers of phenotyping to identify genotypes expressing the desired trait. The role of high throughput systems can be useful; here, we emphasize that this approach is likely to offer useful results at an initial tier of phenotyping and will need to be complemented with more directed tiers of phenotyping.

International audience; Seedling establishment is obviously the first step in having successful crop production. In seasons and locations that experience early drought, potential differences among species in plant survival of drought could result in greater probability of crop establishment. In this study, plant survival capacity during crop establishment was tested for five grain legume species: soybean [Glycine max (L.) Merr.], common bean (Phaseolus vulgaris L.), groundnut (Arachis hypogaea L.), cowpea (Vigna unguiculata L. Walp.), and pigeon pea (Cajanus cajan L. Huth). Experiments were done for four sowing conditions (two soil textures. two initial soil-water contents). Cowpea and pigeon pea had the greatest capacity for survival, while soybean, bean, and groundnut were more sensitive. Differences among species could not be explained by differences in the temporal dynamics of water use. Conversely, plant wilting and senescence in response to soil dehydration varied among species; initial wilting of cowpea and pigeon pea occurred at lower soil-water contents. These two species also had the longest survival after fraction transpirable soil water (FTSW) reached zero. In a comparison of 10 genotypes of each species, genotypic variation in the wilting profile during the survival phase was observed in cowpea, bean, and, to a lesser extent, groundnut and pigeon pea, but no differences were found in soybean.

Grain number has long been considered to be a key determinant of yield in wheat crops and cultivars. In this paper, we argue that both yield and grain number are constrained by a crop's ability to gather resources. In this sense, grain numbers can be considered more as a consequence of yield than a determinant. We examine published literature on the effects of variations in development, light, nitrogen, and water supply on grain number determination and yield. We conclude that most of the variation in both grain number and yield is likely caused by variations in resource accumulation, particularly nitrogen, by the crop.

Currently, the major thrust of plant physiology research is to identify and understand the regulation of genes that might be relevant in plant development and growth. The dominance of a genocentric view of plant behaviour has, unfortunately, resulted in the development of major disconnects in the classical view of plant physiology as a partnership between fundamental and practical research contributing to improved plant production. One disconnect is that much of the genocentric research appears to be organized and executed without regard to the practical needs of enhancing plant performance under applied conditions. Although practical benefits from genocentric research are often claimed, basic assumptions guiding much research and the experimental protocols used are commonly not relevant for real-world plant production. A second disconnect is a failure fully to appreciate the lessons learned in 40 years of classical plant physiology research concerning the role of physiological processes in altering whole plant performance. Regulation of plant systems has proved to be complex and redundant. Alteration of a single physiological process is compensated or dampened so that commonly very little change in plant growth and yield results from modification of a single physiological process. Based on a few successful projects employing classical plant physiology to achieve crop yield increase, key characteristics for research projects that truly seek to increase plant performance in production systems are identified. Basically, the partnership between the fundamental and practical research long espoused for plant physiology needs to be re-established in an intimate and meaningful way.

La producción de forraje puede variar substancialmente durante el ciclo anual en las islas tropicales de Puerto Rico y Santa Cruz. Se ha hipotetizado que las temperaturas frías, los bajos niveles de radiación solar, y la baja precipitación en diciembre y enero son los causantes de las disminuciones en el crecimiento del forraje. Para examinar cuantitativamente estas hipótesis se utilizó un modelo de crecimiento en forraje para simular el rendimiento bajo diferentes ambientes. Los datos climáticos por un periodo de tres a seis años se obtuvieron para tres localidades en Puerto Rico y una localidad en Santa Cruz. Las temperaturas mínimas estuvieron siempre cerca o por encima de los 20°C; por lo tanto, no parecieron causar serias pérdidas en la producción de forraje. El modelo de forraje predijo una reducción en la producción de forraje en los meses de invierno debido a una disminución en los niveles de radiación solar; sin embargo, los rendimientos estimados fueron un 70 a 80% de los rendimientos del verano. Si bien el enraizamiento superficial a una profundidad menor de 45 cm podría causar disminución de los rendimientos en algunas situaciones, la precipitación inadecuada no podría explicar las grandes disminuciones ocurridas en los meses de invierno. Esta investigación indica que un factor adicional a los evaluados contribuye a la pérdida en el rendimiento del forraje durante el invierno. Se especula que la corta duración del día influye directamente sobre la regulación del crecimiento de la planta de forma tal que el rendimiento de forraje disminuye en los meses de invierno.

