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Growth and yield responses of okra (Abelmoschus esculentus(L.) Moench, cv. Clemson Spineless) to 6 different day/night temperature regimes and 3 CO2concentrations were studied in controlled environment chambers of the Duke University Phytotron. The objective of these experiments was to characterize the effects of temperature and CO2concentrations on vegetative growth and fruit yield of okra. The day/ night temperature regimes in the greenhouses at ambient light and CO2concentration (350 ppm) were: 17/11, 20/14, 23/17, 26/20, 29/23 and 32/26 C, and the CO2concentration in the growth chambers at 20/14 C were 450, 675, and 1,000 ppm. In the experiments at ambient CO2concentration, no plants survived at temperatures below 26/20 C. The plants grown at 26/20 C and higher temperatures in the greenhouses grew to maturity and produced fruits. Under CO2enrichment in growth chambers, however, the plants survived to maturity at 20/14 C. Thus at the suboptimal temperature of 20/14 C, CO2enrichment compensated for the adverse effects of cool temperature on the growth of okra. The chamber grown plants at 20/14 C produced the maximumd ry weight in 1,000 ppm CO2. The greenhouse grown plants under ambient CO2and light produced the greatest dry weight at 32/26 C. The results demonstrate why okra is not a productive crop at mean temperature of 26/20 C or below in ambient CO2concentration. However, at increasing atmospheric CO2levels okra becomes much more vigorous and productive at low temperature and therefore may spread into cooler areas if the global atmospheric CO2concentration continues to increase.

Seeds of Gliricidia sepium (Jacq.) Walp., a tree native to seasonal tropical forests of Central America, were inoculated with N-fixing Rhizobium bacteria and grown in growth chambers for 71 days to investigate interactive effects of atmospheric CO2 and plant N status on early seedling growth, nodulation, and N accretion. Seedlings were grown with CO2 partial pressures of 350 and 650 µbar (current ambient and a predicted partial pressure of the mid-21st century) and with plus N or minus N nutrient solutions to control soil N status. Of particular interest was seedling response to CO2 when grown without available soil N, a condition in which seedlings initially experienced severe N deficiency because bacterial N-fixation was the sole source of N. Biomass of leaves, stems, and roots increased significantly with CO2 enrichment (by 32%, 15% and 26%, respectively) provided seedlings were supplied with N fertilizer. Leaf biomass of N-deficient seedlings was increased 50% by CO2 enrichment but there was little indication that photosynthate translocation from leaves to roots or that plant N (fixed by Rhizobium) was altered by elevated CO2. In seedlings supplied with soil N, elevated CO2 increased average nodule weight, total nodule weight per plant, and the amount of leaf nitrogen provided by N-fixation (as indicated by leaf d15N). While CO2 enrichment reduced the N concentration of some plant tissues, whole plant N accretion increased. Results support the contention that increasing atmospheric CO2 partial pressures will enhance productivity and N-fixing activity of N-fixing tree seedlings, but that the magnitude of early seedling response to CO2 will depend greatly on plant and soil nutrient status.

The effects of CO2 enrichment and water stress on gas exchange of Liquidambar styraciflua L. (sweetgum) and Pinus taeda L. (loblolly pine) seedlings were examined for individuals grown from seed under high (1000 µmol·m-2·s-1) and low (250 µmol·m-2·s-1) photosynthetic photon flux density at 350, 675 and 1000 µl·l-1 CO2. At 8 weeks of age, half the seedlings in each CO2-irradiance treatment were subjected to a drying cycle which reduced plant water potential to about -2.5 MPa in the most stressed plants, while control plants remained well-watered (water potentials of -0.3 and -0.7 MPa for sweetgum and loblolly pine, respectively). During this stress cycle, whole seedling net photosynthesis, transpiration and stomatal conductance of plants from each CO2-irradiance-water treatment were measured under respective growth conditions.

