Circadian Rhythms in Photosynthesis : Oscillations in Carbon Assimilation and Stomatal Conductance under Constant Conditions

Net carbon assimilation and stomatal conductance to water vapor oscillated repeatedly in red kidney bean, Phaseolus vulgaris L., plants transferred from a natural photoperiod to constant light. In a gas exchange system with automatic regulation of selected environmental and physiological variables, assimilation and conductance oscillated with a free-running period of approximately 24.5 hours. The rhythms in carbon assimilation and stomatal conductance were closely coupled and persisted for more than a week under constant conditions. A rhythm in assimilation occurred when either ambient or intercellular CO₂ partial pressure was held constant, demonstrating that the rhythm in assimilation was not entirely the result of stomatal effects on CO₂ diffusion. Rhythms in assimilation and conductance were not expressed in plants grown under constant light at a constant temperature, demonstrating that the rhythms did not occur spontaneously but were induced by an external stimulus. In plants grown under constant light with a temperature cycle, a rhythm was entrained in stomatal conductance but not in carbon assimilation, indicating that the oscillators driving the rhythms differed in their sensitivity to environmental stimuli.     

Mechanism of Photosynthesis Decrease by Verticillium dahliae in Potato

Young, visually symptomless leaves from potato (Solanum tuberosum) plants infected with Verticillium dahliae exhibited reduced carbon assimilation rate, stomatal conductance, and intercellular CO₂, but no increase in dark respiration, no change in the relationship between carbon assimilation rate versus intercellular CO₂, and no change in light use efficiency when intercellular CO₂ was held constant. Therefore, the initial decrease in photosynthesis caused by V. dahliae was caused by stomatal closure. Errors in the intercellular CO₂ calculation caused by uneven distribution of carbon assimilation rate across the leaf were tested by ¹⁴CO₂ autoradiography. Patchiness was found at a low frequency. Low stomatal conductance was correlated with low leaf water potentials. Infection did not affect leaf osmotic potentials.     

Stomatal sensitivity to carbon dioxide and humidity: a comparison of two c(3) and two c(4) grass species

The sensitivity of stomatal conductance to changes of CO₂ concentration and leaf-air vapor pressure difference (VPD) was compared between two C₃ and two C₄ grass species. There was no evidence that stomata of the C₄ species were more sensitive to CO₂ than stomata of the C₃ species. The sensitivity of stomatal conductance to CO₂ change was linearly proportional to the magnitude of stomatal conductance, as determined by the VPD, the same slope fitting the data for all four species. Similarly, the sensitivity of stomatal conductance to VPD was linearly proportional to the magnitude of stomatal conductance. At small VPD, the ratio of intercellular to ambient CO₂ concentration, C_i/C_a, was similar in all species (0.8-0.9) but declined with increasing VPD, so that, at large VPD, C_i/C_a was 0.7 and 0.5 (approximately) in C₃ and C₄ species, respectively. Transpiration efficiency (net CO₂ assimilation rate/transpiration rate) was larger in the C₄ species than in the C₃ species at current atmospheric CO₂ concentrations, but the relative increase due to high CO₂ was larger in the C₃ than in the C₄ species.     

C4 photosynthesis and water stress

Background

In contrast to C₃ photosynthesis, the response of C₄ photosynthesis to water stress has been less-well studied in spite of the significant contribution of C₄ plants to the global carbon budget and food security. The key feature of C₄ photosynthesis is the operation of a CO₂-concentrating mechanism in the leaves, which serves to saturate photosynthesis and suppress photorespiration in normal air. This article reviews the current state of understanding about the response of C₄ photosynthesis to water stress, including the interaction with elevated CO₂ concentration. Major gaps in our knowledge in this area are identified and further required research is suggested.

