Chilling and photosynthetic productivity of field grown maize (Zea mays); changes in the parameters of the light-response curve,canopy leaf CO2 assimilation rate and crop radiation-use efficiency

The effect of growth temperatures on the photosynthetic performance of field grown maize (Zea mays cv. LG11) was examined for crops sown on 1 May and 28 June 1991. During the period of growth, 2 May to 10 August, the early-sown crop experienced temperatures below 10 °C on 33 occasions compared with only one for the crop sown on 28 June. The prolonged period of low temperatures throughout May and beginning of June were associated with a marked depression in CO₂ assimilation rates at all light levels in the early-sown treatment. Chill-induced depression of the photosynthetic light-response curve reflected a sustained reduction in canopy leaf photosynthesis and crop radiation-use efficiency (RUE). During the early stages of growth, RUE was 65% lower in the early- than late-sown treatment, with no marked recovery observed in the former treatment until approximately three weeks after chilling conditions had ceased. Data show a close correlation between chill-induced depression of quantum yield () and RUE, with corresponding reductions in the light-saturated rates of CO₂ assimilation (P_max). The convexity of the light-response curve recovered most rapidly from chilling temperatures, and at least three weeks before any improvement in RUE. It is concluded that photosynthetic productivity of immature maize stands is less sensitive to changes in the convexity of the light response, than to changes in either or P_max.     

Adaptation to high CO2 concentration in an optimal environment: radiation capture, canopy quantum yield and carbon use efficiency

The effect of elevated [CO₂] on wheat (Triticum aestivum L. Veery 10) productivity was examined by analysing radiation capture, canopy quantum yield, canopy carbon use efficiency, harvest index and daily C gain. Canopies were grown at either 330 or 1200 μ mol mol^–1[CO₂] in controlled environments, where root and shoot C fluxes were monitored continuously from emergence to harvest. A rapidly circulating hydroponic solution supplied nutrients, water and root zone oxygen. At harvest, dry mass predicted from gas exchange data was 102·8 ± 4·7% of the observed dry mass in six trials. Neither radiation capture efficiency nor carbon use efficiency were affected by elevated [CO₂], but yield increased by 13% due to a sustained increase in canopy quantum yield. CO₂ enrichment increased root mass, tiller number and seed mass. Harvest index and chlorophyll concentration were unchanged, but CO₂ enrichment increased average life cycle net photosynthesis (13%, P < 0·05) and root respiration (24%, P < 0·05). These data indicate that plant communities adapt to CO₂ enrichment through changes in C allocation. Elevated [CO₂] increases sink strength in optimal environments, resulting in sustained increases in photosynthetic capacity, canopy quantum yield and daily C gain throughout the life cycle.     

Heterogeneity of leaf CO2 assimilation during photosynthetic induction

Spatial mid temporal variations in the distribution of photosynthesis over the leaf area were investigated during induction upon illumination of Rosa rubiginosa L. leaves. Gas exchange and maps of relative photosynthetie electron transport activity computed from chlorophyll fluorescence images were simultaneously monitored. In air, after 15 h of dark adaptation, linear electron transport was heterogeneously distributed over the leaf area during the induction. This patchy induction was explained by asynchronous metabolism activation for the first 10 min of illumination, concomitant asynchronous limitation by intrinsic metabolism and stomatal apertures (10–30 min) and finally by only stomatal limitation beyond 30 min. A brief transition to non-photorespiratory conditions after 20 min of illumination under subsaturating irradiance revealed a marked heterogeneity of CO₂ assimilation, presumably as a result of heterogeneous stomatal apertures. The frequency distribution of CO₂ assimilation was unimodal. During the induction, heterogeneity gradually decreased and photosynthesis was uniform at steady-state. After 10 min of dark adaptation, heterogeneity of linear electron transport activity occurred during the first 15 min of a second induction and mainly resulted from metabolic limitation.     

Relationship between photosystem II activity and CO2 fixation in leaves

There is now potential to estimate photosystem II (PSII) activity in vivo from chlorophyll fluorescence measurements and thus gauge PSII activity per CO₂ fixed. A measure of the quantum yield of photosystem II, Φ_II (electron/photon absorbed by PSII), can be obtained in leaves under steady-state conditions in the light using a modulated fluorescence system. The rate of electron transport from PSII equals Φ_II times incident light intensity times the fraction of incident light absorbed by PSII. In C₄ plants, there is a linear relationship between PSII activity and CO₂ fixation, since there are no other major sinks for electrons; thus measurements of quantum yield of PSII may be used to estimate rates of photosynthesis in C₄ species. In C₃ plants, both CO₂ fixation and photorespiration are major sinks for electrons from PSII (a minimum of 4 electrons are required per CO₂, or per O₂ reacting with RuBP). The rates of PSII activity associated with photosynthesis in C₃ plants, based on estimates of the rates of carboxylation (v_o) and oxygenation (v_o) at various levels of CO₂ and O₂, largely account for the PSII activity determined from fluorescence measurements. Thus, in C₃ plants, the partitioning of electron flow between photosynthesis and photorespiration can be evaluated from analysis of fluorescence and CO₂ fixation.     