Atmospheric carbon dioxide and irradiance are important factors affecting growth and yield of plants. Due to the wide variation in irradiance in natural plant stands and the reportedly increasing carbon dioxide concentration in the global atmosphere, it is essential to study the interacting effects of these factors on the growth and production of crop plants. Growth analysis techniques were used to study the interaction of atmospheric CO2concentration (350 and 675 µl/l) and photosynthetic photon flux density (PPFD) (600 and 1200 µEM−2s−1)on four species (including both seed and root crops) grown in controlled environment chambers of the Duke University Phytotron. The plants were soybean (Glycine maxL. Merr.), radish (Raphanus sativusL.), sugarbeet (Beta vulgarisL.), and corn (Zea maysL.). Total dry matter production increased in all species of plants and at all growth stages with both increased CO2concentration and PPFD levels, and the maximum dry matter was produced at the highest combined levels of CO2and PPFD. The dry weight increase varied between the different species and between plant parts within a species. High levels of CO2and PPFD caused a greater increase in net assimilation rate in the plants during early growth stages than in later stages because the first two or three young, rapidly growing leaves were very efficient photosynthetic organs. A high CO2or PPFD level resulted in decreasing leaf area ratios with increasing plant age for all the species due to a rapid increase in stem and root growth later in fruit production, and to a decreasing specific leaf area. Corn, having the C4pathway of photosynthesis, showed less response to increased CO2and PPFD than the three C3species. Increasing the atmospheric CO2concentration from 350 to 675 µl/liter at low and high PPFD levels produced dry matter increases of 72.7 and 76.4%, respectively, in soybean, and 18.9 and 18.6%, respectively, in corn at 50 days after planting. None of the species tested were light saturated at levels available in the standard fluorescent and incandescent lighting as is shown by the increased growth when higher PPFD levels were obtained with a combination of multivapor and sodium lamps.

Most previous studies on the influence of CO2concentration on plant growth have investigated the effect of atmospheric CO2enrichment alone. Little attention has been given to possible interactions between CO2enrichment and nutrient supply. The objective of this study was to characterize the long‐term effects of atmospheric CO2enrichment on growth components of wheat (Triticum aestivumL.) grown under different nutritional levels. Growth and yield responses of spring wheat (cv. GWO1809) to two (350 and 675 ppm) CO2and four (full strength, 1/2, 1/8, or 1/16 strength Hoagland's solution No. 1) nutritional levels were measured in controlled environment chambers at day/night temperature of 26/20 C. Plants were grown from seed and were irrigated with respective nutrient solutions three times daily until maturity. Plants grown in a 675 ppm CO2atmosphere produced more total dry matter at each nutrient level compared to those grown in 350 ppm CO2. The root:shoot ratio decreased with increased nutritional level in both CO2concentrations. Total weight and number of seeds produced in high CO2were greater than those produced in low CO2under similar nutrition. In low CO2, seed weight and number increased with each increase in nutrient concentration up to the one‐half strength level and then decreased with full strength. In high CO2, however, increasing the nutritional level from one‐half to full strength did not decrease seed weight and number significantly. As the plants grew older, the increments of increase in total plant dry weight during harvest intervals were always greater in plants grown in 675 ppm CO2than those grown in 350 ppm CO2.

We measured needle photosynthesis of loblolly pine seedlings grown in a factorial experiment with two CO2 partial pressures (35 and 65 Pa) and three nutrient treatments (7 mM NH4NO3 + 1 mM PO4; 7 mM NH4NO3 + 0.2 mM PO4; 1 mM NH4NO3 + 1 mM PO4). The data were used to parameterize a physiologically based photosynthetic model that included limitations imposed by ribulose-1,5-bisphosphate carboxylase/oxygenase activity, electron transport capacity and inorganic phosphate availability. With nonlimiting nutrients, seedlings grown at 65 Pa CO2 had significantly higher net photosynthesis and lower stomatal conductance than seedlings grown at 35 Pa CO2. Nutrient limitations by either N or P significantly reduced photosynthetic capacity. When either N or P was limiting, there was no effect of growth CO2 partial pressure on photosynthesis, but stomatal conductance was significantly lower for seedlings grown at 65 Pa CO2. Modeled biochemical parameters suggest that, in all cases, photosynthesis was co-limited by carboxylation, electron transport and phosphate regeneration. Acclimation to growth in elevated CO2 involved a reduction in leaf N content. In the low-N and low-P treatments, modeled parameters indicated that the biochemical processes of photosynthesis were down regulated to the point that there was no effect of increasing CO2 partial pressure. The capacity to regenerate phosphate was reduced in both low nutrient treatments, but was only reduced by elevated CO2 when seedlings were grown under low soil P conditions. Increased photosynthetic water use efficiency and nutrient use efficiency in response to CO2 enrichment occurred in all three nutrient treatments and have important implications for whole-plant water and nutrient balance. These data support the contention that soil nutrient status in forest ecosystems will be a critical influence on tree seedling response to increasing atmospheric CO2 partial pressures.