Scope

Evidence indicates that C₄ photosynthesis is highly sensitive to water stress. With declining leaf water status, CO₂ assimilation rate and stomatal conductance decrease rapidly and photosynthesis goes through three successive phases. The initial, mainly stomatal phase, may or may not be detected as a decline in assimilation rates depending on environmental conditions. This is because the CO₂-concentrating mechanism is capable of saturating C₄ photosynthesis under relatively low intercellular CO₂ concentrations. In addition, photorespired CO₂ is likely to be refixed before escaping the bundle sheath. This is followed by a mixed stomatal and non-stomatal phase and, finally, a mainly non-stomatal phase. The main non-stomatal factors include reduced activity of photosynthetic enzymes; inhibition of nitrate assimilation, induction of early senescence, and changes to the leaf anatomy and ultrastructure. Results from the literature about CO₂ enrichment indicate that when C₄ plants experience drought in their natural environment, elevated CO₂ concentration alleviates the effect of water stress on plant productivity indirectly via improved soil moisture and plant water status as a result of decreased stomatal conductance and reduced leaf transpiration.

Conclusions

It is suggested that there is a limited capacity for photorespiration or the Mehler reaction to act as significant alternative electron sinks under water stress in C₄ photosynthesis. This may explain why C₄ photosynthesis is equally or even more sensitive to water stress than its C₃ counterpart in spite of the greater capacity and water use efficiency of the C₄ photosynthetic pathway.Key words: C₃ and C₄ photosynthesis, stomatal and non-stomatal limitation, high CO₂, water stress     

The growth response of C4 plants to rising atmospheric CO2 partial pressure: a reassessment

Despite mounting evidence showing that C₄ plants can accumulate more biomass at elevated CO₂ partial pressure (p(CO₂)), the underlying mechanisms of this response are still largely unclear. In this paper, we review the current state of knowledge regarding the response of C₄ plants to elevated p(CO₂) and discuss the likely mechanisms. We identify two main routes through which elevated p(CO₂) can stimulate the growth of both well-watered and water-stressed C₄ plants. First, through enhanced leaf CO₂ assimilation rates due to increased intercellular p(CO₂). Second, through reduced stomatal conductance and subsequently leaf transpiration rates. Reduced transpiration rates can stimulate leaf CO₂ assimilation and growth rates by conserving soil water, improving shoot water relations and increasing leaf temperature. We argue that bundle sheath leakiness, direct CO₂ fixation in the bundle sheath or the presence of C₃-like photosynthesis in young C₄ leaves are unlikely explanations for the high CO₂-responsiveness of C₄ photosynthesis. The interactions between elevated p(CO₂), leaf temperature and shoot water relations on the growth and photosynthesis of C₄ plants are identified as key areas needing urgent research.     

Benzoxazolin-2-(3<Emphasis Type="Italic">H</Emphasis>)-one reduces photosynthetic activity and chlorophyll fluorescence in soybean

Benzoxazolin-2-(3H)-one (BOA) has been tested in many plants species, but not in soybean (Glycine max). Thus, a hydroponic experiment was conducted to assess the effects of BOA on soybean photosynthesis. BOA reduced net photosynthetic rate, stomatal conductance, and effective quantum yield of PSII photochemistry without affecting intercellular CO₂ concentration or maximal quantum yield of PSII photochemistry. Results revealed that the reduced stomatal conductance restricted entry of CO₂ into substomatal spaces, thus limiting CO₂ assimilation. No change found in intercellular CO₂ concentration and reduced effective quantum yield of PSII photochemistry revealed that CO₂ was not efficiently consumed by the plants. Our data indicated that the effects of BOA on soybean photosynthesis occurred due to the reduced stomatal conductance and decreased efficiency of carbon assimilation. The accumulation of BOA in soybean leaves reinforced these findings.     

Effect of increased salinity on CO2 assimilation,O2 evolution and the δ13C values of leaves of Plantago maritima L. developed at low and high NaCl levels