NOAA AVHRR has been used extensively for monitoring vegetation condition and changes across the United States. Integration of crop growth models with MODIS imagery at 250 m resolution from the Terra Satellite potentially offers an opportunity for operational assessment of the crop condition and yield at both field and regional scales. The primary objective of this research was to evaluate the quality of the MODIS 250 m resolution data for retrieval of crop biophysical parameters that could be integrated in crop yield simulation models. A secondary objective was evaluating the potential use of MODIS 250 m resolution data for crop classification. A field study (24 fields) was conducted during the 2000 crop season in McLean County, Illinois, in the U.S. Midwest to evaluate the applicability of the MODIS 8-day, 250 m resolution composite imagery (version 4) for operational assessment of crop condition and yields. Ground-based canopy and leaf reflectance and leaf area index (LAI) measurements were used to calibrate a radiative transfer model to create a look up table (LUT) that was used to simulate LAI. The seasonal trend of MODIS derived LAI was used to find crop model parameters by adjusting the LAI simulated from the climate-based crop yield model. Other intermediate products such as crop phenological events were adjusted from the LAI seasonal profile. Corn (Zea mays L.) and soybean (Glycine max (L.) Merr.) yield simulations were conducted on a 1.6 × 1.6 km2 spatial resolution grid and the results integrated to the county level. The results were within 10% of county yields reported by the USDA National Agricultural Statistics Service (NASS).

Experimental evidence has been accumulating for a number of years that daily transpiration rates respond to the progressive drying of soil in a similar manner across a wide range of conditions when expressed as a function of available volumetric soil water content. There is little decrease in gas exchange until only about one-third of the available soil water remains in the soil, and then gas exchange decreases in a nearly linear manner until the available soil water is exhausted. There exists no theoretical derivation that explicitly provides a basis for explaining the consistency in this response observed over a wide range of conditions. A relatively simple derivation is presented by defining plant water flux on drying soil relative to a plant on well-watered soil and by examining the response in the relevant range of soil volumetric water content. This derivation resulted in a relatively simple expression that predicted daily transpiration rate response to drying soil that was consistent with experimental observations. Further, the results of this analysis showed the response was essentially independent of root length density, transpiration rate, and soil depth. Also, large decreases in soil hydraulic conductivity as the soil dries had only modest influences on the daily transpiration rate response. Expression of relative transpiration as a function of volumetric soil water content available to support transpiration also minimized the influence of soil texture on the overall response of water flux to drying soil. The derivation offers a theoretical basis to explain the stability in daily transpiration response to drying soil that has been observed over a wide range of conditions.

Soybean production has been expanded into northern Brazil by the introduction of long-juvenile genes. These genes cause flowering to be delayed but specific changes in the sensitivity of plant development to temperature and photoperiod have not been resolved in these commercial cultivars. This study was undertaken to resolve possible changes in long-juvenile plants associated with rates of plant emergence, leaf appearance, and flowering. Eight cultivars, including three long-juvenile cultivars, were sown each week throughout a year in a plastic greenhouse so that the plants would develop under differing temperature and photoperiod. While there were differences among cultivars in the cumulative temperature required for plant emergence and the rate of leaf appearance, these differences were not necessarily associated with the long-juvenile trait. An extended duration to flowering was confirmed for the three long-juvenile cultivars but this delay was not associated with any difference in sensitivity to temperature and photoperiod. The trait that distinguished the long-juvenile cultivars was a much lower maximum development rate towards flowering than that found in the other cultivars. However, cultivar differences were identified among the long-juvenile cultivars indicating the possibility of further selection of genotypes to fit specific environments in low-latitude regions.