The purpose of this paper is to summarize the results of a series of greenhouse and open‐ top chamber studies on the effects of N and elevated atmospheric CO2on ponderosa and loblolly pine (Pinus ponderosaLaws, and P. taedaL.) to evaluate common patterns of response. Growth response to elevated CO2ranged from zero to more than 1000%, depending largely upon N status. In both species, growth response to CO2was greater under moderate N deficiency than under extreme N deficiency or N sufficiency/excess. Elevated CO2generally caused lowered tissue N concentrations in many (but not all) cases, which in turn resulted in smaller increases in N uptake than in biomass. Growth response to N ranged from −50 (in ponderosa pine) to more than 1000%, depending upon the N status of the control medium. Growth response to N was enhanced by elevated CO2when N was in the extreme deficiency range but not when N was in the moderate deficiency range. In two separate studies, ponderosa pine responded negatively to high N inputs, and in each case this response was mitigated by elevated CO2. Collectively, these results show that (i) N deficiency is a continuum rather than a step function, (ii) responses to elevated CO2vary across this continuum of N deficiency, and (iii) elevated CO2greatly enhances growth response to N additions when N is initially in the extremely deficient range.

Two pea (Pisum sativum L.) hybrids differing in the presence or absence of the cyanide-resistant (alternative) pathway of respiration were constructed by reciprocally crossing cv. Alaska and cv. Progress No. 9. The F1 hybrids were grown in greenhouses maintained at either 350 or 650 ppm CO2, and the growth, flowering, and dry matter accumulation were compared. The objective was to assess the significance of the alternative respiratory pathway to whole-plant carbon budgets and further to test the hypothesis that the alternative pathway is important in oxidizing excess carbohydrates such as might accumulate under conditions of CO2 enrichment. More carbohydrates were available in the F1 hybrid lacking the pathway, as evidenced by greater plant height, leaf area, specific leaf weight, and total dry matter compared with the reciprocal hybrid, especially at 650 ppm CO2. Specific leaf weight increased markedly under CO2 enrichment in the hybrid lacking the pathway, while it was the same at 350 and 650 ppm in the reciprocal cross. The hybrid lacking the alternative pathway also outperformed the reciprocal cross in terms of total dry matter and seed production. Increased branching with CO2 enrichment was observed in the hybrid lacking the pathway, while branching in the reciprocal cross was only slightly stimulated. These results suggest that alternative respiration consumes luxury carbohydrate and that respiration via this pathway may be considered energetically wasteful in terms of whole-plant carbon budgets.

