Single Leaf Photosynthesis Research Articles

Microgravity effects on thylakoid, single leaf, and whole canopy photosynthesis of dwarf wheat

The concept of using higher plants to maintain a sustainable life support system for humans during long-duration space missions is dependent upon photosynthesis. The effects of extended exposure to microgravity on the development and functioning of photosynthesis at the leaf and stand levels were examined onboard the International Space Station (ISS). The PESTO (Photosynthesis Experiment Systems Testing and Operations) experiment was the first long-term replicated test to obtain direct measurements of canopy photosynthesis from space under well-controlled conditions. The PESTO experiment consisted of a series of 21-24 day growth cycles of Triticum aestivum L. cv. USU Apogee onboard ISS. Single leaf measurements showed no differences in photosynthetic activity at the moderate (up to 600 micromol m(-2) s(-1)) light levels, but reductions in whole chain electron transport, PSII, and PSI activities were measured under saturating light (>2,000 micromol m(-2) s(-1)) and CO(2) (4000 micromol mol(-1)) conditions in the microgravity-grown plants. Canopy level photosynthetic rates of plants developing in microgravity at approximately 280 micromol m(-2) s(-1) were not different from ground controls. The wheat canopy had apparently adapted to the microgravity environment since the CO(2) compensation (121 vs. 118 micromol mol(-1)) and PPF compensation (85 vs. 81 micromol m(-2) s(-1)) of the flight and ground treatments were similar. The reduction in whole chain electron transport (13%), PSII (13%), and PSI (16%) activities observed under saturating light conditions suggests that microgravity-induced responses at the canopy level may occur at higher PPF intensity.

Effects of Low-growing Perennial Ornamental Groundcovers on the Growth and Fruiting of `Seyval blanc' Grapevines

Greenhouse and field-grown `Seyval blanc' grapevines (Vitis sp.) were grown with low-growing, shallow-rooted, mat-forming, ornamental perennial groundcovers, and the effect of the groundcovers on the vegetative and fruiting growth of the grapevines was evaluated. The groundcovers used in this experiment were `Kentucky-31' tall fescue (Festuca arundinacea); white mazus (Mazus japoonicus albus); english pennyroyal (Mentha pulegium); dwarf creeping thyme (Thymus serpyllum minus); strawberry clover (Trifolium fragiferum); `Heavenly Blue' veronica (Veronica prostrata `Heavenly Blue'); and a companion grass mixture of 75% perennial ryegrass (Lolium perenne) and 25% red fescue (Festuca rubra). A control treatment grown without any groundcover was also used in both the greenhouse and field experiments. All of the groundcovers reduced `Seyval blanc' total shoot length from 22% to 85% in the vineyard. Cluster size was reduced in the field from 7% to 68% by the groundcovers compared to the herbicide control treatment, and from 9% to 66% in the greenhouse experiment, but none of the groundcovers in either the greenhouse or field experiments affected the pH, total acidity, or soluble solids concentrations of the `Seyval blanc' juice. English pennyroyal was the only groundcover that reduced in the leaf area of the grapevine. Single-leaf photosynthesis of the `Seyval blanc' grapevines in the field experiment was reduced by all groundcovers except mazus and creeping thyme. Water infiltration rates were 10 to 50 times higher in the groundcovers compared to the bare soil of the herbicide control treatment. Weed growth in the field caused reduction in shoot length similar to the most competitive groundcovers. Weed growth was reduced in the early season by the english pennyroyal and companion grass, and in the late season by all groundcovers. The reduction in growth of the grapevines caused by groundcovers in the greenhouse was a reasonable screen for the affect of groundcovers in the field. The mazus treatment was the only groundcover in our experiments that coupled fast growth with low competitive ability.