Manganese (Mn) is an essential element of plant growth and has been identified as a cofactor in soybean (Glycine max Merr.) leaves for catabolism of ureides, which are the main products of nitrogen fixation transported from nodules to shoot. Variations among genotypes were observed in the rate of ureide catabolism when plants were grown in nutrient solutions deficient in Mn. One possible explanation is that the differences in ureide catabolism among genotypes might be related to differences in Mn uptake from Mn-deficient nutrient solutions. Therefore, the objective of this study was to compare Mn uptake among six soybean genotypes grown in nutrient solutions with differing Mn and nitrate concentrations. Manganese concentration in various plant tissues was measured, as were concentrations of ureide, α-amino acid, nitrate, and iron (Fe). The genotypes segregated into two distinct groups under Mn-deficient conditions, with one group having higher leaf Mn concentrations and higher leaf Fe concentrati...

Sugarcane (Saccharum spp.) is rather unusual among field crops in that it is not the seed that is of economic value, but rather the stalk. Sucrose is extracted from the large stalks that are produced by sugarcane plants. Considering their economic value, it is rather surprisingly that there is very little information concerning the development and size of the individual internodes over the length of sugarcane stalks. The objective of this study was to document internode volume over the sugarcane stalk of four cultivars and to observe changes in individual internode volume later in the growing season and in response to lodging. When harvested in August, the size of the internodes varied in a continuous manner along the stalk with the largest internode occurring at the bottom of the plant at approximately internode #15. One cultivar (CP80-1743) had much less of a decrease in internode volume below internode #15 than the other cultivars, indicating that growth was more concentrated in the lower internodes of this cultivar. Since the internode volume diminished as the apex was approached, much of the harvested material was concentrated in the bottom nodes. Comparison of a mid-season harvest (August) and a final harvest (January) indicated both that additional internodes were added to the stalk and that the size of all internodes increased. That is, plant growth was distributed all along the stalk and not focused on one particular zone. Similarly, the negative impact of plant lodging was uniformly distributed over the entire stalk, although there appeared to be differences in cultivars in the extent to which the growth of the stalk was decreased.

Leaf area development is critical in the establishment of a full leaf canopy to maximize interception of solar radiation and achieve high crop productivity. In sugarcane, leaf area development is especially important because the rate of leaf area increase is relatively slow. Previous studies have highlighted the fact that the rate of emergence of individual leaves is highly dependent on temperature. These previous studies have been limited, however, to only very few cultivars, and these cultivars were adapted for production in environments different from the climate of the continental USA. Hence, there is little information for cultivars selected for production in other environments and little basis for resolving which variable contributing to leaf area development has the greatest impact on canopy leaf area. The objective of this 2-year field study was to examine the contribution of rate of leaf emergence, leaf shape, and individual leaf area to the development of plant leaf area. Four cultivars developed for the subtropical climate of Florida were compared. The dependence of leaf emergence as a function of temperature was confirmed in this study and the leaf appearance rate of CP88-1762 was significantly greater than CP72-2086. Leaf shape was found to be nearly uniform among the four cultivars although the shape factor (0.72) was different from that previously reported for sugarcane. Cultivars differed in the area of successive emerged leaves on the stalk. Leaves produced early in development were found to be larger for one cultivar (CP88-1762) as compared to the other cultivars. These results indicated that area of earliest leaves produced by sugarcane cultivars might be a variant that could be exploited to achieve more rapid development of crop leaf area.