Effects of photoinhibitory treatment on CO2 assimilation, the quantum yield of CO2 assimilation, D1 protein, ascorbate, glutathione and xanthophyll contents and the electron transport rate in vine leaves

The responses of photosynthesis to high light and low temperature were studied in vines cultivated in the greenhouse in low light. Exposure to high light (1000 /umol m^?2 s^?1) or low temperature (5 °C) alone had no measurable effect on the photosynthetic processes, but the combination of high light and low temperature caused rapid loss of photosynthetic capacity and a decrease in the efficiency of photosynthetic energy conversion. After a 15 h exposure to 5°C at high light, the F_v/sb/F_mratio had decreased by 80% and the apparent quantum yield by 75%. Nevertheless, when the leaves were returned to low light at 22°C, these parameters recovered rapidly. The foliar pools of ascorbate and glutathione decreased in the first hours of photoinhibitory treatment while the zeaxanthin content increased from negligible levels to about 50% of the total foliar xanthophyll pool. There was a clear correlation between the zeaxanthin content of the leaves and their F_v/F_m ratio during both photoinhibition and recovery. However, there was also a good correlation between the decrease in theF_v F_m ratio and the measured decrease in the total foliar levels of the antioxidants ascorbate and glutathione. The amount of D, protein diminished over the same period as the zeaxanthin levels were increasing. This approach, involving simultaneous measurements of several parameters considered to influence photosystemy II activity, clearly demonstrates that measured decreases in F_v/F_m may not simply be related to zeaxanthin levels or to amounts of D₁ protein alone but result from multifactoral influences.     

Estimation of the carbon dioxide (CO2) fertilization effect using growth rate anomalies of CO2 and crop yields since 1961

The effect of elevated carbon dioxide (CO₂) on crop yields is one of the most uncertain and influential parameters in models used to assess climate change impacts and adaptations. A primary reason for this uncertainty is the limited availability of experimental data on CO₂ responses for crops grown under typical field conditions. However, because of historical variations in CO₂, each year farmers throughout the world perform uncontrolled yield ‘experiments’ under different levels of CO₂. In this study, measurements of atmospheric CO₂ growth rates and crop yields for individual countries since 1961 were compared to empirically determine the average effect of a 1 ppm increase of CO₂ on yields of rice, wheat, and maize. Because the gradual increase in CO₂ is highly correlated with major changes in technology, management, and other yield controlling factors, we focused on first differences of CO₂ and yield time series. Estimates of CO₂ responses obtained from this approach were highly uncertain, reflecting the relatively small importance of year‐to‐year CO₂ changes for yield variability. Combining estimates from the top 20 countries for each crop resulted in estimates with substantially less uncertainty than from any individual country. The results indicate that while current datasets cannot reliably constrain estimates beyond previous experimental studies, an empirical approach supported by large amounts of data may provide a potentially valuable and independent assessment of this critical model parameter. For example, analysis of reliable yield records from hundreds of individual, independent locations (as opposed to national scale yield records with poorly defined errors) may result in empirical estimates with useful levels of uncertainty to complement estimates from experimental studies.     

The temperature acclimation of photosynthetic responses to CO2 in Zea mays and its relationship to the activities of photosynthetic enzymes and the CO2-concentrating mechanism of C4 photosynthesis

Abstract Associations between photosynthetic responses to CO₂ at rate-saturating light and photosynthetic enzyme activities were compared for leaves of maize grown under constant air temperatures of 19, 25 and 31°C. Key photosynthetic enzymes analysed were ribulose bisphosphatc (RuBP) carboxylase, phosphoenolpyruvate (PEP) carboxylase, NADP-malic enzyme and pyruvate, P_i dikinasc. Rates of CO₂-saturated photosynthesis were similar in leaves developed at 19°C and 25°C but were decreased significantly by growth at 31°C. In contrast, carboxylation efficiency differed significantly between all three temperature regimes. Carboxylation efficiency was greatest in leaves developed at 19°C and decreased with increasing temperature during growth. The changes of carboxylation efficiency were highly correlated with changes in the activity of pyruvate, P_i dikinase (r= 0.95), but not with other photosynthetic enzyme activities. The activities of these latter enzymes, including that of RuBP carboxylase, were relatively insensitive to temperature during growth. The sensitivity of quantum yield to O₂ concentration was lower in leaves grown at 19°C than in leaves grown at 31°C. These observations support the novel hypothesis that variation in the capacity for CO₂ delivery to the bundle sheath by the C₄ cycle, relative to the capacity for net assimilation by the C₂ cycle, can be a principal determinant of C₄ photosynthetic responses to CO₂.     