Phenology-Dependent Effects of Foliar Injury and Herbivory on the Growth and Photosynthetic Capacity of NonbearingVitis labrusca(Linnaeus) var. Niagara

The response of potted fruitless <i>Vitis labrusca</i> (L.) var. Niagara grapevines to early and late season mechanical and insect defoliation was recorded. Twenty percent of the leaf area was removed with a hole puncher to simulate insect injury early and/or late in the season. Measurements of growth, single-leaf photosynthesis, and whole-vine photosynthesis indicated a higher tolerance to foliar injury late in the season than early in the season. Because defoliation was made on a percentage basis, the results were due to the stage of vine development at the time of damage rather than to the amount of leaf area injured. In a second experiment, measurements of vines injured early in the season by the rose chafer, <i>Macrodactylus subspinosus</i> (F.), and late in the season by the Japanese beetle, <i>Popillia japonica</i> (N.), indicated qualitatively similar impacts of beetle feeding on growth parameters compared to vines injured mechanically. Feeding injury by the Japanese beetle had a negative impact on whole-vine carbon assimilation not observed in mechanically injured vines. A final experiment comparing the effect of mechanical and beetle injury early and late in the season on single leaves indicated no significant differences between rose chafer and mechanical injury early in the season. However, 17 days after injury, leaves injured late in the season by the Japanese beetle had lower carbon assimilation rates than mechanically injured leaves. This response was detected on both injured and uninjured sections of injured leaves.

The effect of leaf removal and canopy height on whole-vine gas exchange and fruit development of Vitis vinifera L. Sauvignon Blanc.

Canopy topping and leaf removal are management practices commonly used in New Zealand vineyards to increase light and pesticide penetration to the fruit zone, thus, reducing disease incidence. Previous research has suggested that an increase in photosynthesis occurs when leaves are removed, and this may compensate for the reduced leaf area. However, it is difficult to extrapolate single-leaf photosynthesis measurements to a whole-plant scale. Therefore the extent of the compensation is unknown. To evaluate the impact of leaf removal and canopy height on whole-vine photosynthesis, treatments were imposed during the lag phase of berry growth. Leaves were removed from the lower quarter of the canopy, or vines were topped to three quarters of the height of control plants, in a two-by-two-factorial design. Both topping and leaf removal caused a decrease in whole-vine photosynthesis immediately after the treatments were imposed. Leaf removal, but not topping height, reduced photosynthesis on a per unit leaf area basis. This suggests that the lower portion of the canopy contributes more than the upper portion of the canopy to whole-vine photosynthesis. When measurements were made again approximately two months later, tall vines without leaf removal had a higher photosynthesis rate than the other treatments. Fruit yield, sugar content, vine carbohydrate reserves and pruning weights followed trends similar to those observed for photosynthesis, suggesting that although some photosynthetic compensation occurred, the defoliation treatments had a negative effect on vine growth.

Ionic and Osmotic Effects of Salinity on Single-Leaf Photosynthesis in Two Wheat Cultivars with Different Drought Tolerance

The effects of iso-osmotic salinity and drought stresses on leaf net photosynthetic rate (PN) in two wheat (Triticum aestivum L.) cultivars BR 8 and Norin 61, differing in drought tolerance, were compared. In drought-sensitive Norin 61, the decline of PN was larger than that in drought-tolerant BR 8. Under NaCl treatment, PN decreased in two phases similarly in both cultivars. In the first phase, photosynthetic depression was gradual without any photochemical changes. In the second phase, photosynthetic depression was rapid and accompanied with a decline of the energy conversion efficiency in photosystem 2 (&Phi;PS2). Our observations suggest that the osmotic factor may induce a gradual depression of photosynthesis due to stomatal closure under both stress treatments. However, under NaCl treatment, a ionic factor (uptake and accumulation of excess Na+) may have direct effects on electron transport and cause more severe photosynthetic depression. The drought tolerance mechanism of BR 8 was insufficient to maintain single-leaf photosynthesis under salinity.