The effect of increased salinity on photosynthesis was studied in leaves of Plantago maritima L. that developed while plants were at low and high NaCl levels. In leaves that developed while plants were grown at 50 mol·m^-3, exposure to 200 and 350 mol·m^-3 NaCl resulted in reductions in net CO₂ assimilation and stomatal conductance. The decline in CO₂ assimilation in plants at 200 and 350 mol·m^-3 NaCl occurred almost exclusively at high intercellular CO₂ concentrations. The initial slope of the CO₂ assimilation-intercellular CO₂ (A-C _i) curve, determined after salinity was increased, was identical or very similar to that measured initially. In contrast to the reductions observed in CO₂ assimilation, there were no significant differences in O₂ evolution rates measured at 5% CO₂ among leaves from plants exposed to higher salinity and plants remaining at low salinity.Leaves that developed while plants were at increased salinity levels also had significantly lower net CO₂ assimilation rates than plants remaining at 50 mol·m^-3 NaCl. The lower CO₂ assimilation rates in plants grown at 200 and 350 mol·m^-3 NaCl were a result of reduced stomatal conductance and low intercellular CO₂ concentration. There were no significant differences among treatments for O₂ evolution rates measured at high CO₂ levels. The increased stomatal limitation of photosynthesis was confirmed by measurements of the ¹³C/¹²C composition of leaf tissue. Water-use efficiency was increased in the plants grown at high salinity.Abbreviations and symbols A net CO₂ assimilation rate - C _a ambient CO₂ concentration - C _i intercellular CO₂ concentration - ¹³C isotopic ratio (¹³C/¹²C) expressed relative to a standard - RuBP ribulose-1,5-bisphosphate     

An improved dynamic model of photosynthesis for estimation of carbon gain in sunfleck light regimes

A dynamic model of leaf photosynthesis for C₃ plants has been developed for examination of the role of the dynamic properties of the photosynthetic apparatus in regulating CO₂ assimilation in variable light regimes. The model is modified from the Farquhar-von Caemmerer-Berry model by explicitly including metabolite pools and the effects of light activation and deactivation of Calvin cycle enzymes. It is coupled to a dynamic stomatal conductance model, with the assimilation rate at any time being determined by the joint effects of the dynamic biochemical model and the stomatal conductance model on the intercellular CO₂ pressure. When parametrized for each species, the model was shown to exhibit responses to step changes in photon flux density that agreed closely with the observed responses for both the understory plant Alocasia macrorrhiza and the crop plant Glycine max. Comparisons of measured and simulated photosynthesis under simulated light regimes having natural patterns of lightfleck frequencies and durations showed that the simulated total for Alocasia was within ±4% of the measured total assimilation, but that both were 12–50% less than the predictions from a steady–state solution of the model. Agreement was within ±10% for Glycine max, and only small differences were apparent between the dynamic and steady–state predictions. The model may therefore be parametrized for quite different species, and is shown to reflect more accurately the dynamics of photosynthesis than earlier dynamic models.     

Gas Exchange, Stomatal Behavior, and deltaC Values of the flacca Tomato Mutant in Relation to Abscisic Acid

The relationship between stomatal conductance and capacity for assimilation was investigated in flacca, a mutant of tomato (Lycopersicon esculentum Mill.) that has abnormal stomatal behavior and low abscisic acid (ABA) content. The assimilation capacity, determined by measuring assimilation rate as a function of intercellular CO₂ pressure, did not differ in leaves of flacca and its parent variety, Rheinlands Ruhm (RR). On the other hand, stomatal conductance of flacca leaves was greater than that of RR, and could be phenotypically reverted by spraying with 30 micromolar ABA. Stomatal conductance of flacca leaves was also reduced by increasing CO₂ pressure, increasing leaf to air vapor pressure difference, and decreasing quantum flux, irrespective of ABA treatment.

The high conductance of flacca leaves resulted in a high intercellular CO₂ pressure. This allowed greater discrimination against ¹³CO₂, as evidenced by more negative δ ¹³C values for flacca as compared to RR. The δ ¹³C values of both flacca and RR plants as influenced by ABA treatment were consistent with predictions based on gas exchange measurements, using a recent model of discrimination.

     

Regulation of mesophyll photosynthesis in intact wheat leaves by cytoplasmic phosphate concentrations