The present experiment was an attempt to study the differences between photosynthetic responses of pot‐grown and field‐grown soybean [Glycine max(L.) Merr., ‘Bragg’] plants to atmospheric CO2enrichment at six levels of irradiance and four leaf temperatures. Plants were grown in field soil or in pots containing an artificial substrate at five elevated atmospheric CO2levels and two water regimes in open top field chambers. Under well‐watered conditions plants grown in the pots had lower stomatal conductances and lower rates of net photosynthesis than plants grown in the field. However, the field‐grown plants responded to a less extent to increasing atmospheric CO2concentration than pot‐grown plants. The photosynthesis‐ irrdiance relationships were linear in the field‐grown plants and curvilinear in the pot‐grown plants. At the highest irradiance level tested (1400 µmol m−2s−1), the field‐grown plants had maximum net photosynthetic rates of 43 and 48 mg CO2dm−2h−1at 332 and 623 µL L−1CO2respectively. The pot‐grown plants were irradiance saturated at a level of 700 µmol m−2s−1and net leaf photosynthetic rates were 18 and 28 mg CO2dm−2h−1at 332 and 623 µL L−1CO2, respectively. Water stress caused a reduction in both stomatal conductance and net photosynthetic rate in the leaves of pot‐grown plants. The effect of leaf temperature on net photosynthethis and stomatal conductance depended upon the atmospheric CO2concentration and rooting media. High temperature increased net photosynthetic response to CO2enrichment at high CO2concentrations but not at low CO2concentrations.

The water relations of Pentaclethra macroloba (Willd.) Kuntze, a dominant, shade-tolerant, tree species in the Atlantic lowlands of Costa Rica, were examined within the forest canopy. Pressure-volume curves and diurnal courses of stomatal conductance and leaf water potential were measured in order to assess differences in water relations between understory, mid-canopy and canopy leaves. Leaves in the canopy had the smallest pinnules but the largest stomatal frequencies and stomatal conductances of the three forest levels. Osmotic potentials at full turgidity decreased with height in the forest; in the canopy and midcanopy they were reduced relative to those in the understory just enough to balance the gravitational component of water potential. Consequently, maximum turgor pressures were similar for leaves from all three canopy levels. Bulk tissue elastic modulus increased with height in the canopy. Leaf water potentials were lowest in the canopy and highest in the understory, even when the gravitational component was added to mid-canopy and canopy values. As a result, minimum turgor pressures were also lowest in the canopy compared to those at lesser heights, and approached zero in full sunlight on clear days.

Translocation of assimilates in plants of Echinochloa crus-galli, from Quebec and Mississippi, and of Eleusine indica from Mississippi was monitored, before and after night chilling, using radioactive tracing with the short-life isotope 11C. Plants were grown at 28°/22°C (day/night temperatures) under either 350 or 675 µl·l-1 CO2. Low night temperature reduced translocation mainly by increasing the turn-over times of the export pool. E. crus-galli plants from Mississippi were the most susceptible to chilling; translocation being completely inhibited by exposure for one night to 7°C at 350 µl·l-1 CO2. Overall, plants from Quebec were the most tolerant to chilling-stress. For plants of all three populations, growth under CO2 enrichment resulted in higher 11C activity in the leaf phloem. High CO2 concentrations also seemed to buffer the transport system against chilling injuries.

Plants of the salt marsh grass Spartina alterniflora Loisel were collected from North Carolina and grown under controlled nutrient, temperature, and photoperiod conditions. Plants were grown at two different illumination levels; substrate salinity was varied, and leaf photosynthesis, transpiration, total chlorophyll, leaf xylem pressure, and specific leaf weight were measured. Conditions were controlled so that gaseous and liquid phase resistances to CO2 diffusion could be calculated. Growth at low illumination and high salinity (30 ppt) resulted in a 50% reduction in photosynthesis. The reduction in photosynthesis of plants grown at low illumination was correlated with an increase in gaseous resistance. Photosynthetic rates of plants grown at high salinity and high illumination were reduced only slightly compared to rates of plants grown, in 10 ppt and Hoagland's solution. Both high salinity and high illumination were correlated with increases in specific leaf weight. Chlorophyll data indicate that specific leaf weight differences were the result of increases in leaf thickness. It is therefore hypothesized that photosynthetic response can be strongly influenced by salinity-induced changes in leaf structure. Similarities in photosynthetic rate on an area basis at high, illumination were apparently the result, of increases in leaf thickness at high salinity. Photosynthetic rates were generally quite high, even at salinities close to open ocean water, and it is concluded that salinity rarely limits photosynthesis in S. alterniflora.