Predicting dry matter production of cauliflower ( Brassica oleracea L. botrytis) under unstressed conditions : Part II. Comparison of light use efficiency and photosynthesis–respiration based modules

Six different modules for dry matter production of cauliflower were parameterised and evaluated using a database of 22 cauliflower crops originating from 15 independent field experiments. The evaluation included a light use efficiency, LUE, based module assuming LUE to be constant, an LUE based module assuming a linear decrease of LUE with increasing daily photosynthetically active radiation sum, I, two photosynthesis–respiration based modules using an analytical integration of the rectangular hyperbola over the canopy, assuming either the light saturated photosynthesis rate of single leaves, P max, to be constant or to decrease proportionally to irradiance within the canopy. Furthermore two slightly modified versions of the light interception and photosynthesis algorithms of the SUCROS model were evaluated, where the negative exponential equation for single leaf photosynthesis was replaced by the rectangular hyperbola. In order to make these modules comparable with the analytical integration approach, P max was also assumed to be either constant or to decrease proportionally to irradiance within the canopy. The results indicate that an estimated constant LUE (3.15 (±0.04) g MJ −1) is only poorly able to predict total dry matter production for cauliflower (modelling efficiency EF=0.69) of an independent data set. Using a linear decline of LUE with I (LUE=6.66 (±0.80)–0.36 (±0.08) I) drastically increased the predictive value (EF=0.88) of the LUE approach. The descriptive and predictive value of the photosynthesis based modules was higher when assuming that P max declines within the canopy. Then the predictive value of the photosynthesis/respiration based approach was better than the simple LUE approach but not generally better than the LUE approach assuming a linear decrease of LUE with increasing daily radiation sum.

Sustained growth and increased tolerance to glyphosate observed in a C3 perennial weed, quackgrass (Elytrigia repens), grown at elevated carbon dioxide

Although the response of crop plants to rising atmospheric carbon dioxide concentration ([CO2]) has been well characterized, little is known concerning the long-term growth and/or photosynthetic response of peren-nial weeds. The growth and photosynthetic characteristics of three cohorts of a perennial C3 weedy species, quack-grass (Elytrigia repens (L.) Nevski) were examined at ~380 µmol mol−1 (ambient) and 720 µmol mol−1 (elevated) [CO2] in temperature-controlled greenhouses during 1998 and early 1999. Different cohorts were used to assess the sensitivity of growth, photosynthesis and glyphosate tolerance to elevated [CO2] for different stages in the life cycle of quackgrass. For the ‘old’ cohort, planted on Day of Year (DOY) 187, elevated [CO2] resulted in a consistent stim-ulation of single leaf photosynthesis, vegetative and whole plant biomass relative to the ambient [CO2] condition over a 231-d period. Data from the ‘intermediate’ (DOY 268) and ‘young’ cohorts (DOY 350) indicated that the stimula-tion of biomass at the elevated [CO2] was time-dependent. To determine if the observed stimulation of growth at ele-vated [CO2] altered tolerance to chemical weed control, glyphosate [(N-phosphonomethyl)glycine] was applied to each cohort and each [CO2] treatment at rates of 0 (control) and 2.24 kg ai ha−1 (sprayed). Tolerance was determined by following the growth and slope of each cohort at the growth [CO2] treatment for a 28-d period following glyphosate application. For the young cohort, [CO2] had no affect on glyphosate tolerance; however, an application rate of 2.24 kg ai ha−1, reduced but did not eliminate growth for the intermediate and old cohorts grown at elevated [CO2]. The basis for increased glyphosate tolerance at elevated [CO2] for these cohorts was unclear, but was not related to plant size at the time of glyphosate application. Data from this experiment indicate that sustained stimula-tion of photosynthesis and growth in perennial weeds could occur as atmospheric [CO2] increases, with a reduction in chemical control effectiveness and potential increases in weed/crop competition.

Sensitivity of field-grown soybean to future atmospheric CO2: selection for improved productivity in the 21st century.