The effect of D-(+)-mannose, inorganic phosphate (Pi) and mannose-6-phosphate on net mesophyll CO₂ assimilation rate (A) and stomatal conductance (g_s) of wheat (Triticum aestivum L.) leaves was studied. The compounds were supplied through the transpiration stream of detached leaves from plants grown in sand in growth cabinets or glasshouses, with different concentrations of Pi (0.25, 1.0 and 4.0 mM) supplied during growth. In all cases, 10 mM D-(+)mannose caused 40–60% reduction of A within 30 min, though the time courses differed for flag leaves and the sixth leaf on the mainstem of glasshouse- and cabinet-grown plants. D-(+)Mannose had a similar effect on A in leaves having a fourfold range in total phosphate content. Effects of D-(+)mannose in reducing g_s were always slower than on A. When the CO₂ concentration in the leaf chamber was adjusted to maintain intercellular CO₂ concentration (C_i) constant as A declined after mannose supply, g_s still declined indicating that stomatal closure was not caused by changing C_i. Supplying mannose-6-phosphate at 10 and 1 mM and Pi at 5 and 10 mM concentrations caused rapid reductions in g_s and also direct reductions in A. The observed effects of mannose and Pi on assimilation are consistent with the proposed regulatory role of cytoplasmic Pi in determining mesophyll carbon assimilation that has been derived previously using leaf discs, protoplasts and chloroplasts.Abbreviations and symbols A net mesophyll CO₂-assimilation rate - C_a, C_i external (assimilation-chamber) and intercellular CO₂ concentration, respectively - g_s stomatal conductance - Man6P mannose-6-phosphate - Pi orthophosphate     

Analysis of Stomatal and Nonstomatal Components in the Environmental Control of CO(2) Exchange in Leaves of Welwitschia mirabilis

In well-watered plants of Welwitschia mirabilis, grown in the glass-house under high irradiance conditions, net CO₂ assimilation was almost exclusively observed during the daytime. The plants exhibited a very low potential for Crassulacean acid metabolism, which usually resulted in reduced rates of net CO₂ loss for several hours during the night. In leaves exposed to the diurnal changes in temperature and humidity typical of the natural habitats, CO₂ assimilation rates in the light were markedly depressed under conditions resembling those occurring during midday, when leaf temperatures and the leaf-air vapor pressure differences were high (36°C and 50 millibars bar⁻¹, respectively). Studies on the relationship between CO₂ assimilation rate and intercellular CO₂ partial pressure at various temperatures and humidities showed that this decrease in CO₂ assimilation was largely due to stomatal closure. The increase in the limitation of photosynthesis by CO₂ diffusion, which is associated with the strong decline in stomatal conductance in Welwitschia exposed to midday conditions, may significantly contribute to the higher ¹³C content of Welwitschia compared to the majority of C₃ species.     

Changes in gas exchange characteristics and water use efficiency of mangroves in response to salinity and vapour pressure deficit

Summary Measurements were made of the photosynthetic gas exchange properties and water use efficiency of 19 species of mangrove in 9 estuaries with different salinity and climatic regimes in north eastern Australia and Papua New Guinea. Stomatal conductance and CO₂ assimilation rates differed significantly between species at the same locality, with the salt-secreting species, Avicennia marina, consistently having the highest CO₂ assimilation rates and stomatal conductances. Proportional changes in stomatal conductance and CO₂ assimilation rate resulted in constant and similar intercellular CO₂ concentrations for leaves exposed to photon flux densities above 800 mol·m^-2·s^-1 in all species at a particular locality. In consequence, all species at the same locality had similar water use efficiencies. There were, however, significant differences in gas exchange properties between different localities. Stomatal conductance and CO₂ assimilation rate both decreased with increasing salinity and with increasing leaf to air vapour pressure deficit (VPD). Furthermore, the slope of the relationship between assimilation rate and stomatal conductance increased, while intercellular CO₂ concentration decreased, with increasing salinity and with decreasing ambient relative humidity. It is concluded from these results that the water use efficiency of mangroves increases with increasing environmental stress, in this case aridity, thereby maximising photosynthetic carbon fixation while minimising water loss.Contribution No. 459 from the Australian Institute of Marine Science     

A stomatal optimization theory to describe the effects of atmospheric CO2 on leaf photosynthesis and transpiration

Background and Aims

Global climate models predict decreases in leaf stomatal conductance and transpiration due to increases in atmospheric CO₂. The consequences of these reductions are increases in soil moisture availability and continental scale run-off at decadal time-scales. Thus, a theory explaining the differential sensitivity of stomata to changing atmospheric CO₂ and other environmental conditions must be identified. Here, these responses are investigated using optimality theory applied to stomatal conductance.