Potential effects of elevated CO2 and changes in temperature on tropical plants

Abstract. Very little attention has been directed at the responses of tropical plants to increases in global atmospheric CO₂ concentrations and the potential climatic changes. The available data, from greenhouse and laboratory studies, indicate that the photosynthesis, growth and water use efficiency of tropical plants can increase at higher CO₂ concentrations. However, under field conditions abiotic (light, water or nutrients) or biotic (competition or herbivory) factors might limit these responses. In general, elevated atmospheric CO₂ concentrations seem to increase plant tolerance to stress, including low water availability, high or low temperature, and photoinhibition. Thus, some species may be able to extend their ranges into physically less favourable sites, and biological interactions may become relatively more important in determining the distribution and abundance of species. Tropical plants may be more narrowly adapted to prevailing temperature regimes than are temperate plants, so expected changes in temperature might be relatively more important in the tropics. Reduced transpiration due to decreased stomatal conductance could modify the effects of water stress as a cue for vegetative or reproductive phenology of plants of seasonal tropical areas. The available information suggests that changes in atmospheric CO₂ concentrations could affect processes as varied as plant/herbivore interactions, decomposition and nutrient cycling, local and geographic distributions of species and community types, and ecosystem productivity. However, data on tropical plants are few, and there seem to be no published tropical studies carried out in the field. Immediate steps should be undertaken to reduce our ignorance of this critical area.     

The effects of increasing CO2 on crop photosynthesis and productivity: a review of field studies

Abstract. Only a small proportion of elevated CO₂ studies on crops have taken place in the field. They generally confirm results obtained in controlled environments: CO₂ increases photosynthesis, dry matter production and yield, substantially in C₃ species, but less in C₄, it decreases stomatal conductance and transpiration in C₃ and C₄ species and greatly improves water-use efficiency in all plants. The increased productivity of crops with CO₂ enrichment is also related to the greater leaf area produced. Stimulation of yield is due more to an increase in the number of yield-forming structures than in their size. There is little evidence of a consistent effect of CO₂ on partitioning of dry matter between organs or on their chemical composition, except for tubers. Work has concentrated on a few crops (largely soybean) and more is needed on crops for which there are few data (e.g. rice). Field studies on the effects of elevated CO₂ in combination with temperature, water and nutrition are essential; they should be related to the development and improvement of mechanistic crop models, and designed to test their predictions.     

Salinity effects on CO2 assimilation and diffusive conductance of cowpea leaves

The effect of NaCl salinity at concentrations of 43–173 mM in nutrient solution on net gas exchange of attached cowpea [Vigna unguiculata (L.) Walp cv. California Black-eye No. 5 (CB5)] leaves was investigated under both greenhouse and growth chamber conditions.
There was a marked decrease in leaf conductance to water vapor after exposure to low salinity levels and a slighter decrease when salinity levels were higher. The decrease in net assimilation was much more gradual throughout the entire salinity range. The altered responses of net assimilation and leaf conductance to salinity were more evident at a high light intensity. A decrease in intercellular partial CO₂ pressure [p(CO₂)] was found at the low and intermediate salinity levels but not at the high level. These findings suggest that CO, assimilation was mainly controlled by stomatal conductance and the fixation of CO, might have been increased due to stimulated biochemical activity or to higher chlorophyll concentration per unit leaf area. A decrease in assimilation was already found one day after salinization and pro-ceeded up to 4 days when it was inhibited by 50% at 43 mM NaCl and up to 85% at 173 mM. The decrease in transpiration was larger than the decrease in net assimila-tion, and both were attributed to osmotic stress. Partial recovery was found thereaf-ter and new steady-state rates, in the range of 55 to 100% of the control, were then obtained for salinity levels between 43 and 130 mM. Inhibition of net CO, assimila-tion at this stage was attributed partly to a specific sodium effect and partly to plant water status. A linear relationship between leaf sodium content and net photosynthe-sis was also evident at this stage. Net CO, assimilation recovered more completely than transpiration when salt stress was removed, but at 173 mM NaCl recovery was neglible.     