Although genotypic differences among soybean (Glycine max (L.) Merr.) cultivars in their response to future CO2 partial pressures have been observed in the glasshouse, it is unclear if similar responses would occur among cultivars when grown under field conditions at normal stand densities. To determine variation in the sensitivity of soybean growth and seed yield to CO2, we grew two contrasting cultivars of the same maturity group, Ripley (semi-dwarf, determinate) and Spencer (standard, indeterminate), to reproductive maturity at ambient and elevated (30 Pa above ambient) CO2 partial pressures for two field seasons. Spencer had been previously selected in glasshouse trials as responsive to increased CO2. Significant cultivar x CO2 interaction was observed for both vegetative biomass and seed yield, with Spencer demonstrating a consistently greater yield enhancement at elevated CO2 than Ripley (60 vs 35%, respectively). Differences in CO2 sensitivity between cultivars were not evident in measurements of single leaf photosynthesis taken during anthesis, nor early or late pod-fill. Analysis of reproductive characteristics indicated that the sensitivity of the seed yield response to CO2 in Spencer was associated with the ability to form additional seed on axillary branches in response to elevated CO2. Data from this experiment suggest that screening of soybean germplasm at the glasshouse level, when combined with field trials, may be an effective strategy to begin selecting soybean lines that will maximize yield in a future, higher CO2 environment.

The impact of elevated carbon dioxide on the growth and gas exchange of three C4 species differing in CO2 leak rates

Recent work has suggested that the photosynthetic rate of certain C4 species can be stimulated by increasing CO2 concentration, [CO2], even under optimal water and nutrients. To determine the basis for the observed photosynthetic stimulation, we tested the hypothesis that the CO2 leak rate from the bundle sheath would be directly related to any observed stimulation in single leaf photosynthesis at double the current [CO2]. Three C4 species that differed in the reported degree of bundle sheath leakiness to CO2, Flaveria trinervia, Panicum miliaceum, and Panicum maximum, were grown for 31–48 days after sowing at a [CO2] of 350 μl l−1 (ambient) or 700 μl l−1 (elevated). Assimilation as a function of increasing [CO2] at high photosynthetic photon flux density (PPFD, 1 600 μmol m−2 s−1) indicated that leaf photosynthesis was not saturated under current ambient [CO2] for any of the three C4 species. Assimilation as a function of increasing PPFD also indicated that the response of leaf photosynthesis to elevated [CO2] was light dependent for all three C4 species. The stimulation of leaf photosynthesis at elevated [CO2] was not associated with previously published values of CO2 leak rates from the bundle sheath, changes in the ratio of activities of PEP‐carboxylase to RuBP carboxylase/oxgenase, or any improvement in daytime leaf water potential for the species tested in this experiment. In spite of the simulation of leaf photosynthesis, a significant increase in growth at elevated [CO2] was only observed for one species, F. trinervia. Results from this study indicate that leaf photosynthetic rates of certain C4 species can respond directly to increased [CO2] under optimal growth conditions, but that the stimulation of whole plant growth at elevated carbon dioxide cannot be predicted solely on the response of individual leaves.

A Chamber Design for Measuring Net CO 2 Exchange on Rangeland

Net carbon exchange of terrestrial ecosystems will likely change as atmospheric CO2 concentration increases. Currently, little is known of the annual dynamics or magnitude of CO2 flux on many native and agricultural ecosystems. Remoteness of many ecosystems has limited our ability to measure CO2 flux on undisturbed vegetation. Today, many plant ecologists have portable photosynthesis systems with which they make single-leaf photosynthesis measurements. Utility of this equipment is enhanced when canopy-level CO2 flux is also measured. We designed a portable 1-m3 closed chamber for use in measuring CO2 exchange in short statured vegetation with widely varied canopy structure. The design includes external ductwork equipped with doors which are used to open the chamber for ventilation with outside air between measurements. The chamber was tested on a Wyoming big sagebrush (Artemisia tridentata ssp. Wyomingensis Nutt.)/Thurber's needlegrass (Stipa thurberiana Piper) community using 10 plots equally divided between shrub and interspace. The ductwork and doors provided adequate ventilation to allow consecutive measurements of CO2 flux without removing the chamber from the plot. The chamber could differentiate CO2 flux between plots with sagebrush and those with grass only, even at relatively low fluxes. Net CO2 uptake per unit ground area was greater (P = 0.04) on sagebrush-grass plots (7.6 +/- 1.4 micromoles m-2 s-1) than on interspace plots without sagebrush (3.1 +/- 1.0 micromole m-2 s-1). Chamber and leaf temperature increased by an average of 0.5 and 1.2 degrees C, respectively, during measurements.

