Internal CO2 Partial Pressure Research Articles

Short-term salt stress reduces photosynthetic oscillations under triose phosphate utilization limitation in tomato.

Triose phosphate utilization (TPU) limitation is one of the three biochemical limitations of photosynthetic CO2 assimilation rate in C3 plants. Under TPU limitation, abrupt and large transitions in light intensity cause damped oscillations in photosynthesis. When plants are salt-stressed, photosynthesis is often down-regulated particularly under dynamic light intensity, but how salt stress affects TPU-related dynamic photosynthesis is still unknown. To elucidate this, tomato (Solanum lycopersicum) was grown with and without sodium chloride (NaCl, 100 mM) stress for 13 d. Under high CO2 partial pressure, rapid increases in light intensity caused profound photosynthetic oscillations. Salt stress reduced photosynthetic oscillations in leaves initially under both low- and high-light conditions and reduced the duration of oscillations by about 2 min. Besides, salt stress increased the threshold for CO2 partial pressure at which oscillations occurred. Salt stress increased TPU capacity without affecting Rubisco carboxylation and electron transport capacity, indicating the up-regulation of end-product synthesis capacity in photosynthesis. Thus salt stress may reduce photosynthetic oscillations by decreasing leaf internal CO2 partial pressure and/or increasing TPU capacity. Our results provide new insights into how salt stress modulates dynamic photosynthesis as controlled by CO2 availability and end-product synthesis.

A resistant cowpea (Vigna unguiculata [L.] Walp.) genotype became susceptible to cowpea severe mosaic virus (CPSMV) after exposure to salt stress

In nature, plants are simultaneously challenged by biotic and abiotic stresses. However, little is known about the effects of these combined stresses for most crops. This work aimed to evaluate the responsed of the virus-resistant cowpea genotype BRS-Marataoã to the exposure of salt stress combined with CPSMV infection. Cowpea plants were exposed to 200 mM NaCl either simultaneously (SV plant group) or 24 h prior to the CPSMV infection [S(24 h)V plant group]. Physiological, biochemical, and proteomic analyses at 2 and 6 days post salt stress (DPS) revealed that cowpea significantly reprogrammed its cellular metabolism. Indeed, plant size, photosynthetic parameters (net photosynthesis, transpiration rate, stomatal conductance, and internal CO2 partial pressure) and chlorophyll and carotenoid contents were reduced in S(24 h)V compared to SV. Moreover, accumulation of viral particles at 6 DPS in S(24 h)V was observed indicating that the salt stress imposed prior to virus infection favors viral particle proliferation. Proteomic analysis showed differential contents of 403 and 330 proteins at 2 DPS and 6 DPS, respectively, out of 733 differentially abundant proteins between the two plant groups. The altered leaf proteins are involved in energy and metabolism, photosynthesis, stress response, and oxidative burst. Biological significanceThis is an original study in which a virus-resistant cowpea genotype (BRS-Marataoã) was (i) exposed simultaneously to 200 mM NaCl and inoculation with CPSMV (SV plant group) or (ii) exposed to 200 mM NaCl stress 24 h prior to inoculation with CPSMV [S(24 h)V plant group]. The purpose was to shed light on how this CPSMV resistant cowpea responded to the combined stresses. Numerous key proteins and associated pathways were altered in the cowpea plants challenged with both stresses, but unexpectedly, the salt stress imposed 24 h prior to CPSMV inoculation allowed viral proliferation, turning the cowpea genotype from resistant to susceptible.