Response of wild C4 crop progenitors to subambient CO2 highlights a possible role in the origin of agriculture

The synchronous origin of agriculture in at least four independent climatic regions at the end of the last glacial period (c10 kyr bp ) points to a global limitation for crop domestication. One hypothesis proposes that a rapid carbon dioxide (CO₂) increase from 18 Pa to ～27 Pa during deglaciation caused significant increases in the growth rates of wild crop progenitors, thereby removing a productivity barrier to their successful domestication. However, early C₄ crops present a challenge to this hypothesis, because they were among the first domesticates, but have a carbon‐concentrating mechanism that offsets the limitation of photosynthesis by CO₂. We investigated the CO₂‐limitation hypothesis using the wild progenitors of five C₄ founder crops from four independent centres of domestication. Plants were grown in controlled environment chambers at glacial (18 Pa), postglacial (28 Pa) and current ambient (38 Pa) CO₂ levels, and photosynthesis, transpiration and biomass were measured. An increase in CO₂ from glacial to postglacial levels caused a significant gain in vegetative biomass of up to 40%, but the equivalent rise in CO₂ from postglacial to modern levels generally had no effect on biomass. Investigation into the underlying mechanisms showed C₄ photosynthesis to be limited more by glacial than postglacial CO₂ levels, matching theoretical expectations. Moreover, the increase in CO₂ from glacial to postglacial levels caused a reduction in the transpiration rate via decreases in stomatal conductance of ～35%. In combination, these physiological changes conferred a large improvement in water‐use efficiency at the postglacial CO₂ partial pressure compared with the glacial level. Our data, therefore, provide experimental support for the CO₂‐limitation hypothesis, suggesting that these key physiological changes could have greatly enhanced the productivity of wild crop progenitors after deglaciation.     

Stomatal acclimation over a subambient to elevated CO2 gradient in a C3/C4 grassland

An investigation to determine whether stomatal acclimation to [CO₂] occurred in C₃/C₄ grassland plants grown across a range of [CO₂] (200–550 µmol mol^?1) in the field was carried out. Acclimation was assessed by measuring the response of stomatal conductance (g_s) to a range of intercellular CO₂ (a g_s–C_i curve) at each growth [CO₂] in the third and fourth growing seasons of the treatment. The g_s–C_i response curves for Solanum dimidiatum (C₃ perennial forb) differed significantly across [CO₂] treatments, suggesting that stomatal acclimation had occurred. Evidence of non–linear stomatal acclimation to [CO₂] in this species was also found as maximum g_s (g_smax; g_s measured at the lowest C_i) increased with decreasing growth [CO₂] only below 400 µmol mol^?1. The substantial increase in g_s at subambient [CO₂] for S. dimidiatum was weakly correlated with the maximum velocity of carboxylation (V_cmax; r² = 0·27) and was not associated with CO₂ saturated photosynthesis (A_max). The response of g_s to C_i did not vary with growth [CO₂] in Bromus japonicus (C₃ annual grass) or Bothriochloa ischaemum (C₄ perennial grass), suggesting that stomatal acclimation had not occurred in these species. Stomatal density, which increased with rising [CO₂] in both C₃ species, was not correlated with g_s. Larger stomatal size at subambient [CO₂], however, may be associated with stomatal acclimation in S. dimidiatum. Incorporating stomatal acclimation into modelling studies could improve the ability to predict changes in ecosystem water fluxes and water availability with rising CO₂ and to understand their magnitudes relative to the past.     

CO2 assimilation, respiration and chlorophyll fluorescence in peach leaves infected by Taphrina deformans

Photosynthesis, respiration and chlorophyll fluorescence parameters were determined in peach ( Prunus persica L. cv. Dixired) leaves naturally infected by Taphrina deformans (Berk.) Tul. and in healthy leaves (controls), in two successive springs. A drastic decrease in net photosynthesis and an evident increase in respiration in curled leaves were noted. The instantaneous PSII fluorescence yield, with no (F₀) and with (F₀) quenching component, and steady state fluorescence yield (under actinic light, F_s) were essentially unchanged. Maximal fluorescence in dark-adapted (F_m) and illuminated (F'_m) leaves and the corresponding variable fluorescence (F_v and F_v) clearly decreased. The indicators of PSII quantum yield (F_v/F_m) in dark-adapted leaves, and the potential PSII excitation capture efficiency (F'_v/F'_m) and the quantum yield of PSII (q_p [F'_v/F'_m]) in the light were also significantly lower in curled leaves. Decreasing tendencies were also noted for the PSII photochemical yield (photochemical quenching, q_p) and in the energy status of the chloroplast (non-photochemical quenching, q_N, and Stern-Vollmer value, NPQ) although the differences were not always significant. In curled leaves the main alteration documented is the imbalance between the drastic inhibition of CO₂ fixation and the moderate decrease in photochemical reactions (i.e. F_v/F_m and ΔF/F'_m), indicating changes in the energy flux.     