In order study C3 and C4 plant growth in atmospheric CO2 levels ranging from past through predicted future levels, Abutilon theophrasti (C3) and Amaranthus retroflexus (C4) were grown from seed in growth chambers controlled at CO2 partial pressures of 15 Pa (below Pleistocene minimum), 27 Pa (pre-industrial), 35 Pa (current) and 70 Pa (predicted future). After 35 days of growth, CO2 had no effect on the relative growth rate, total biomass or partitioning of biomass in the C4 species. However, the C3 species had greater biomass accumulation with increasing CO2 partial pressure. C3 plants grown in 15 Pa CO2 for 35 days had only 8% of the total biomass of plants grown in 35 Pa CO2, C3 plants had lower relative growth rates and lower specific leaf mass than plants grown in higher CO2 partial pressures, and aborted reproduction. C3 plants grown in 70 Pa CO2 had greater root mass and root-to-shoot ratios than plants grown in lower CO2 partial pressures. These findings, support other studies that show C3 plant growth is more responsive to CO2 partial pressure than C4 plant growth. Differences in growth responses to CO2 levels of the Pleistocene through the future suggest that competitive interactions of C3 and C4 annuals have changed through geologic time. This study also provided evidence that C3 annuals may be operating near a minimum CO2 partial pressure for growth and reproduction at 15 Pa CO2.

Abutilon theophrasti (C3) and Amaranthus retroflexus (C4), were grown from seed at four partial pressures of CO2: 15 Pa (below Pleistocene minimum), 27 Pa (pre-industrial), 35 Pa (current), and 70 Pa (future) in the Duke Phytotron under high light, high nutrient, and wellwatered conditions to evaluate their photosynthetic response to historic and future levels of CO2. Net photosynthesis at growth CO2 partial pressures increased with increasing CO2 for C3 plants, but not C4 plants. Net photosynthesis of Abutilon at 15 Pa CO2 was 70% less than that of plants grown at 35 Pa CO2, due to greater stomatal and biochemical limitations at 15 Pa CO2. Relative stomatal limitation (RSL) of Abutilon at 15 Pa CO2 was nearly 3 times greater than at 35 Pa CO2. A photosynthesis model was used to estimate ribulose-1,5-bisphosphate carboxylase (rubisco) activity (Vcmax), electron transport mediated RuBP regeneration capacity (Jmax), and phosphate regeneration capacity (PiRC) in Abutilon from net photosynthesis versus intercellular CO2 (A-Ci) curves. All three component processes decreased by approximately 25% in Abutilon grown at 15 Pa compared with 35 Pa CO2. Abutilon grown at 15 Pa CO2 had significant reductions in total rubisco activity (25%), rubisco content (30%), activation state (29%), chlorophyll content (39%), N content (32%), and starch content (68%) compared with plants grown at 35 Pa CO2. Greater allocation to rubisco relative to light reaction components and concomitant decreases in Jmax and PiRC suggest co-regulation of biochemical processes occurred in Abutilon grown at 15 Pa CO2. There were no significant differences in photosynthesis or leaf properties in Abutilon grown at 27 Pa CO2 compared with 35 Pa CO2, suggesting that the rise in CO2 since the beginning of the industrial age has had little effect on the photosynthetic performance of Abutilon. For Amaranthus, limitations of photosynthesis were balanced between stomatal and biochemical factors such that net photosynthesis was similar in all CO2 treatments. Differences in photosynthetic response to growth over a wide range of CO2 partial pressures suggest changes in the relative performance of C3 and C4 annuals as atmospheric CO2 has fluctuated over geologic time.

Summary An integrated approach to study the effects of environmental factors on plants is described. The central theme of the system is the use of CO2 labelled with11C supplied continuously to the plant and following the emitted radiation in vivo in the leaf, the phloem and the various sinks. The system consists of six components and with minor changes can be used for other tracers such as13N for nitrogen fixation studies. Because of the short half life of the isotope, several experiments can be carried out on the same plant under the same environmental conditions without disturbing the plant. A host of ecologically, agriculturally and genetically important questions can be answered using this technique.