Oil and protein are the most valuable components of soybean seed. Evidence indicates that growth and composition of soybean seed are controlled by supplies of carbon and nitrogen provided by the maternal plant to the seed, but it is difficult experimentally to control and quantify the precise amount of carbon and nitrogen provided to the seed by the whole plant. To examine whether oil and protein concentrations are affected by the supply of nitrogen to the seed, immature soybean seeds (Glycine max cv. Williams 82) were grown in vitro in nutrient solutions containing 20, 40, 60 or 80 mM of glutamine. The seeds were incubated in Erlenmeyer flasks for 8 days at 25°C. The rate of dry matter accumulation changed from 7.2 to 8.3 mg seed -1 day -1 as the glutamine concentration increased from 20 to 80 mM but the differences were not significant (P > 0.05). Seed protein concentration increased as glutamine concentration increased from 294 mg g -1 at 20 mM glutamine to as high as 445 mg g -1 at 80 mM glutamine. Typical in vivo protein concentration of mature soybean seeds is about 400 mg g -1 . Oil and protein concentrations were negatively correlated (r 2 = 0.44), which indicates that oil and protein synthesis are interrelated. Protein synthesis was favoured over oil synthesis when nitrogen became more abundant. The seeds used in this study clearly demonstrated a capacity to respond to nitrogen availability with changes in seed protein concentration.

Increasing N2 fixation tolerance to drought has been hindered by the labor and costs of quantifying N2 fixation using 15N methodologies. The relative abundance of ureides (RAU) in plant tissues has been used for estimating N2 fixation in soybean [Glycine max (L.) Merr.] grown under well-watered conditions, but it has not been evaluated for drought conditions. The present research evaluated the response of N accumulation to N fertilization, the ability of the RAU technique to predict N2 fixation under drought conditions, and the response of yield to N fertilization under well-watered and drought conditions. Under drought, shoot N accumulation rate during vegetative growth approximately doubled as the amount of N fertilizer was increased from 10 to 200 kg N ha(-1), indicating a greater sensitivity of N2 fixation to drought than uptake and assimilation of inorganic N. Under well-watered conditions, the relationship between estimates of N2 fixation made by 15N-dilution and RAU agreed within 15% of published reports. Under drought conditions, however, this relationship was greatly different (13 to 43%) from published reports. Fertilization with inorganic N in 1 yr increased grain yield 15 to 25% for the drought treatment and 12 to 15% for the well-watered treatment. In a second year, N fertilization increased yield of both drought and well-watered treatments approximately 9%. This research indicates that the RAU technique for estimating N2 fixation under drought conditions may be invalid without further refinement, that N2 fixation is more sensitive to drought than the uptake and assimilation of inorganic soil N, and that increasing the tolerance of N2 fixation to drought would likely result in yield increases.

Summary Much of the economic value of soybean (Glycine max) is based on the amount of protein and oil produced in the seeds. To examine the influence of temperature on seed oil and protein concentration, immature soybean seeds (cv. Williams 82) were grown in vitro at temperatures of 17°C, 21°C, 25°C, 29°C and 33°C. Dry growth rate (DGR) was calculated to be maximal at 23.7°C. Oil and protein concentration and seed growth rate did not show statistical difference (P > 0.05) within the temperature range from 21–29°C. Across all temperatures, however, a quadratic regression on oil concentration (R2 = 0.66) showed a minimum at 24.1°C and a quadratic regression on protein concentration (R2 = 0.59) showed a minimum at 24.3°C. Dilution by increased dry matter accumulation in the seed accounted for much of the variation in oil and protein concentration and the two concentrations were equally affected across temperatures. Consequently, oil and protein concentrations were positively related over the tested range of temperature. It was concluded that under these conditions the rate of dry matter accumulation by soybean seeds was critical in influencing seed oil and protein concentrations.