Performance of Three Species of Salad-type Plants Grown under Narrow-spectrum Light-emitting Diodes (LEDs) in a Controlled Environment

Light-emitting diodes (LEDs) are a promising light source for growing crop plants in spaceflight systems because of their superior safety, durability, small mass and volume, wavelength specificity, and long life. To address the usefulness of LEDs as a plant lighting source, radish (Raphanus sativus L. cv. Cherriette), lettuce (Lactuca sativa L. cv. Waldmann's Green), and spinach (Spinacea oleracea L. cv. Nordic IV) were grown using nutrient film technique hydroponics under cool-white fluorescent (CWF) lamps, red LEDs (peak emission 660 nm), and red LEDs + 30 μmol·m–2·s–1 supplemental blue fluorescent (BF) light (400–500 nm). Each lighting treatment was maintained in a separate controlled-environment chamber with total photosynthetic photon flux (PPF) maintained near 300 μmol·m–2·s–1 and a photoperiod of 18-h light/06-h dark. Results showed significant reductions in total dry mass (TDM) accumulation for radish plants grown under red LEDs + 30 μmol·m–2·s–1 BF or red LEDs only as compared to CWF-grown plants. Although TDM tended to decrease with decreasing BF for lettuce, treatment differences were not significant. TDM for spinach plants was significantly less under red LEDs compared to CWF or red LEDs + 30 μmol·m–2·s–1 BF. Interestingly, single-leaf photosynthesis and leaf conductance were not significantly different for any treatment, regardless of species. The data suggest a species-specific response for the amount of blue necessary for normal plant growth.

Relationship between thylakoid electron transport and photosynthetic CO2 uptake in leaves of three maize (Zea mays L.) hybrids

The introduction of a more efficient means of measuring leaf photosynthetic rates under field conditions may help to clarify the relationship between single leaf photosynthesis and crop growth rates of commercial maize hybrids. A large body of evidence suggests that gross photosynthesis (AG) of maize leaves can be accurately estimated from measurements of thylakoid electron transport rates (ETR) using chlorophyll fluorescence techniques. However, before this technique can be adopted, it will first be necessary to determine how the relationship between chlorophyll fluorescence and CO2 assimilation is affected by the non-steady state PPFD conditions which predominate in the field. Also, it must be determined if the relationship is stable across different maize genotypes, and across phenological stages. In the present work, the relationship between ETR and AG was examined in leaves of three maize hybrids by making simultaneous measurements of leaf gas exchange and chlorophyll fluorescence, both under controlled environment conditions and in the field. Under steady-state conditions, a linear relationship between ETR and AG was observed, although a slight deviation from linearity was apparent at low AG. This deviation may arise from an error in the assumption that respiration in illuminated leaves is equivalent to respiration in darkened leaves. The relationship between chlorophyll fluorescence and photosynthetic CO2 assimilation was not stable during fluctuations in incident PPFD. Since even minor (e.g. 20%) fluctuations in incident PPFD can produce sustained ( > 20 s) departures from the mean relationship between ETR and AG, chlorophyll fluorometry can only provide an accurate estimate of actual CO2 assimilation rates under relatively stable PPFD conditions. In the field, the mean value of ETR / AG during the early part of the season (4.70 ± 0.07) was very similar to that observed in indoor-grown plants in the vegetative stage (4.60 ± 0.09); however, ETR / AG increased significantly over the growing season, reaching 5.00 ± 0.09 by the late grain-filling stage. Differences in ETR / AG among the three genotypes examined were small (less than 1% of the mean) and not statistically significant, suggesting that chlorophyll fluorometry can be used as the basis of a fair comparison of leaf photosynthetic rates among different maize cultivars.