Summary Water-stress experiments withPhaseolus vulgaris L. were undertaken to determine the transpiration rate dependency of the naturally occurring leaf H218O fractionation process. Water-stress leaf H218O levels were observed to be unexpectedly higher than controls. Speculations on the cause of this phenomenon are discussed. Since transpiration rate variations should theoretically affect only the rate and not the extent of leaf H218O fractionation, the respective time courses for water-stressed and control leaf H218O accumulations were compared. Water-stressed leaves displayed a slower rate of isotopic enrichment relative to controls, as was predicted from their reduced transpiration rates. In an absolute sense, however, both control and water-stress leaf H218O fractionation rates were markedly greater than projected values from the existing model. Consequently, transpiration rates cannot be derived accurately at present from the observed rates of leaf H218O discrimination. Several modifications of the theory are also considered.

Growth and yield responses of a semi‐dwarf spring wheat (Triticum aestivumL.; cv. GWO 1809) to two CO2concentrations and two water stress regimes were studied in controlled environment chambers of the Duke Univ. Phytotron. Groups of plants in low (350 ppm), or high (1,000 ppm) CO2environments were subjected to water stress By withholding irrigation starting at the 10th day after the beginning of anthesis. A second drying cycle beginning 5 days after termination of the first cycle was also given to some of the plants. Water potential of the flag leaves of the main stem of the plants in each CO2environment reached a minimum of −13 bars at the end of the first drying cycle and −17 bars at the end of the second cycle. Under well‐watered conditions high CO2enhanced the rate of tiller production by 43% and significantly increased grain yield, total dry matter, and number and size of the grains. As water stress developed, the osmotic potentials of the high CO2. plants decreased at a faster rate and resulted in maintenance of higher turgor pressures at the end of each stress cycle compared to the low CO2plants. Osmotic potentials of the leaves of both high and low 2plants decreased faster in the second drying cycle than in the first. Significantly fewer and smaller grains were produced on the plants grown under water stress in both CO2concentrations compared to unstressed plants. In general, high CO2plants under water stress conditions had a grain yield and total dry matter production equal to the unstressed, low CO2plants. Thus, CO2enrichment increased the yield potential of the water limited wheat plants due probably to osmotic adjustment by an increased concentration of solutes in their leaves.

The relationship between carbon exchange rate (CER) and internal CO(2) concentration was measured in leaves of saplings of Liquidambar styraciflua L. (sweetgum) and Pinus taeda L. (loblolly pine) grown from seed for more than 14 months at atmospheric CO(2) concentrations of either 350 or 500 microl l(-1). An elevated concentration of CO(2) during growth reduced CER at any given internal CO(2) concentration in sweetgum, but not in loblolly pine. Stomatal limitation of CER showed little response to concentration of CO(2) during measurement, but was higher in both species when grown at 500 than at 350 microl l(-1) CO(2). The net effect of a long-term increase in CO(2) concentration from 350 to 500 microl l(-1) was an increase in CER of loblolly pine, but a slight decrease in CER of sweetgum. It is suggested that the depression of CER in sweetgum resulted from a reduction in the activity of ribulose-1,5-bisphosphate carboxylase-oxygenase.

Effects of phosphorus supply and mycorrhizal status on the response of photosynthetic capacity to elevated CO(2) were investigated in loblolly pine (Pinus taeda L.) seedlings. Seedlings were grown in greenhouses maintained at either 35.5 or 71.0 Pa CO(2) in a full factorial experiment with or without mycorrhizal inoculum (Pisolithus tinctorius (Pers.) Coker & Couch) and with an adequate or a limiting supply of phosphorus. Assimilation versus internal CO(2) partial pressure (C(i)) curves were used to estimate maximum Rubisco activity (V(c,max)), electron transport mediated ribulose 1,5-bisphosphate regeneration capacity (J(max)), phosphate regeneration capacity (PiRC) and daytime respiration rates (R(d)). Nonmycorrhizal seedlings grown with limiting phosphorus had significantly reduced V(c,max) and PiRC compared to seedlings in other treatments. Elevated CO(2) increased photosynthetic capacity in nonmycorrhizal seedlings in the low phosphorus treatment by increasing PiRC, whereas it induced phosphorus limitation in mycorrhizal seedlings in the low phosphorus treatment and did not affect the photosynthetic capacity of seedlings in the high phosphorus treatment. Despite the variety of effects on photosynthetic capacity, seedlings in the elevated CO(2) treatments had higher net assimilation rates than seedlings in the ambient CO(2) treatments. We conclude that phosphorus supply affects photosynthetic capacity during long-term exposure to elevated CO(2) through effects on Rubisco activity and ribulose 1,5-bisphosphate regeneration rates.