Kaolin particle film alleviates adverse effects of light and heat stresses and improves nut and kernel quality in Persian walnut

Abstract Spraying with kaolin is becoming a feasible and cheap treatment to alleviate the destructive effects of light and heat stresses. Here, the effects of spraying with kaolin were investigated on leaf temperature, photosynthesis and gas exchange rate, sunburn and quality of nut and kernel, potassium (K) and sodium (Na) concentrations and proline content of the walnut. Three spraying levels of kaolin (0%, 3% and 6%) were used in 5 replicates during two consecutive years. Results indicate that the kaolin spray increases leaf area and chlorophyll content (up to 14.71% and 11.99%, respectively) comparing to the control. The severity of leaf, husk, and kernel sunburn of kaolin treated trees was reduced up to 50% comparing to the control plants. Considering physiological parameters kaolin treatment reduced leaf temperature (4.4 °C and 5.6 °C, respectively in kaolin 3% and 6% compared to the control), but increased photosynthesis rate (Amax), gas exchange (gs), internal CO2 partial pressure (Ci) and the water use efficiency (up to 29.85%, 30%, 10.97% and 19.88%, respectively) comparing to the control. Kaolin application also improved nut and kernel weight, kernel percentage and kernel oil content (up to 8.2%, 16.1%, 7.7% and 11.29%, respectively) comparing to the control. Kaolin treatments increased potassium level and K:Na ratio, but decreased Na and proline content, both in leaf and kernel, in comparison with the control. However, there was no significant difference between the results of applying kaolin at concentrations of 3% and 6% in almost all of the measured characteristics, but improved results were obtained with higher levels of kaolin, except for nuts and kernel quantities. Considering global warming, using kaolin application can be recommended as a suitable and cheap method for the acclimation of walnut to high temperatures and solar radiation.

Physiological characteristics of grapevine under irrigation and fertilization management1

ABSTRACT The viticulture in the semiarid region of Brazil stands out in the country due to the expansion of its cultivated area and increased production, especially in the sub-mid São Francisco Valley. However, the supplying of water and an efficient replacement of nutrients to the soil are essential for these crops, due to the intrinsic climatic conditions of this region, such as low rainfall and high evapotranspiration. This study aimed at evaluating the physiological characteristics of ‘Itália’ grapevine (Vitis vinifera L.) under irrigation and fertilization management. The experiment was conducted in a randomized block design, with four replications, in a split-plot arrangement, with four irrigation water depths (0.75, 1.00, 1.25 and 1.5 times the crop coefficient) in the plots and four fertilizer rates (100 %, 75 %, 50 % and 25 %, based on the recommended fertilizer rates for this crop) in the subplots, with addition of organic compounds. The leaf water potential, leaf chlorophyll content, relative water content, photosynthesis, stomatal conductance, internal CO2 partial pressure, leaf temperature and biomass accumulation in the berries were evaluated. The highest leaf chlorophyll contents were found in treatments with fertilizer rates of 70-85 %. Stomatal conductance decreased with the increasing irrigation water depths, thus reducing the internal CO2 partial pressure and increasing the leaf temperature and air vapor pressure deficit. The reduction of 25 % in the conventional fertilization and irrigation of 135 % of the recommended coefficient resulted in the highest average weight of berries. The highest yields were obtained with the lowest irrigation water depths and fertilizer rates of 50-65 %.

Photosynthetic induction and its diffusional, carboxylation and electron transport processes as affected by CO2 partial pressure, temperature, air humidity and blue irradiance.

Plants depend on photosynthesis for growth. In nature, factors such as temperature, humidity, CO2 partial pressure, and spectrum and intensity of irradiance often fluctuate. Whereas irradiance intensity is most influential and has been studied in detail, understanding of interactions with other factors is lacking. We tested how photosynthetic induction after dark-light transitions was affected by CO2 partial pressure (20, 40, 80 Pa), leaf temperatures (15·5, 22·8, 30·5 °C), leaf-to-air vapour pressure deficits (VPDleaf-air; 0·5, 0·8, 1·6, 2·3 kPa) and blue irradiance (0-20%) in tomato leaves (Solanum lycopersicum). Rates of photosynthetic induction strongly increased with CO2 partial pressure, due to increased apparent Rubisco activation rates and reduced diffusional limitations. High leaf temperature produced slightly higher induction rates, and increased intrinsic water use efficiency and diffusional limitation. High VPDleaf-air slowed down induction rates and apparent Rubisco activation and (at 2·3 kPa) induced damped stomatal oscillations. Blue irradiance had no effect. Slower apparent Rubisco activation in elevated VPDleaf-air may be explained by low leaf internal CO2 partial pressure at the beginning of induction. The environmental factors CO2 partial pressure, temperature and VPDleaf-air had significant impacts on rates of photosynthetic induction, as well as on underlying diffusional, carboxylation and electron transport processes. Furthermore, maximizing Rubisco activation rates would increase photosynthesis by at most 6-8% in ambient CO2 partial pressure (across temperatures and humidities), while maximizing rates of stomatal opening would increase photosynthesis by at most 1-3%.

