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Effects of N in crops are profound, but much understanding of crop growth responses to N is empirical. This review attempts to develop a mechanistic understanding of the effects of N on crop biomass accumulation by elucidating quantitative relationships among leaf N content, CO2 assimilation rate, and crop radiation use efficiency. Three crop species were considered: soybean (Glycine max [L.] Merr.), rice (Oryza sativa L.), and maize (Zea mays L.). The correlation beween leaf N content and leaf CO2 assimilation rates was high within each species, although the response functions were markedly different among species. A relationship was developed predicting crop radiation use efficiency (biomass accumulated per unit solar radiation intercepted) for each of the crops as a function of both leaf CO2 assimilation rate and leaf N content. Radiation use efficiency within each species was nearly constant at high leaf CO2 assimilation rates, but decreased appreciably at low leaf CO2 assimilation rates. At the leaf CO2 assimilation rates typical of a species, the radiation use efficiency was predicted to be about 1.2 g MJ-1 for soybean, 1.4 g MJ-1 for rice, and 1.7 g MJ-1 for maize. Simple calculations during early crop growth examined the competitive use of N for the construction of either large leaf area or high leaf N content. Maize had the greatest biomass accumulation because it had low leaf N contents that allowed the most crop leaf area growth, and it had high radiatiion use efficiencies. For each rate of N supply to leaves, an optimum leaf N content existed to maximize crop biomass accumulation

A system for employing open-ended root chambers to measurein situ acetylene reduction rates under field conditions is described. Gas mixtures containing about 2 mbar acetylene were continuously flowed through the chambers providing a continuous record of acetylene reduction. These chambers have been used to measure acetylene reduction rates of soybeans during three growing seasons. The system has proved to be reliable with a high degree of precision. The large amount of plant-to-plant variability observed in N2 fixation research has been confirmed by the data collected with this system. However, such variability in physiological studies can be reduced by using a non-destructive system to compare the response of an individual plant with its rates before treatment.

Previous work had indicated that the harvest index within cultivars of soybeans [Glycine max (L.) Merr.] might be fairly stable. Such stability of harvest index would greatly simplify the conceptualization of the influence of seed growth processes on yield. In the studies reported here harvest index of field-grown soybean plants was measured in response to four types of environmental influence: interplant competition, extended photoperiod, timing of drought stress, and degrees of drought stress (line source irrigation). The experiments included both indeterminate and determinate cultivars, and included both a northern (Ithaca, N.Y.) and southern location (Gainesville, Fla). These experimental conditions resulted in considerable variability in plant sizes within a cultivar but the harvest index (ratio of seed mass to mass of mature plant without fallen leaves and petioles) was stable. Total masses of individual mature ‘Chippewa 64’ and ‘Bragg’ plants varied substantially in response to interplant competition, but harvest index was essentially constant (0.54 and 0.50 for Chippewa 64 and Bragg, respectively) across most of the range of plant sizes. Harvest index of smaller plants was less than for larger plants, but these smaller plants have little influence on total crop yield and harvest index. Extending the photoperiod to 14.5 h throughout the life cycle of six cultivars increasingly delayed ontogenic events as the distance from the photoperiod lamps decreased. However, no influence on harvest index was observed except at the position nearest the lamps where the standard errors of the harvest indices were increased. Harvest index did not respond to differences in the timing of severe drought stress. When rain was withheld from ‘Wilkin’ plants during vegetative growth, flowering, or seed setting, harvest index of individual plants was essentially 0.60. Only those plants stressed late in pod filling had lower harvest index and increased variability in harvest index. Seed yields (Chippewa 64, Wilkin, and ‘Evans’ for three planting dates) decreased substantially as degree of drought stress, induced by increasing distance from a line source irrigation system, increased yet, harvest index changed negligibly with distance from the line source. Thus, we conclude that harvest index is a conservative characteristic within each soybean cultivar.

The components of the energy balance at the surface of spherical fruits are presented and their relevance in the field situation is discussed. Of these, forced convection is the most complicated, varying over the sphere's surface, depending on wind direction, velocity and turbulent intensity, and the presence of adjacent bluff bodies. The flux density of solar radiation and wind velocity are the two most important environmental parameters in determining fruit temperature. Of lesser importance are fruit size and albedo, wind direction, fruit transpiration and thermal exchange by long-wave radiation. Based on an energy balance at the surface, simple relationships are proposed to predict maximum and minimum temperature increases on the surfaces of sunlit fruits. These relationships are functions of flux density of absorbed radiation, fruit size and thermal conductivity, and a convective heat transfer coefficient that may be calculated from wind velocity using equations developed by Nobel (1975). Good agreement is found between the simple model and an infinite series analytical model of the system, and also with laboratory measurements of berry temperatures. Measurements on green grape berries in the field showed maximum skin temperatures of the order of 12°C above air temperature, with a gradient of 3°C across sunlit berries. Berries on tight clusters heated more than those on loose clusters.