Growth and Photosynthetic Carbon Metabolism in Tobacco Plants under an Oscillating CO2 Concentration in the Atmosphere

Abstract: The hypothesis for the present work was that photosynthetic acclimation to increased atmospheric CO₂ in Nicotiana tabacum could be prevented by an oscillating supply of CO₂. This was tested by growing half of the plants (for the 20 day period after sowing) at 700 μmol mol^‐1 CO₂ (S+ plants) and half at 350 μmol mol^‐1 CO₂ (S‐ plants) and thereafter switching them every 48 h from high to low CO₂ and vice versa. These plants were compared with plants continuously kept (from sowing onwards) at 350 μmol mol^‐1 CO₂ (C‐ plants) and 700 μmol mol^‐1 CO₂ (C+ plants). Switching plants from high to low CO₂ and vice versa (S+ and S‐) did not improve plant growth efficiency, as hypothesized. The extra carbon fixed by the leaves under increased CO₂ in the atmosphere, supplied either continuously or intermittently, was mostly stored as starch and not used to build additional structural biomass. The differences in final plant biomass, observed between S+ and S‐ plants, are explained by the CO₂ concentration in the atmosphere during the first 20 days after sowing, the oscillation in CO₂ supply thereafter is playing a smaller role in this response. Switching plants from high to low CO₂ and vice versa, also did not prevent down‐regulation of photosynthesis, despite lower leaf sugar concentrations than in C+ plants. Nitrate concentration decreased dramatically in C+, S+ and S‐ plants. The leaf C/N ratio was highest in C+ plants (ranging from 8 to 13), intermediate in S+ and S‐ plants (from 7 to 11) and lowest in C‐ plants (from 6 to 8). This supports the view that the balance between carbohydrates and nitrogen may have a triggering role in plant response under elevated CO₂. Carbon export rates by the leaves seem to be independent of total carbon assimilation, suggesting a sink limiting effect on tobacco growth and phototsynthesis under elevated CO₂.     

Sensitivity of stomata and water use efficiency to high CO2

Abstract The observed responses of stomata to carbon dioxide are reviewed, and the interaction of other known factors on the sensitivity to CO₂ are summarized. The role of stomatal response to CO₂ is discussed, and it is argued that while the effect of the CO₂ response in normal daily stomatal behaviour is presently poorly understood the stomatal response to CO₂ will have major impact in improving water use efficiency in future CO₂ atmospheres. However, the attenuation of this increase is emphasized so that increases at the crop level will probably be much smaller than those observed at the single leaf assimilation level.     

CO2 uptake by the cultivated hemiepiphytic cactus, Hylocereus undatus

The climate of the native tropical forest habitats of Hylocereus undatus, a hemiepiphytic cactus cultivated in 20 countries for its fruit, can help explain the response of its net CO2 uptake to environmental factors. Under wet conditions, about 85% of the total daily net CO₂ uptake occurs at night via Crassulacean acid metabolism, leading to a high water‐use efficiency. Total daily net CO₂ uptake is reduced 57% by only 10 days of drought, possibly involving stomatal closure induced by abscisic acid produced in the roots, which typically occupy a small substrate volume. Total daily net CO₂ uptake for H. undatus is maximal at day/night air temperatures of 30/20°C, optimal temperatures that are higher than those for desert cacti but representative of ambient temperatures in the tropics; its total daily net CO₂ uptake becomes zero at day/night air temperatures of 42/32°C. Stem damage occurs at 45°C for H. undatus, whose photosynthetic cells show little acclimation to high temperatures compared with other cacti and are also sensitive to low temperatures, ‐1.5°C killing half of these cells. Consistent with its shaded habitat, total daily net CO2 uptake is appreciable at a total daily PPF of only 2 mol m2 day' and is maximal at 20 mol m^?2 day^?1, above which photoinhibition reduces net CO₂ uptake. Net CO₂ uptake ability, which is highly correlated with stem nitrogen and chlorophyll contents, changes only gradually (halftimes of 2–3 months) as the concentration of applied N is changed. Doubling the atmospheric CO₂ concentration raises the total daily net CO₂ uptake of H. undatus by 34% under optimal conditions and by even larger percentages under adverse environmental conditions.