Drought increases cowpea (Vigna unguiculata [L.] Walp.) susceptibility to cowpea severe mosaic virus (CPSMV) at early stage of infection

The physiological and biochemical responses of a drought tolerant, virus-susceptible cowpea genotype exposed to drought stress (D), infected by Cowpea severe mosaic virus (CPSMV) (V), and to these two combined stresses (DV), at 2 and 6 days post viral inoculation (DPI), were evaluated. Gas exchange parameters (net photosynthesis, transpiration rate, stomatal conductance, and internal CO2 partial pressure) were reduced in D and DV at 2 and 6 DPI compared to control plants (C). Photosynthesis was reduced by stomatal and biochemical limitations. Water use efficiency increased at 2 DPI in D, DV, and V, but at 6 DPI only in D and DV compared to C. Photochemical parameters (effective quantum efficiency of photosystem II and electron transport rate) decreased in D and DV compared to C, especially at 6 DPI. The potential quantum efficiency of photosystem II did not change, indicating reversible photoinhibition of photosystem II. In DV, catalase decreased at 2 and 6 DPI, ascorbate peroxidase increased at 2 DPI, but decreased at 6 DPI. Hydrogen peroxide increased at 2 and 6 DPI. Peroxidase increased at 6 DPI and chitinase at 2 and 6 DPI. β-1,3-glucanase decreased in DV at 6 DPI compared to V. Drought increased cowpea susceptibility to CPSMV at 2 DPI, as verified by RT-PCR. However, at 6 DPI, the cowpea plants overcome this effect. Likewise, CPSMV increased the negative effects of drought at 2 DPI, but not at 6 DPI. It was concluded that the responses to combined stresses are not additive and cannot be extrapolated from the study of individual stresses.

The carbon fertilization effect over a century of anthropogenic CO2 emissions: higher intracellular CO2 and more drought resistance among invasive and native grass species contrasts with increased water use efficiency for woody plants in the US Southwest.

From 1890 to 2015, anthropogenic carbon dioxide emissions have increased atmospheric CO2 concentrations from 270 to 400mol mol-1 . The effect of increased carbon emissions on plant growth and reproduction has been the subject of study of free-air CO2 enrichment (FACE) experiments. These experiments have found (i) an increase in internal CO2 partial pressure (ci ) alongside acclimation of photosynthetic capacity, (ii) variable decreases in stomatal conductance, and (iii) that increases in yield do not increase commensurate with CO2 concentrations. Our data set, which includes a 115-year-long selection of grasses collected in New Mexico since 1892, is consistent with an increased ci as a response to historical CO2 increase in the atmosphere, with invasive species showing the largest increase. Comparison with Palmer Drought Sensitivity Index (PDSI) for New Mexico indicates a moderate correlation with Δ13 C (r2 =0.32, P<0.01) before 1950, with no correlation (r2 =0.00, P=0.91) after 1950. These results indicate that increased ci may have conferred some drought resistance to these grasses through increased availability of CO2 in the event of reduced stomatal conductance in response to short-term water shortage. Comparison with C3 trees from arid environments (Pinus longaeva and Pinus edulis in the US Southwest) as well as from wetter environments (Bromus and Poa grasses in New Mexico) suggests differing responses based on environment; arid environments in New Mexico see increased intrinsic water use efficiency (WUE) in response to historic elevated CO2 while wetter environments see increased ci . This study suggests that (i) the observed increases in ci in FACE experiments are consistent with historical CO2 increases and (ii) the CO2 increase influences plant sensitivity to water shortage, through either increased WUE or ci in arid and wet environments, respectively.