Rice Responses to Elevated CO2 Concentrations and High Temperatures

Long-term CO2 enrichment experiments under various temperature conditions on “Akihikari” rice were conducted using Temperature Gradient Chambers (TGCs) over 4 growing seasons (1992-1995). Measurements were made of crop biomass production, yield, and water use efficiency (WUE), and also of single leaf photosynthesis and transpiration, to evaluate interactive effects of CO2 and temperature on those attributes. Temperature conditions created in the TGC ranged from 21-30°C on average over entire growth season.Doubling CO2 concentration increased total biomass at maturity by 26% on average. The relative enhancement rate in biomass by doubling CO2 tended to increase slightly with a rise in temperature. The doubling CO2 upward shifted the optimum temperature for single leaf photosynthesis by 5°C. Also it increased yield under near outdoor temperature conditions to a similar extent as the dry matter production. However, the yield decreased drastically with a temperature increase both under the ambient and the doubling CO2, due to spikelet sterility induced by high temperature at the flowering period. Doubling CO2 treatments enhanced WUE by 34% on average over the entire range of temperature conditions. The enhancement rate in WUE was maximal (49%) at 26°C, and decreased with a temperature rise at a rate of 7% per°C. These results suggest that interactive effects of CO2 and temperature on various processes of crop production are key issues for understanding and evaluating crop production under global climate change.

Use of 13C and 15N isotopes to investigate O 3 effects on C and N metabolism in soybeans. Part I. C fixation and translocation

Short- and long-term C uptake/transaction dynamics were studied using stable isotope techniques and leaf gas exchange to investigate the mechanism(s) of O 3 action on plant yield and C partitioning. Glycine max (L.) Merr. (soybean) plants were grown in open-top chambers and exposed to one of three O 3 regimes: half-ambient, ambient, or 2 × ambient for nearly the entire growing season. The seasonal 7 h average O 3 concentrations (nl l −1) were 25, 43, and 76 nl l −1, respectively. Whole plant C translocation was measured using pulse-labeled 13CO 2 (99 atom % 13C) at two distant growth stages (R2 and R5). Translocation parameters were as follows: % 13C (sink strength), % 13C/g dry weight (sink intensity), and % 13C/% organ dry weight (relative specific uptake). Single leaf photosynthesis ( P n) was measured at four growth stages (V7, R2, R3, and R4). Ozone significantly affected translocation, but the effect was dependent upon growth stage and the time following the 13C pulse. At the stage of rapid seed fill within the pods (R5), and at 42 h post-labeling, all three leaf translocation parameters had a significant positive linear relationship with O 3 exposure. Conversely, root nodule values were all inversely related to O 3 exposure. Generally, at 0.5 h post-labeling, no significant effects were observed for leaves and nodule translocation patterns, with the exception of an inverse relationship between sink strength and O 3 exposure. No significant differences were observed for single leaf P n among treatments. Our results indicate that the mechanism of chronic O 3 action involves inhibition of translocation, implying reduced phloem loading and the inhibition may be occurring without a concomitant reduction in the amount of C fixed. In addition, 13C pulse labeling appears to be a very useful technique for investigating integrated long-term C translocation dynamics which might not otherwise be evident using instantaneous methods such as short-term labeling or limited leaf gas exchange measurements.