Drought effects on the final leaf area of individual leaves were hypothesized to depend on the leaf developmental stage at which drought occurred. To evaluate this hypothesis, final leaf area and cell number were measured for soybean (Glycine max (L.) Merr.) leaves that were at different stages of development when single or cyclical drought treatment was imposed. Leaf emergence rate from the meristem, as depicted by changes in the plastochron index, was not as sensitive as leaf expansion to cyclical droughts. For leaf expansion, small leaves, once they emerged from the meristem, suffered larger decreases in growth than leaves undergoing rapid leaf area expansion. Decreases in final leaf area as a result of a cyclical drought were correlated with decreases in final cell number. Decreases resulting from a single 8-d drought were dependent on the age of the leaf at the time of drought, because small leaves were found to have proportionately larger decreases in final cell number and area than larger leaves. These results indicated that age-dependent leaf responses to drought are based on the relative activity of cell division and expansion at the time stress was imposed.

Virtually no information exists on the fate of nodule numbers once drought is imposed on a legume crop. This study was undertaken on field-grown soybean (Glycine max (L.) Merr.) to observe possible short-term changes in nodule number and dry weight in response to four irrigation treatments. Two years of study showed that mild drought treatments had no significant effect on nodule number or dry weight. Only when severe drought was imposed were significant (P

The low gas permeability of a diffusion barrier in the cortex of soybean nodules plays a significant role in the protection of nitrogenase from oxygen inactivation. It may also set an upper limit on nodule respiration and nitrogen fixation rates. Two methods which have been used to quantify the gas permeability of leguminous nodules are reviewed and found to be unreliable. A new assay technique for determining both the nodule activity and gas permeability is developed and tested. This ‘lag-phase’ assay is based on the time nodules require to reach steady-state ethylene production after being exposed to acetylene. The technique is rapid, insensitive to errors in biochemical parameters associated with nitrogenase, and is non-destructive. The method was tested with intact aeroponically grown soybean plants for which the mean nodule gas permeability was found to be 13.3×10−3 mms−1. This corresponds to a layer of cells approximately 35 um thick and is consistent with previously reported values.

A novel phenomenon in which wilted cabbage leaves appeared to regain positive turgor pressures without additional water uptake has been previously reported (J Levitt [1986] Plant Physiol 82: 147-153). These experiments were replicated and the biophysical nature of turgor recovery characterized. Leaf water potential and its components were assayed in hydrated, wilted, and desiccated leaves which appeared to regain turgor after wilting. The hypotheses that turgor recovery was due to an increased volumetric elastic modulus (epsilon), or alternatively the result of solute redistribution were tested. Quantitative evidence that turgor recovery occurs in excised leaves was found. Leaf turgor pressure in hydrated leaves ( approximately 0.6 megapascal) decreased to zero upon wilting. After continued desiccation, turgor pressure returned to approximately 0.3 megapascal even though leaf relative water content declined. The epsilon of hydrated leaves was large and there was no evidence of an increased epsilon in the turgor-recovered leaves. Solute mobilization occurred during desiccation. The apoplastic osmotic potential decreased from -0.15 to -0.44 megapascal in hydrated and turgor-recovered leaves, respectively, and solutes were transported from the lamina to the midrib tissue. Solute redistribution coupled with the high epsilon may have resulted in localized turgor recovery in specific cells in the desiccated leaves.

While diurnal cycles in nitrogen fixation rates are sometimes assumed to result from diurnal variation in photosynthetically active radiation, contradicting evidence exists that indicate soil temperature is the primary environmental influence. These studies assessed the significance of temperature on soybean nitrogen fixation under field conditions. Two groups of intact field-grown soybean plants, one at ambient and the other exposed to a 10°C diurnal variation in soil temperature, were nondestructively assayed for acetylene reduction rates. Activity was closely associated with soil temperature (R2=0.85), even when temperature was 12 h out of phase with ambient. Data were also obtained to determine if the effects of rhizosphere temperature on nitrogen fixation are mediated through an effect on the nodule oxygen permeability. Nodule oxygen permeability of intact, aeroponically grown soybean was closely correlated with the diurnal changes in temperature (R2=0.90).