A 520 year record of summer sunshine for the eastern European Alps based on stable carbon isotopes in larch tree rings

A 520-year stable carbon isotope chronology from tree ring cellulose in high altitude larch trees (Larix decidua Mill.), from the eastern European Alps, correlates more strongly with summer temperature than with summer sunshine hours. However, when instrumental records of temperature and sunshine diverge after AD1980, the tree ring time series does not follow warming summer temperatures but more closely tracks summer sunshine trends. When the tree ring stable carbon isotope record is used to reconstruct summer temperature the reconstruction is not robust. Reconstructed temperatures prior to the twentieth century are higher than regional instrumental records, and the evolution of temperature conflicts with other regional temperature reconstructions. It is concluded that sunshine is the dominant control on carbon isotope fractionation in these trees, via the influence of photosynthetic rate on the internal partial pressure of CO2, and that high summer (July–August) sunshine hours is a suitable target for climate reconstruction. We thus present the first reconstruction of summer sunshine for the eastern Alps and compare it with the regional temperature evolution.

Thermal Decomposition of Silver Carbonate: Phenomenology and Physicogeometrical Kinetics

Thermal decomposition of Ag2CO3 to Ag2O was investigated to identify the physicochemical events that occur during the reaction and to reveal the interactions that cause the complex kinetic behavior of the reaction. Based on comparative investigation of thermal decomposition behavior of six different commercially available Ag2CO3, a physicogeometrical reaction model of two partially overlapping reaction stages is proposed. The reaction stages involve the formation of a surface product layer and an internal reaction in the as-produced core–shell structure of the reacting particles. Immediately after the formation of the surface product layer, the structural phase transitions of Ag2CO3 to two different high-temperature phases occur. Under these conditions, the thermal decomposition behavior is controlled by the diffusional removal of CO2 through the surface product layer and/or the increase in internal partial pressure of CO2. The growth of Ag2O particles in the surface product layer produces possible channe...

Effect of preharvest bagging on fruit quality and postharvest physiology of pears (Pyrus communis)

Preharvest bagging of pear fruit (Pyrus communis L. ‘Doyenne du Comice') with micro‐perforated polyethylene bags c. 30 days after full bloom did not affect fruit size and weight, density, maturity, and flesh content of N, P, K, Ca, and Mg. Bagged fruit had a greener and lighter skin colour than non‐bagged fruit, whereas the development of blush on the sunny side was not different between treatments. Bagged fruit had less cuticle deposition, whereas the wax content of the cuticle was not different between treatments. Fruit skin permeance to water (P‘H2O) and fruit weight loss were not different between treatments. Permeance to CO2 (P‘co2) and O2 (P‘co2) were lower for bagged than for non‐bagged fruit. Accordingly, internal partial pressure of O2 (pi o2) was lower and internal partial pressure of CO2 (pi co2) was higher for bagged than for non‐bagged fruit. However, these differences in fruit internal atmosphere between treatments were not large enough to result in significant differences in rates of skin background colour change and respiration during shelf life after 8 weeks in cold storage. With prolonged cold storage the differences between treatments in fruit skin background colour were reduced (bagged fruit de‐greened more than non‐bagged fruit) while the differences in firmness were increased (bagged fruit had more substantial softening than non‐bagged fruit). Fruit bagging did not affect the incidence of senescent scald, senescent breakdown, and diseases after 6 months in cold storage followed by 7 days of shelf life.

Effect of heat stress on the inhibition and recovery of the ribulose-1,5-bisphosphate carboxylase/oxygenase activation state.