Possibility of Increasing Yield Potential of Rice by Reducing Panicle Height in the Canopy. II. Canopy Photosynthesis and Yield of Isogenic Lines

Reduced panicle height in a rice crop canopy may have beneficial effects of increasing yield potential through reduced shading of leaves leading to greater canopy photosynthesis. Effects of different panicle height in the canopy were evaluated in glasshouse and field experiments using isogenic lines with elongated upper internodes (EUI lines) from two cultivars. Isogenic lines of IR36 and IR50 with elongated upper internodes (IR36EUI and IR50EUI) had panicle heights at the top of the canopy of 96-100% of canopy height, while lines with low panicle heights had panicles which were 74 and 82% of canopy height respectively. Lines with low panicle height had about 10% more of the total leaf area index (LAI) above panicles and this resulted in up to 35% greater light interception by leaves above panicles relative to high panicle height plants. At 5 days before flowering IR36 and IR36EUI had equal canopy photosynthesis, while at flowering, lines had equal shoot nitrogen percentage and LAI. At maturity spikelets per mainstem were not significantly different. At 0, 7, 14 and 21 days after flowering (DAF), IR36, with low panicle height, had 10-30% greater canopy photosynthesis than IR36EUI; greater canopy photosynthesis was observed for IR50 relative to IR50EUI. These beneficial effects of low panicle height on canopy photosynthesis occurred even though the maximum single leaf photosynthesis and respiration rates were similar in both isogenic lines during grain filling. In the field and in a glasshouse experiment where plants were arranged into canopies, IR36, with low panicle heights had 15-40% greater yields than the isogenic line IR36EUI with high panicle heights; greater yields also occurred for IR50 than IR50EUI.

Growth and photosynthetic response of three soybean cultivars to simultaneous increases in growth temperature and CO2

Three soybean (Glycine max L. Merr.) cultivars (Maple Glen, Clark and CNS) were exposed to three CO2 concentrations (370, 555 and 740 μmol mol−1) and three growth temperatures (20/15°, 25/20° and 31/26°C, day/night) to determine intraspecific differences in single leaf/whole plant photosynthesis, growth and partitioning, phenology and final biomass. Based on known carboxylation kinetics, a synergistic effect between temperature and CO2 on growth and photosynthesis was predicted since elevated CO2 increases photosynthesis by reducing photorespiration and photorespiration increases with temperature. Increasing CO2 concentrations resulted in a stimulation of single leaf photosynthesis for 40–60 days after emergence (DAE) at 20/15°C in all cultivars and for Maple Glen and CNS at all temperatures. For Clark, however, the onset of flowering at warmer temperatures coincided with the loss of stimulation in single leaf photosynthesis at elevated CO2 concentrations. Despite the season‐long stimulation of single leaf photosynthesis, elevated CO2 concentrations did not increase whole plant photosynthesis except at the highest growth temperature in Maple Glen and CNS, and there was no synergistic effect on final biomass. Instead, the stimulatory effect of CO2 on growth was delayed by higher temperatures. Data from this experiment suggest that: (1) intraspecific variation could be used to select for optimum soybean cultivars with future climate change; and (2) the relationship between temperature and CO2 concentration may be expressed differently at the leaf and whole plant levels and may not solely reflect known changes in carboxylation kinetics.

Physiological Basis for the Interaction of Mite and Crop Load on `Starkrimson Red Delicious' Apple Trees

Most research on the effects of European red mite (ERM) [Panonychus umli(Koch)] on apples has shown highly variable results. Those variations were probably due to different genotypes, internal status of the tree (e.g., crop load), environment and timing and severity of mite stress. We propose that limitations in carbon availability for fruit growth is the primary mechanism of foliar feeder injury to apple trees. Heavy-cropping tress should be more sensitive to ERM stress than low-cropping trees and differences in responses of apple trees should be better-related to carbon supply/demand than to cumulative mite-days (CMD) along. Eighteen mature `Starkrimson Red Delicious'/M26 trees were hand-thinned to low (≈20 t·ha–1) and high (≈40 t·ha–1) crop loads and submitted to low (&lt;400 CMD), medium (400–1000 CMD), and high (&gt;1000 CMD) target levels of ERM. A range of crop loads and CMDs was obtained. Mite population, fruit growth, single-leaf photosynthesis, and whole-tree gas exchange were monitored through the season. ERM reduced leaf and whole-tree net CO2 exchange (Pn). Heavy-cropping trees showed fruit weight reduction earlier and more severely than low-cropping trees with high ERM injury. The main effect, variations in final fruit weight, was much better related to whole-tree Pn/tree than to CMD.