A prime concern in cultivating crops has always been water availability. The earliest crops may have been seeded about 18,000 years ago on the high dunes area of the Nile floodplain immediately after the flood waters receded (Wendorf et al. 1982). This practice assured adequate moisture for plants to grow and produce grain. Plant water-use efficiency was a topic for early scientific study (Briggs and Shantz 1913, Lawes 1850, Woodward 1699). Knowledge of the factors influencing crop water-use efficiency and a hope to improve the efficiency has continued to be an objective in many modern investigations. Wittwer (1975) identified water as the second-most limiting factor, behind land area, to increasing food production. He argued that a high research priority should be an improvement in the efficiency of water use by plants. Considerable research has been done on crop water-use efficiency during the past century, but much work resulted in empirical conclusions that seemed confusing or contradictory. However, recent developments in the understanding of the physical and physiological processes regulating crop growth and water loss allow crop water-use efficiency to be analyzed in quantitative, mechanistic terms.

a b s t r a c t Plant phenology is a critical component of crop adaptation, especially under environmental conditions that don't allow crop growth for unlimited periods. In chickpea (Cicer arietinum L.), which faces terminal drought and increasing temperature at the end of its growing season, it is widely considered that longer duration genotypes are needed for the higher latitudes of India and shorter duration genotypes for lower latitudes. Here, we compare two sets of genotypes bred in two locations varying in latitude (high latitude: Hisar, Haryana, India; low latitude: ICRISAT, Andhra Pradesh, India) for the number of biological days from emergence to flowering (EMR1) and for the grain filling period (R5R7). Biological days referred to days where the phenological development was optimal and therefore provides a measure of thermal time. Using a robust crop simulation model, the optimum EMR1 and R5R7 were determined for various locations. As expected, EMR1 and R5R7 values of genotypes bred for low latitude were lower than those bred for high latitude. However, predicted yields of these two sets of genotypes were similar when simulated for each of the two environments, yields being overall higher at Hisar. Results for the combined set of genotypes at each location predicted a similar optimum EMR1 to achieve maximum yield at each location: 44.3 biological days at Hisar and 43.5 biological days at ICRISAT. Derivation of optimum EMR1 across a total of ten locations in India indicated a wider range (37.2-51.8 biological days), although in eight locations the optimum EMR1 was in a narrower range (39.4-47.3 biological days). The differences in EMR1 across locations did not correspond to their latitudinal differences. Instead, rainfall through the growing season was significantly and positively related (R 2 = 0.55) to optimum EMR1. These results indicate that the breeding for optimum EMR1 of chickpea in India needs to be focused on expected rainfall for a region, and that an optimum EMR1 of about 43 biological days would likely fit most of the environments.

For active nodule nitrogen fixation, O(2), N(2), and carbohydrate must be transported throughout the nodule. No quantitative analysis of these transport processes in the nodules has been presented. By invoking several simplifying assumptions, a second-order differential equation for the various gradients and concentrations in the nodule was solved. Even though the nodule can only be approximated in this analysis, it indicates clearly that intercellular gas spaces must exist in nodules for adequate O(2) distribution. To preserve low O(2) concentrations and protect the nitrogenase, these gas spaces cannot be in direct contact with the ambient atmosphere. It is hypothesized that a gas barrier exists in the cortical region of the nodule to limit O(2) diffusion. This barrier would not substantially inhibit N(2) transport. Carbohydrate transport from the vascular tissue via diffusion in the liquid phase can adequately accommodate the requirements within the nodule.

Leaf senescence in field-grown soybean (Merrill) as defined by the period after full expansion, was studied by measuring abscisic acid (ABA), total soluble protein, and chlorophyll in leaves through the later part of the growing season. ABA concentrations increased significantly at the end of the season when leaves had started to turn yellow, well after total soluble protein and chlorophyll had started to decline. The results indicate that events occurring before leaf yellowing are more significant in evaluating leaf senescence since the yellowing condition and rise in ABA are effects of changes in physiological activity beginning when leaves are still green.

Nitrogenase, the enzyme responsible for nitrogen fixation, is inactivated by oxygen (Bergersen 1962), yet leguminous nitrogen fixation is a highly energy-dependent reaction. A high energy requirement necessitates high rates of oxygen consumption (Tjepkema 1971). Nodules must be able to support high respiration rates and at the same time provide a nearly oxygen-free environment in which nitrogen fixation is sequestered. This oxygen paradox has led to a wide range of hypotheses concerning the nature and physiology of oxygen protection in leguminous nodules.

The rate of nitrogen fixation per unit dry weight of nodules (specific nodule activity) typically increases with increasing rhizosphere pO2 but is not ordinarily limited by photosynthate supply. Therefore, oxygen supply for nodule respiration is the most direct limiting factor of specific nodule activity. Nodule permeability to oxygen varies with temperature and water status, and may be under physiological control. Adequate soil moisture can increase the rate and duration of nitrogen fixation.