Experiments were conducted to determine the relative contributions of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco; EC 4.1.1.39) activation state vis-a-vis Rubisco activase and metabolite levels to the inhibition of cotton (Gossypium hirsutum L.) photosynthesis by heat stress. Exposure of leaf tissue in the light to temperatures of 40 or 45 degrees C decreased the activation state of Rubisco to levels that were 65 or 10%, respectively, of the 28 degrees C control. Ribulose-1,5-bisphosphate (RuBP) levels increased in heat-stressed leaves, whereas the 3-phosphoglyceric acid pool was depleted. Heat stress did not affect Rubisco per se, as full activity could be restored by incubation with CO2 and Mg2+. Inhibition and recovery of Rubisco activation state and carbon dioxide exchange rate (CER) were closely related under moderate heat stress (up to 42.5 degrees C). Moderate heat stress had negligible effect on Fv/Fm, the maximal quantum yield of photosystem II. In contrast, severe heat stress (45 degrees C) caused significant and irreversible damage to Rubisco activation, CER, and Fv/Fm. The rate of Rubisco activation after alleviating moderate heat stress was comparable to that of controls, indicating rapid reversibility of the process. However, moderate heat stress decreased both the rate and final extent of CER activation during dark-to-light transition. Treatment of cotton leaves with methyl viologen or an oxygen-enriched atmosphere reduced the effect of heat stress on Rubisco inactivation. Both treatments also reduced tissue RuBP levels, indicating that the amount of RuBP present during heat stress may influence the degree of Rubisco inactivation. Under both photorespiratory and non-photorespiratory conditions, the inhibition of the CER during heat stress could be completely reversed by increasing the internal partial pressure of CO2 (Ci). However, the inhibition of the CER by nigericin, a K+ ionophore, was not reversible when the Ci was increased at ambient or high temperature. Our results indicate that inhibition of photosynthesis by moderate heat stress is not caused by inhibition of the capacity for RuBP regeneration. We conclude that heat stress inhibits Rubisco activation via a rapid and direct effect on Rubisco activase, possibly by perturbing Rubisco activase subunit interactions with each other or with Rubisco.

Effects of micro‐environmental factors on photosynthetic CO2 uptake and carbon fixation metabolism in a spring ephemeral, Erythronium japonicum, growing in native and open habitats

Microenvironmental factors and physiological parameters (such as water potential, activity of ribulose 1,5‐bisphosphate carboxylase (RuBPcase), levels of ribulose 1,5‐bisphosphate (RuBP), 3‐phosphoglyceric acid (PGA) and sucrose in leaves) affecting photosynthetic processes of the typical vernal species Erythronium japonicum Decne. were examined on the floor of a deciduous broad‐leaved Quercus mongolica forest (Q.m. stand) and on bare land left undisturbed for 7 years after forest clearing (bare stand). Daytime solar radiation and the air and leaf temperatures at the bare stand were significantly higher than those at the Q.m. stand. The relative air humidity was very low and did not differ much between the stands, whereas the leaf–air vapor pressure difference (VPD) at the bare stand was twice as high as that at the Q.m. stand. The water potential in leaves at the bare stand was lower than two times that at the Q.m. stand. Therefore, the aboveground parts of the plants at the bare stand were subjected to much more severe heat stress than those at the Q.m. stand. When these environmental factors observed at the bare stand were reproduced in an assimilation chamber, the rate of photosynthetic CO2 uptake, stomatal conductance and water potential in leaves were significantly low in comparison with those when the factors at the Q.m. stand were simulated. The internal CO2 partial pressure in leaves at the bare stand was considerably lower than that at the Q.m. stand. Consequently, the decrease in the photosynthetic rate of the plants at the bare stand was caused mainly by a decrease in stomatal conductance through a lowering of water potential due to subjection of the aboveground parts to much more severe heat stress than that at the Q.m. stand. The possibility that an inhibition of the photosynthetic carbon fixation metabolism induced by the decrease in water potential contributes to the reduction in photosynthetic CO2 uptake in the plants at the bare stand is also discussed in light of physiological characteristics such as the activity of RuBPcase and levels of PGA, RuBP and sucrose in the leaves.