While plant sensitivity to extended photoperiod has been well documented, virtually no attention has been given to the possibility that low-level photosynthetic photon flux density (PPFD) during the extended photoperiod has an influence on the expression of the photoperiodic response. This study was undertaken with four grasses (Pensacola bahiagrass, Paspalum notatum Flugge, Tifton 85 bermudagrass, Cynodon dactylon L., Florakirk bermudagrass, Florona stargrass, Cynodon nlemfuensis Vanderyst var. nlemfuensis ) grown under field conditions to examine their photoperiodic responses under a PPFD gradient. Individual lamps were positioned above each experimental plot to impose a PPFD gradient (from 0.1 to 30 μmol m −2 s −1 ) during the extended photoperiod treatment. The experiment was undertaken during two seasons of short daylengths and plant responses were measured as mass accumulation and height. There was a linear increase in both measured variables with increasing PPFD up to a level where the plant responses were light saturated. Saturating PPFD was generally consistent between mass accumulation and height within a species, but there were substantial differences among grasses. Grasses with the greatest relative increase in mass accumulation under saturating PPFD for the extended photoperiod also were found to require the greatest PPFD for saturation. Overall, these results provided original observations on the quantitative response of plants to PPFD showing that there was a linear increase in response at very low PPFD and a saturating PPFD that varied among the grasses.

Molecular transformation is commonly offered as a hope to overcome the apparent stagnation in crop yield potential. A basic understanding of the resource limits imposed on crops and the yield hierarchy going from gene expression to harvestable yield leads to a rather negative view that transformations of a few, or even of a complex of genes will result directly in major yield increases. Forty years of biochemical and physiological research illustrate the great difficulty in translating research at the basic level into improvements in crop yield. However, there are a few cases where physiological research has led to improved crop cultivars with increased yield. These successes are instructive in highlighting key elements required to achieve success in developing crop cultivars for increased yield.

Seasonal rainfall is a key factor determining yield of nonirrigated, summer crops. In temperate regions, however, systematic analyses of long-term weather data have not been used for directing breeding programs or for crop management options. We evaluated long-term weather data (36-98 yr) for 16 sites in four geographical regions in the USA to assess the potential for drought avoidance. For each day of year when the probability (P) of having a minimum temperature 50 mm was defined as a drought. For the Midsouth, there were approximately 62 d at both the beginning and end of the growing season with P ≤ 0.20 of drought. In the Southeast, there were approximately 48 d and 121 d at the beginning and end of the growing season, respectively, with P ≤ 0.20 of drought. For the Midwest, P of drought was ≤0.20 throughout the growing season for three of the four sites, and it was concluded that a 50-mm water deficit was not likely to be a production constraint on the deep soils of the Midwest. For the Northern Great Plains, P of drought was >0.20 for more than half of the region's growing season. This meteorological approach for assessing drought may provide insights for drought avoidance in breeding and crop management.

Nitrogen fixation in soybean (Glycine max Merr.) is especially sensitive to soil drying. The basis of this sensitivity appears to be related to the fact that ureides are transported from the nodules, and the ureide concentrations increase with water deficits in leaves resulting in an apparent feedback to nodules involving ureides that inhibit activity. Therefore, sustained ureide catabolism in the leaves under water deficit appears to be critical for N fixation tolerance. Recently, eight plant introduction lines were identified that expressed substantial N fixation tolerance of water deficits. The focus of this study was to explore the basis for the tolerance previously observed in the eight lines. Specifically, the objective of this study was to evaluate in these genotypes the dependence for ureide catabolism on allantoate amidinohydrolase, which in other cultivars appears to be related to N fixation tolerance of water deficits. Since allantoate amidinohydrolase does not require Mn as a co-factor in contrast to the alternate enzyme for allantoic acid catabolism, ureide accumulation was measured in leaves of these genotypes after the plants were fed allantoic acid following growth on low Mn hydroponic solutions. This treatment confirmed that ureide accumulation was independent of Mn nutrition level in six of the eight tolerant lines. Ureide accumulation in PI 429328 was consistently the most insensitive to Mu nutrition level. Overall, these results indicated that ureide catabolism independent of Mn is active in six of the eight plant introduction lines identified to express N fixation tolerance to soil drying.