Methods

An analytical model for stomatal conductance is proposed based on: (a) Fickian mass transfer of CO₂ and H₂O through stomata; (b) a biochemical photosynthesis model that relates intercellular CO₂ to net photosynthesis; and (c) a stomatal model based on optimization for maximizing carbon gains when water losses represent a cost. Comparisons between the optimization-based model and empirical relationships widely used in climate models were made using an extensive gas exchange dataset collected in a maturing pine (Pinus taeda) forest under ambient and enriched atmospheric CO₂.

Key Results and Conclusion

In this interpretation, it is proposed that an individual leaf optimally and autonomously regulates stomatal opening on short-term (approx. 10-min time-scale) rather than on daily or longer time-scales. The derived equations are analytical with explicit expressions for conductance, photosynthesis and intercellular CO₂, thereby making the approach useful for climate models. Using a gas exchange dataset collected in a pine forest, it is shown that (a) the cost of unit water loss λ (a measure of marginal water-use efficiency) increases with atmospheric CO₂; (b) the new formulation correctly predicts the condition under which CO₂-enriched atmosphere will cause increasing assimilation and decreasing stomatal conductance.     

PHOTOSYNTHETIC CHARACTERISTICS OF C3-C4 INTERMEDIATE FLAVERIA FLORIDANA (ASTERACEAE) IN NATURAL HABITATS: EVIDENCE OF ADVANTAGES TO C3-C4 PHOTOSYNTHESIS AT HIGH LEAF TEMPERATURES

Measurements of leaf gas exchange were conducted in situ for the C₃-C₄ intermediate plant Flaveria floridana. Leaves exhibited measurable CO₂ assimilation at atmospheric CO₂ concentrations as low as 20 μmol/mol. This result demonstrates that the low CO₂ compensation points observed in past studies of greenhouse-grown C₃-C₄ intermediate plants also exist in plants growing in their natural habitat. Photosynthesis rates in F. floridana were near their maximum at intercellular CO₂ concentrations as low as 112 μmol/mol. The existence of near-maximum photosynthesis rates at such low intercellular CO₂ concentrations is interpreted as evidence for the existence of a CO₂-concentrating mechanism in F. floridana. Such a mechanism would also explain the observed lack of response in photosynthesis rates to reductions in stomatal conductance and intercellular CO₂ concentration as the leaf-to-air water vapor concentration gradient is increased. Photosynthetic rates were relatively high at leaf temperatures between 35 and 40 C, compared to most C₃ plants. At midday during May, when leaf temperatures were between 35 and 42 C, F. floridana leaves exhibited photosynthesis rates that were four times higher than a sympatric C₃ species (Eustoma exaltatum) of similar growth form and ecological habit. The high photosynthesis rates at high leaf temperatures in F. floridana were not due to higher leaf nitrogen contents, but rather to its reduced rate of photorespiration. These results confirm that C₃-C₄ intermediate photosynthesis can provide plants with an advantage at high leaf temperatures, compared to C₃ plants.     

Stomatal responses to light and humidity in sugarcane: prediction of daily time courses and identification of potential selection criteria

Abstract Stomatal sensitivities to light and VPD have potential as quantitative selection criteria in programs designed to enhance water-use efficiency of sugarcane and other crops. These responses were characterized using gas exchange techniques and then simulated by a mathematical relationship describing conductance as a function of photon fluence rates and VPD values. The same form of relationship simulated stomatal responses of well-watered greenhouse- and field-grown plants. A comparison between simulated and measured conductance values showed a close correlation, indicating that light and VPD responses of stomata are dominant input signals modulating stomatal conductance in sugarcane. Observed conductance of Hawaiian sugarcane in a commerical production area appeared larger than required to support prevailing rates of carbon assimilation, since predicted intercellular CO₂ was greater than required to saturate its C₄ photosynthesis. Manipulation of the relationship describing stomatal conductance allowed us to simulate the responses of plants with hypothetically altered stomatal sensitivities to VPD or to light, using micrometeorological data collected in the field. Further simulation indicated that selection for clones with altered stomatal sensitivity to either light or VPD could improve the water-use efficiency of sugarcane without inhibiting current high levels of productivity.     