Plant Growth and Soil Processes in Temperate Grassland Communities at Elevated CO 2

Turves of a Mollic Psammaquent soil were used in controlled environment rooms to examine the response of managed temperate pasture communities to 350, 525 or 700 p.p.m. CO2. Yield of herbage (regrowth over 3-week intervals) increased only slightly with higher C02; however, the botan- ical composition was markedly different. At elevated CO2 Paspalum dilatatum (C4) and Lolium perenne (C3) declined as a proportion of harvested yield despite a stimulation of single leaf photosynthesis that was comparable to that found in Trifolium repens, a species that increased in abundance. Changes in species composition were largely a consequence of C02-induced differences in axillary bud activity. Net primary productivity below-ground was stimulated by CO2. Soil C02-C production was greater in elevated CO2 treatments, and was consistent with a greater input of herbage and root mass and/or metabolites and of more readily decom- posable material. Levels of microbial biomass were un- changed, but enchytraeids were more abundant at elevated CO2. Tracking of 14CO2 into the various C pools also indicated a more rapid turnover of C at elevated CO2 but no change in pool sizes. No consistent effects on net mineralization of N were observed.

Influence of Crop Load on Photosynthesis and Dry Matter Partitioning of Seyval Grapevines. III. Seasonal Changes in Dry Matter Partitioning, Vine Morphology, Yield, and Fruit Composition

Single leaf photosynthesis, whole vine photosynthesis measured in a whole plant chamber, dry matter partitioning, and several morphological indices were measured at four phenological stages of development on two-year-old, own-rooted Seyval grapevines adjusted to five different crop loads. Crop loads were 0, 1, 2, 4, or 6 clusters/vine with all laterals removed to eliminate intra-vine shading. Leaf area was inversely related to crop load, with differences evident as early as fruit set. Differences in dry weight partitioning did not become evident until mid-season. Leaf and shoot dry weight were inversely correlated, and fruit dry weight positively correlated with crop load, respectively. Root dry weight was inversely related to crop load only at harvest. Leaf area per vine, leaf size, shoot length, node number, and internode length were inversely related to crop load. Berries per cluster and cluster weight were inversely correlated with clusters per vine. Yield and berries per vine were correlated with clusters per vine. Sugar accumulation and shoot maturation were delayed by high crop loads. Implications of the crop load treatments on vine photosynthesis are discussed.

Influence of Crop Load on Photosynthesis and Dry Matter Partitioning of Seyval Grapevines. II. Seasonal Changes in Single Leaf and Whole Vine Photosynthesis

Photosynthesis (Pn), leaf area/vine and dry matter partitioning were measured at four phenological stages of development on two-year-old, own-rooted Seyval grapevines adjusted to five different crop loads. Crop loads were 0, 1, 2, 4, or 6 clusters/vine with all laterals removed to eliminate intra-vine shading. Yield/vine was positively correlated and leaf area/vine negatively correlated with clusters/vine. Total dry weight/vine was similar for all crop loads, but increased at each phenophase. Whole vine Pn (WVPn) was measured in a whole plant chamber and expressed as WVPn per unit leaf area (WVPn/L) and WVPn per vine (WVPn/V). Single leaf Pn (SLPn) was also measured on leaves at four node positions. SLPn was highest at the basal node position (node four or five) at fruit set. SLPn at all other node positions was highest mid-season or veraison. WVPn increased through veraison, then declined. SLPn was positively correlated with crop load in at least one leaf position at each phenophase. WVPn/L was positively correlated with crop load only at harvest. WVPn/V was inversely correlated with crop load at mid-season, indicating that vegetative, as well as fruiting sinks, can strongly influence whole vine photosynthetic rates. Over the season, SLPn of the most recently fully expanded leaf was best correlated with WVPn/L. However, there was no general relationship between SLPn and WVPn/V.