Extended daylength substantially increases the total mass of subtropical forages. An experiment was designed with four forages to determine the result of altered plant growth as a result of extended daylength on concentrations of β-carotene and α-tocopherol. Pure stands of Florakirk and Tifton-85 bermudagrass (Cynodon dactylon), Florona stargrass (Cynodon nlemfuensis Vanderyst) and Pensacola bahiagrass (Paspalum notatum Fluegge) were established in central Florida during the summer of 1997. Individual grass plots were exposed to two daylength treatments, extended (artificial light used to maintain daylength at 15 h) and normal daylength, conducted over a seven-month period. Samples were harvested at distances of 4 m ‘behind’ the light and 1.0 m ‘in front’ of the light. Samples were collected and analyzed for β-carotene and α-tocopherol at six sample dates from late fall to early spring. Extended daylength increased forage yields from 32 to 258% over normal daylength. However, there was no difference betwe...

One constraint on cattle production in the southeastern USA is the low productivity of perennial forage grasses during the short-daylength months. Evidence indicates that total growth during these months could be enhanced by exposing these grasses to extended photoperiod. A detailed analysis of their year-round productivity and nutritive value is needed to understand fully the consequences of overcoming the photoperiod-induced decline in growth. A 2-yr experiment was established at Ona, FL, in which photoperiod was extended to 15 h throughout the short-daylength months. Four grasses were studied:'Pensacola' bahiagrass, Paspalum notatum Flugge var. Saurde Parodi; 'Tifton 85' bermudagrass, Cynodon spp. L. Pers.; 'Florakirk' bermudagrass; and 'Florona' stargrass, Cynodon nlemfuensis Vanderyst var. nlemfuensis. Growth increases were observed in all grasses during the short-daylength months as a result of the extended-photoperiod treatment, with increases in the January through March harvests of 3-fold or more for Pensacola bahiagrass and 1.5- to 2.5-fold for Tifton 85 bermudagrass. Generally, there was no evidence of adverse consequences from sustained growth during the short-daylength months either in the subsequent spring and summer growth or in traits measured in below-ground tissue. With one exception, there was no major influence of the sustained growth on forage nutritive value during the short-daylength months for any of the year-round harvests. Pensacola bahiagrass had decreased crude protein under the extended-photoperiod treatment relative to the natural-daylength treatment. Overall, these results indicated that the selection and genetic incorporation of photoperiod insensitivity into these grasses could enhance productivity without adverse consequences.

Simulation of soybean growth and yield requires an accurate representation of nitrogen accumulation and distribution in the developing crop. Approaches to simulate nitrogen accumulation by soybean have been complicated by the need to account for both soil nitrate uptake and symbiotic nitrogen fixation. Past approaches to simulating soybean have generally been empirical requiring ‘calibration’ for each new environment. Recently, Jamieson and Semonov [Field Crops Res. 68 (2000) 21] proposed a relatively simple approach for simulating nitrogen accumulation by wheat based on the nitrogen demand imposed separately by the development of leaf area and the growth of stems. This paper assesses this approach for soybean. This approach was further simplified by assuming that nitrogen availability to the plant was limited only by soil moisture conditions and a maximum uptake rate and that it was not necessary to distinguish between nitrate uptake and symbiotic nitrogen fixation. The simplified model generally resulted in simulations that corresponded to experimental observations on nitrogen and mass accumulation through the growing season, and on yield over a number of years. Sensitivity analysis of the model indicated that storage of nitrogen in the stem might be a key trait in increasing soybean yield potential.

Crop modelling has evolved over the last 30 or so years in concert with advances in crop physiology, crop ecology and computing technology. Having reached a respectable degree of acceptance, it is appropriate to review briefly the course of developments in crop modelling and to project what might be major contributions of crop modelling in the future. Two major opportunities are envisioned for increased modelling activity in the future. One opportunity is in a continuing central, heuristic role to support scientific investigation, to facilitate decision making by crop managers, and to aid in education. Heuristic activities will also extend to the broader system-level issues of environmental and ecological aspects of crop production. The second opportunity is projected as a prime contributor in understanding and advancing the genetic regulation of plant performance and plant improvement. Physiological dissection and modelling of traits provides an avenue by which crop modelling could contribute to enhancing integration of molecular genetic technologies in crop improvement.