Evaluation of the potential to measure photosynthetic rates in C3 plants (Flaveria pringlei and Oryza sativa) by combining chlorophyll fluorescence analysis and a stomatal conductance model

The response curves of leaf photosynthesis to varying light, temperature and leaf-to-air vapour pressure deficit were measured in the C₃ plants Flaveria pringlei and Oryza sativa in normal air with a computerized open infrared gas analysis (IRGA) system, and the photochemical efficiency of photosystem II, described as (1–F,/F′_m) after Genty. Briantais & Baker (1989, Biochimica et Biophysica Acta 990, 87–92), was simultaneously measured with a modulated fluorometer. A model was written for rates of CO₂ fixation as a function of the true rate of O₂ evolution measured by fluorescene analysis (Jo₂), mesophyll conductance and intercellular CO₂ partial pressure. A second model was developed for rates of CO₂ fixation as a function of Jo₂, mesophyll conductance and stomatal conductance. In the latter case, leaf stomatal conductance was simulated using the stomatal model proposed by Leuning (1995, Plant, Cell and Environment 18 , 339–355). The rates of CO₂ fixation predicted from the models were similar to rates measured by IRGA. The results indicate that there is potential to measure CO₂ fixation in C₃ plants by combining the non-invasive measurement of Jo₂ by chlorophyll fluorescence analysis with the stomatal conductance model.     

Control of photosynthesis by the carbohydrate level in leaves of the C4 plant Amaranthus edulis L.

Photosynthesis was studied in relation to the carbohydrate status in intact leaves of the C₄ plant Amaranthus edulis. The rate of leaf net CO₂ assimilation, stomatal conductance and intercellular partial pressure of CO₂ remained constant or showed little decline towards the end of an 8-h period of illumination in ambient air (340 bar CO₂, 21% O₂). When sucrose export from the leaf was inhibited by applying a 4-h cold-block treatment (1°C) to the petiole, the rate of photosynthesis rapidly decreased with time. After the removal of the cold block from the petiole, further reduction in photosynthetic rate occurred, and there was no recovery in the subsequent light period. Although stomatal conductance declined with time, intercellular CO₂ partial pressure remained relatively constant, indicating that the inhibition of photosynthesis was not primarily caused by changes in stomatal aperture. Analysis of the leaf carbohydrate status showed a five- to sixfold increase in the soluble sugar fraction (mainly sucrose) in comparison with the untreated controls, whereas the starch content was the same. Leaf osmotic potential increased significantly with the accumulation of soluble sugars upon petiole chilling, and leaf water potential became slightly more negative. After 14 h recovery in the dark, photosynthesis returned to its initial maximum value within 1 h of illumination, and this was associated with a decline in leaf carbohydrate levels overnight. These data show that, in Amaranthus edulis, depression in photosynthesis when translocation is impaired is closely related to the accumulation of soluble sugars (sucrose) in source leaves, indicating feedback control of C₄ photosynthesis. Possible mechanisms by which sucrose accumulation in the leaf may affect the rate of photosynthesis are discussed with regard to the leaf anatomy of C₄ plants.Abbreviations and symbols A net CO₂ assimilation rate - Ci intercellular CO₂ partial pressure - PEP phosphoenolpyruvate - RuBP ribulose-1,5-bisphosphate - water potential - osmotic pressure     

Effects of Various Levels of CO(2) on the Induction of Crassulacean Acid Metabolism in Portulacaria afra (L.) Jacq

In response to water stress, Portulacaria afra (L.) Jacq. (Portulacaceae) shifts its photosynthetic carbon metabolism from the Calvin-Benson cycle for CO₂ fixation (C₃) photosynthesis or Crassulacean acid metabolism (CAM)-cycling, during which organic acids fluctuate with a C₃-type of gas exchange, to CAM. During the CAM induction, various attributes of CAM appear, such as stomatal closure during the day, increase in diurnal fluctuation of organic acids, and an increase in phosphoenolpyruvate carboxylase activity. It was hypothesized that stomatal closure due to water stress may induce changes in internal CO₂ concentration and that these changes in CO₂ could be a factor in CAM induction. Experiments were conducted to test this hypothesis. Well-watered plants and plants from which water was withheld starting at the beginning of the experiment were subjected to low (40 ppm), normal (ca. 330 ppm), and high (950 ppm) CO₂ during the day with normal concentrations of CO₂ during the night for 16 days. In water-stressed and in well-watered plants, CAM induction as ascertained by fluctuation of total titratable acidity, fluctuation of malic acid, stomatal conductance, CO₂ uptake, and phosphoenolpyruvate carboxylase activity, remained unaffected by low, normal, or high CO₂ treatments. In well-watered plants, however, both low and high ambient concentrations of CO₂ tended to reduce organic acid concentrations, low concentrations of CO₂ reducing the organic acids more than high CO₂. It was concluded that exposing the plants to the CO₂ concentrations mentioned had no effect on inducing or reducing the induction of CAM and that the effect of water stress on CAM induction is probably mediated by its effects on biochemical components of leaf metabolism.     