Soybean (Glycine max (L.) Merr) seed coat respiration rates in response to changing O(2) concentration and temperature were examined experimentally and with a mathematical analysis. The experimental observations showed seed coat respiration rates were sensitive to O(2) concentration below 0.25 micromole O(2) cm(-3). There was a steady decline in respiration rates from the saturating O(2) concentration down to about 0 to 0.03 micromole O(2) per cubic centimeter. Seed coat respiration rates were found to change linearly with temperature between 8 and 28 degrees C. The explanation for these results was sought by examining the diffusion of O(2) into the vascular bundles of the soybean seed coat. Differential equations describing O(2) uptake in two distinct zones of the vascular bundle were solved. The outer zone was assumed to be O(2) saturated and respiration proceeded at a constant rate per unit volume. The inner zone was assumed to have respiration rates which were linearly dependent on O(2) concentration. The solution of this mathematical model showed considerable similarity with the experimental results. Respiration rates were predicted to saturate at about 0.31 micromole O(2) per cubic centimeter and to decrease curvilinearly below that concentration. While the mathematical model predicted an exponential response in respiration rate to temperature, it was found that the exponential response is difficult to distinguish from a linear response in the temperature range studied experimentally. Consequently, both the experimental and theoretical studies showed the importance of O(2) diffusion into soybean seed coat vascular bundles as a potential restriction on respiration rates. In particular, it was suggested that increases in the total length of the vascular bundles in the soybean seed coat was the major option for increasing the total respiratory capability.

Nodule nitrogen fixation rates are regulated by a mechanism which is responsive to the rhizosphere oxygen concentration. In some legumes, this oxygen-sensitive mechanism appears to involve changes in the gas permeability of a diffusion barrier in the nodule cortex. In soybean evidence for such a mechanism has not been found. The purpose of this research was to make quantitative measurements of soybean nodule gas permeability to test the hypothesis that soybean nodule gas permeability is under physiological control and responsive to the rhizosphere oxygen concentration. Intact hydroponically grown soybean plants were exposed to altered rhizosphere oxygen concentrations, and the nodule gas permeability, acetylene reduction and nodule respiration rates were repeatedly assayed. After a change in the external oxygen concentration, nitrogenase activity and nodule respiration rates displayed a short-term transient response after which the values returned to rates similar to those observed under ambient oxygen conditions. In contrast to steady-state nitrogenase activity and nodule respiration, nodule gas permeability was dramatically affected by the change in oxygen concentration. Decreasing the external oxygen concentration to 0.1 cubic millimeter per cubic millimeter resulted in a mean increase in nodule gas permeability of 63%. Increasing the rhizosphere oxygen concentration resulted in decreased nodule gas permeability. These data are consistent with the hypothesis that soybean nodules are capable of regulating nitrogen fixation and nodule respiration rates in response to changes in the rhizosphere oxygen concentration and indicate that the regulatory mechanism involves physiological control of the nodule gas permeability.

A convenient and stable method for expressing crop seed growth is important both in experimental and simulation studies. The conventional approach has been to measure the seed growth rate (SGR) of individual seeds and multiply this by the number of seeds per unit ground area. An alternative to measurements of SGR has emerged from the observation that during seed growth, harvest index (HI) increases linearly over time. The objective of this study was to examine and compare SGR and dHI/dt as the appropriate description of crop seed growth on the basis of 22 data sets representing four crop species for which there were intensive measures of seed growth and HI. Seed growth was analyzed by linear, quadratic, cubic, and logistic models, in regression analysis. These analyses showed that SGR and dHI/dt were not constant for the entire seed growth period. However, constant values for these two measures of seed growth were statistically equivalent to a cubic function of seed growth for much of the seed growth period. A linear increase in SGR and dHI/dt was found to be statistically appropriate when seed weight and harvest index were increasing from 10 to 90% of their maximum value. It was found in comparisons of the coefficients of variability that the stability of dHI/ dt was greater than for SGR for individual cultivars grown under a range of conditions. Overall, this analysis indicated that a constant dHI/dt is an attractive approach for expressing crop seed growth.

Short-term studies have indicated that alterations in the oxygen partial pressure (pO(2)) around developing soybean (Glycine max [L.] Merr.) seeds may alter seed growth characteristics. A 2-year field study was undertaken to determine the effects on seed development of long-term exposures of individual pods to either sub-ambient or supra-ambient pO(2). Pod chambers were used through which fixed pO(2) were continuously flowed throughout seed development. No effects on maturity date were observed from exposures to either sub-ambient or supra-ambient pO(2). On the other hand, seed weight was reduced by 0.10 pO(2) in both years of the study implicating an O(2) limitation on seed growth rate at this fairly high pO(2). In 1 of the 2 years, supra-ambient pO(2) resulted in increased seed weight.