Pure stands of the Florakirk and Tifton 85 bermudagrass [Cynodon dactylon (L) Pers.], Florona stargrass (Cynodon nlemfuensis Vanderyst), and Pensacola bahiagrass (Paspalum notatum Fluegge) were established in central Florida during the summer of 1997. Individual grass plots were exposed to an artificial light, which extended the daylength. There were two daylength treatments, extended (artificial light used to maintain daylength at 15 h) and normal daylength conducted over a two-year period. Samples were harvested at distances of 4.0 m ‘behind’ the light and 1.0 m ‘in front’ of the light. Samples were collected and analyzed for iron (Fe), copper (Cu), zinc (Zn), manganese (Mn), and selenium (Se) at six sample dates from the late fall to early spring for two consecutive years, but only one year for Se. There was no difference between normal and extended daylength for Fe, Cu, Zn, and Mn. Forage Cu concentrations increased with extended daylength for Florona stargrass (from 20.3 to 23.1 ppm). Florona stargra...

There are large increases in leaf ureides, allantoin, and allantoic add upon water deficit in N 2 fixing soybean [Glycine max (L.) Merr.], which are likely to trigger a feedback inhibition of nodule activity. The degradation of ureides in the leaves appears to be a major factor associated with N 2 fixation tolerance to water deficit. Since one of the possible enzymes responsible for allantoic acid degradation depends on Mn as a cofactor, we investigated the possibility that the previously demonstrated N 2 fixation tolerance to water deficit of the cultivar Jackson may result from a superior ability to accumulate Mn. Indeed, Jackson was found in field and greenhouse experiments to have higher leaf Mn concentrations than other genotypes over a range of Mn availability. Acetylene reduction activity after treating Jackson plants grown on hydroponic solutions with ureide did not vary, however, with leaf Mn concentration. This was in contrast to a soybean line with N 2 fixation sensitive to water deficit in which acetylene reduction activity following ureide treatment was greater as the leaf Mn concentration increased. In addition, ureide degradation rates in Jackson leaf samples with differing Mn concentrations were insensitive to Mn concentration. These results indicate that ureide degradation likely did not involve Mn as a cofactor and that ureide degradation in Jackson is catalyzed by an enzyme not requiring Mn as a cofactor. It was concluded that the capacity to maintain a high leaf Mn concentration in Jackson under sufficient Mn availability was a trait with no causal relationship to N 2 fixation tolerance of water deficit.

Osmolyte accumulation (OA) is frequently cited as a key putative mechanism for increasing yields of crops subjected to drought conditions. The hypothesis is that OA results in a number of benefits that sustain cell and tissue activity under water-deficit conditions. It has been proposed as an effective tolerance mechanism for water deficits, which could be enhanced in crops by traditional plant breeding, marker-assisted selection or genetic engineering, to generate drought-tolerant crops. However, field studies examining the association between OA and crop yield have tended to show no consistent benefit. The few, often-cited, investigations with positive associations were obtained under severe water deficits with extremely low yields or conditions with special water-supply scenarios when much of the benefit is plant survival. Under conditions where water deficits threaten crop survival, yields are so low that even large fractional yield gains offer little practical benefit to growers. Indeed, the often-cited benefit of turgor maintenance in cells is likely to result in crop behaviour that is exactly opposite to what is beneficial to crops. The one clear mechanism identified in this review for beneficial yield responses to OA is in the maintenance of root development in order to reach water that may be available deeper in the soil profile.