Tuber growth of sweet potato (Ipomoea batatas) is a sink that may be limited by source capacity under present ambient CO2levels. Hence, sweet potato may demonstrate more response to predicted increases in atmospheric CO2than many other annual plants. The present investigation was undertaken to determine the long‐term effects of CO2enrichment on some physiological parameters, growth, and yield, as well as on the source‐sink relationship in sweet potato at different stages of growth. Plants of the cultivar Georgia Jet were grown from stem cuttings in a mixture of gravel and vermiculite in controlled environment chambers at 350, 675, and 1000 μL L−1CO2and were irrigated with one‐half strength Hoagland's solution. The temperature was 28°C during 14‐h days and 20°C during 10‐h nights. The photosynthetic photon flux density was 550 μmol m−2s−1. The length of main stem, total branch length, number of branches, and leaf area were increased for plants grown at 675 or 1000 μL L−1COs. The production of total dry matter of plants increased at each harvest interval in response to CO2enrichment but it was greatest in 1000 μL L−1CO2. Specific leaf weight also increased with increased CO2concentration. The number and diameter of tubers increased at high CO2concentration. At the final harvest, the dry weight of roots and tubers increased 1.8 and 2.6 times in plants grown at 675 and 1000 μL L−1CO2, respectively, compared to those grown at 350 μL L−1CO2. Carbon dioxide enrichment resulted in the modulation of sink capacity to enhance the production of tubers in sweet potato.

Toxicity symptoms of carbon dioxide (CO2) have been observed at 10,000 ppm concentration after six days in seven species. Maize is an indicator plant for C-toxicity, developing zebra-like stripes. Full recovery from C-toxicity occurred only in wheat and maize plants. No deficiency symptoms of carbon have been determined. Root exposure during six days to 10,000 ppm CO2 or near zero CO2 had no visible effect, and plants develop normally.

Tissue water potential is the most important factor throughout the seasons controlling phenological events, photosynthesis, and productivity of Larrea divaricata growing in Deep Canyon near Palm Desert, California. Growth of reproductive structures was initiated at the time of highest tissue water potential and ceased as water potential decreased. Percentage foliation correlated strongly with dawn water potential (r = 0.89). The elongation rate of stems and the rate of node production were both dependent on tissue water potential. Leaf growth and node growth proceeded at varying rates throughout the year, providing a continuous sink for photosynthates. Photosynthesis rates ranged from 9.02 mg CO2incorporated per day per gram dry weight of leaf tissue in September to an estimated 74.7 mg CO2in early February. Net photosynthesis and relative productivity correlated very strongly with dawn water potential (r = 0.93 and r = 0.97, respectively). Larrea plants were labeled at 1— to 2—month intervals with photosynthetically incorporated 14CO2to determine the utilization in growth and storage of photosynthate fractions produced at various times throughout the year. Tissue was subsampled at similar intervals, and the activity in various metabolic compounds (sugar, starch, lipid, organic acid, amino acid, protein, cellulose, and cell—wall materials) was analyzed. The utilization of photosynthates in the various fractions was similar in all seasons. No appreciable mobilization into and out of storage materials was apparent. Never dormant, Larrea remains metabolically active and forms new tissue throughout the year. This growth pattern may be an important adaptation allowing Larrea to exist in a wide range of geographical and climatic areas, and, perhaps owing to the species' tropical affinities, it might have been a preadaptation to the desert environment.