Physiological Site of Ethylene Effects on Carbon Dioxide Assimilation in Glycine max L. Merr

The physiological site of ethylene action on CO₂ assimilation was investigated in intact plants of Glycine max L., using a whole-plant, open exposure system equipped witha remotely operated single-leaf cuvette. The objective of the study was met by investigating in control and ethylene-treated plants the (a) synchrony in response of CO₂ assimilation, stomatal conductance to water vapor, and substomatal CO₂ partial pressure; (b) response of CO₂ assimilation as a function of a range of substomatal CO₂ partial pressures; and (c) response of CO₂ assimilation as a function of a range of photon flux densities. After exposure to 410 micromoles per cubic meter of ethylene for 2.0 hours, CO₂ assimilation and stomatal conductance declined in synchrony, while substomatal CO₂ partial pressure remained unchanged until exposure times equaled and exceeded 3.0 hours. Because incipient changes in CO₂ assimilation occurred without a change in the CO₂ partial pressure in the leaf interior, it is concluded that both stomatal physiology and the chloroplast's CO₂ assimilatory capacity were initial sites of ethylene action. After 3.5 hours the effect of ethylene on stomatal conductance and CO₂ assimilation exhibited saturation kinetics, and the effect was substantially more pronounced for stomatal conductance than for CO₂ assimilation. Based on the response of CO₂ assimilation to a range of substomatal CO₂ partial pressures, ethylene did not affect either the CO₂ compensation point or carboxylation efficiency at subsaturating CO₂ partial pressures. Above-ambient supplies of CO₂ did not alleviate the diminished rates of CO₂ assimilation. In partitioning the limitations imposed on CO₂ assimilation in control and ethylene-treated plants, the stomatal component accounted for only 16 and 4%, respectively. The response of CO₂ assimilation to a range of photon flux densities suggests that ethylene reduced apparent quantum yield by nearly 50%. Thus, the pronounced decline in net photosynthetic CO₂ assimilation in the presence of ethylene was due more to a loss in the mesophyll tissue's intrinsic capacity to assimilate CO₂ than to a reduction in stomatal conductance.     

Ecophysiological Significance of CO(2)-Recycling via Crassulacean Acid Metabolism in Talinum calycinum Engelm. (Portulacaceae)

High levels of variability in gas exchange characteristics and degree of CAM-cycling were found in the same and different individuals of Talinum calycinum Engelm. collected from rock outcrops in Missouri. Differences in CO₂ assimilation were mostly correlated with differences in shoot conductance to CO₂ not shoot internal CO₂ concentration. As found previously, CAM acid fluctuations were evident in well-watered plants exhibiting C₃ gas exchange patterns (CAM-cycling) and also in drought-stressed plants with stomata closed, or nearly so, day and night (CAM-idling). Drought stress also resulted in rapid stomatal closure, conserving water during droughts. Maximal CO₂ uptake rates occurred below 35°C; higher temperatures induced decreases in CO₂ assimilation and conductance while shoot internal CO₂ concentrations remained similar. Plant water-use-efficiency was severely curtailed at temperatures above 30°C. Tissue acid fluctuations were the result of changes in malic acid concentrations. Calculations of the amount of water potentially conserved by CAM-cycling yielded values of approximately 5 to 44% of daytime water loss. Thus, CAM-cycling may be an important adaptation minimizing water loss by perennial succulents growing in shallow soil on rock outcrops.