A decline in plant transpiration has been widely observed to occur within a fairly stable range of threshold values of fraction transpirable soil water (FTSW), usually 0.3–0.4. However, the stability of this function has not been compared at various levels of atmospheric vapor pressure deficit (VPD). Soil hydraulic conductivity is likely to be involved in determining the threshold where water supply is limiting. Thus, it was hypothesized that at a high VPD resulting in increased transpiration rates, the FTSW threshold for the decline of transpiration rates as a result of drying soil would be increased. This study was undertaken in controlled environment chambers with two maize (Zea mays L.) hybrids (Pioneer Brand Hybrids `3165' and `3737') so as to subject plants to four VPD levels (1.1, 2.0, 2.9 and 3.6 kPa) during a soil drying experiment. In contrast to the original hypothesis, there was little (≤ 0.05 FTSW) change in the threshold FTSW in response to increased VPD for either hybrid. In fact, over the narrow 0.31–0.38 FTSW range observed, the two hybrids showed opposite trends in FTSW threshold as VPD increased. These results supported the view that the FTSW threshold for the decline in transpiration with drying soil is stable, showing little sensitivity to changes in VPD.

We compare predictions of a simple process-based crop model (Soltani and Sinclair 2012), a simple statistical model (Schlenker and Roberts 2009), and a combination of both models to actual maize yields on a large, representative sample of farmer-managed fields in the Corn Belt region of the United States. After statistical post-model calibration, the process model (Simple Simulation Model, or SSM) predicts actual outcomes slightly better than the statistical model, but the combined model performs significantly better than either model. The SSM, statistical model and combined model all show similar relationships with precipitation, while the SSM better accounts for temporal patterns of precipitation, vapor pressure deficit and solar radiation. The statistical and combined models show a more negative impact associated with extreme heat for which the process model does not account. Due to the extreme heat effect, predicted impacts under uniform climate change scenarios are considerably more severe for the statistical and combined models than for the process-based model.

Nos travaux de recherche apportent la preuve que l'activite fixatrice de N 2 peut etre potentiellement amelioree en conditions de deficit hydrique. Les ressources genetiques utilisees pour exploiter cette possibilite ont ete obtenues en developpant une approche unique de criblage de materiel genetique pour un caractere physiologique complexe. L'etude physiologique des effets du deficit hydrique sur la fixation symbiotique nous a permis d'identifier la concentration en ureides des petioles comme critere associe a la sensibilite de la fixation de N 2 , malgre un faible coefficient de correlation. Ce critere specifique nous a servi a etablir un criblage initial comme premiere etape de selection. Les lignees identifiees comme tolerantes dans cette premiere etape ont ete ensuite soumises a deux autres etapes de criblage avec une intensite et une precision plus grandes pour la quantification de la capacite fixatrice de N 2 . Nous avons finalement reussi a identifier huit lignees introduites montrant une tolerance elevee de la fixation de N 2 au deficit hydrique. Ces lignees sont actuellement utilisees pour etudier en detail les mecanismes physiologiques de la tolerance et poursuivre nos efforts d'amelioration genetique des cultivars commerciaux de soja pour augmenter le rendement en conditions de deficit hydrique.

Une large variabilite genetique a ete mise en evidence parmi les especes et cultivars de legumineuses pour la sensibilite de la fixation de N 2 au deficit hydrique. Les especes de legumineuses qui transportent l'azote sous forme d'ureides se sont averees plus sensibles que les especes a amides. Il en a ete deduit qu'un mecanisme de regulation de type « feedback » impliquant les ureides serait responsable de l'inhibition de la fixation de N, par deficit hydrique. Des resultats experimentaux montrant une inhibition de l'activite nitrogenase par l'application d'ureides exogenes constituent une preuve supplementaire en faveur de l'hypothese de regulation « feedback ». Le mecanisme physiologique responsable de l'accumulation des ureides est suppose resulter d'une diminution du catabolisme foliaire des ureides, mais la nature exacte de la molecule chimique impliquee dans le signal reste encore inconnue. Nous avons montre recemment que l'asparagine (Asn) pourrait induire une inhibition de la degradation des ureides dans les feuilles, et par consequent, leur accumulation dans les differents tissus de la plante. Ces resultats soutiennent l'hypothese de l'intervention de Asn comme signal de regulation du metabolisme des ureides et de la fixation symbiotique de N 2 sous deficit hydrique.