Surveys relating soil wettability and the establishment of annual plants were conducted on soil hummocks under burned and unburned shrubs and trees in a desert scrub community 4 years following a fire. Hummocks under burned and unburned individuals of Larrea divaricata, Prosopis juliflora, and Cercidium floridum were nearly devoid of established annual vegetation, whereas the surrounding soil was densely populated with several species of annual plants, predominately Sphaeralcea emoryi. Hydrophobic (water—repellent) layers of soil were found at various depths in the barren hummocks. In general, the layers were further below the soil surface under burned shrubs or trees, and the degree of water repellency was greater. The soil surrounding the hummocks did not contain hydrophobic layers. The mosaic of annuals in the desert scrub vegetation is probably the result of a reduction of available soil moisture due to the formation of water—repellent soils under the shrubs. The effect was more pronounced following fire.

Encelia farinosa is one of a number of species of desert perennial shrubs in which individuals exhibit considerable seasonal variation in the amount and structure of leaf tissue. The function of seasonal leaf variability in adapting this species to the desert environment was investigated. Field observations demonstrated that leaf quantity and structure are controlled by the moisture status of the environment. The field observations also established the influence which leaf quantity and structure have on the CO2exchange capacity and water status of the shrub. These observations were corroborated by laboratory experiments. The laboratory investigations also indicated that the influences of leaf structure are brought about by alterations in the resistances to CO2and water vapor diffusion.

Following preconditioning to three temperature regimes (5°—16°C, 21°—32°C, 32°—43°C), dark respiration was measured over a 35°C range in three geographically isolated populations of Atriplex polycarpa (Torr.) S. Wats. Intraspecific differences in carbon dioxide exchange were found which correlated with the climate of the area of origin of the population.

Field measurements of carbon dioxide exchange and carbohydrate resources show that physiological adaptations occur in the shrubs Larrea divaricata, Encelia farinosa, Hymenoclea salsola, and Acacia greggii growing in situ in the Colorado Desert of California. L. divaricata, the most widely distributed shrub of the North American deserts, exhibited the greatest seasonal adaptation in the field. This study supports the hypothesis that adaptive temperature acclimations occur in plants.

The effect of temperature preconditioning on the net photosynthesis and dark respiration of four woody desert perennials (Larrea divaricata, Hymenoclea salsola, Encelia farinosa, and Chilopsis linearis) was analyzed. After the establishment of stem cuttings or field transplants in a greenhouse, the plants were preconditioned at daytime temperatures of 15°, 30°, or 40°C in a growth chamber for 7—17 days. Carbon dioxide exchange capacities wer then determined at various temperatures. There were distinct species differences in response to the preconditioning treatments which could be correlated with differences in distribution and ecology.

Plants of Erigeron clokeyi, Haplopappus apargioides, Chamaebatiaria millefolium, and Encelia californica were grown at elevations of 1,250 and 3,094 m for at least 4 weeks in the White Mountains of California. During this period the photosynthetic temperature response was different at the two elevations. However, there was no apparent photosynthetic acclimation to differences in the CO2tensions of the two growth environments.

Rising atmospheric carbon dioxide may alter plant/herbivore interactions. The projected rise in atmospheric carbon dioxide is expected to increase plant productivity, but little evidence is available regarding effects on insect feeding or growth. Leaves of soybean plants grown under three carbon dioxide regimes (350, 500, and 650 µl/liter)were fed to soybean looper larvae. Larvae fed at increasingly higher rates on plants from elevated carbon dioxide atmospheres: 80% greater rates on leaves from the 650 µl/liter treatment than on leaves from the 350 µl/liter treatment. Variation in larval feeding was related to the leaf content of nitrogen and water and to the leaf-specific weight. each of which was altered by the carbon dioxide growth regime of the soybean plants. This study suggests that the impact of herbivores may increase as the level of atmospheric carbon dioxide rises.