Leaf Gas Exchange, Dry Matter Partitioning, and Mineral Element Concentrations in Mango as Influenced by Elevated Atmospheric Carbon Dioxide and Root Restriction

The effects of atmospheric CO2 enrichment and root restriction on net CO2 assimilation (A), dry mass partitioning, and leaf mineral element concentrations in `Kensington' and `Tommy Atkins' mango (Mangifera indica L.) were investigated. Trees were grown in controlled-environment glasshouse rooms at ambient CO2 concentrations of 350 or 700 μmol·mol-1. At each CO2 concentration, trees were grown in 8-L containers, which restricted root growth, or grown aeroponically in 200-L root mist chambers, which did not restrict root growth. Trees grown in 350 μmol·mol-1 CO2 were more efficient at assimilating CO2 than trees grown in 700 μmol·mol-1 CO2. However, total plant and organ dry mass was generally higher for plants grown at 700 μmol·mol-1 CO2 due to increased A as a result of a greater internal partial pressure of CO2 (Ci) in leaves of plants in the CO2 enriched environment. Root restriction reduced A resulting in decreased organ and plant dry mass. In root-restricted plants, reduced A and dry matter accumulation offset the increases in these variables resulting from atmospheric CO2 enrichment. Atmospheric CO2 enrichment and root restriction did not affect dry mass partitioning. Leaf mineral element concentrations were generally lower for trees grown at the higher ambient CO2 concentration, presumably due to a dilution effect from an increased growth rate.

Photosynthesis and micro‐environmental factors in a spring ephemeral, Erythronium japonicum, from native and open habitats

AbstractEnvironmental factors affecting photosynthetic activity of the typical vernal species Erythronium japonicum Decne were examined on the floor of a deciduous broad‐leaved Quercus mongolica forest (Q.m. stand) and on bare land left undisturbed for 4 years after forest clearing (bare stand). Daytime solar irradiation and air and leaf temperatures at the bare stand were significantly higher than those at the Q.m. stand. The relative air humidity was very low and did not differ much between the stands, although the leaf‐air vapor pressure differences (VPD) at the bare stand were about twice as high as those at the Q.m. stand. The plants at both stands were supplied with sufficient soil water throughout their growing season by a large snowmelt. However, the aboveground parts of the plants at the bare stand were subjected to much more severe heat stress, caused by the strong radiations and high leaf temperatures, and water stress, caused by the highly transpiring conditions, than those at the Q.m. stand. When the radiation on leaves, leaf temperatures and VPD in the assimilation chamber were changed from those observed at the Q.m. stand to those at the bare stand, the photosynthetic rate and stomatal conductance fell significantly. However, the rate and conductance were immediately restored to the respective values near those measured under the conditions at the Q.m. stand when only VPD was dropped to the value similar to that observed at the Q.m. stand. These results indicate that the photosynthetic rate of the plants at the bare stand was lowered largely by a decrease in stomatal conductance. The internal CO2 partial pressure was considerably greater in leaves placed under environmental conditions similar to those at the bare stand.

558 PB 377 SALINITY AND LEAF GAS-EXCHANGE OF ANNONA SOUAMOSA

Leaf gas-exchange responses of A. souamosa seedlings to salinity were studied in sand culture in a series of glasshouse experiments. Trees were irrigated with a complete nutrient solution as the control, or with this solution amended to 3 or 6 dS/m with sea salt. Inhibition of net CO2 assimilation, stomatal conductance of CO2, and transpiration was apparent 14 days after treatments were imposed, and continued to decline until day 30 to 35. The diurnal pattern of leaf gas exchange was not altered by salinity. Salinity reduced CO2, light energy, and water use efficiencies. Dark respiration and internal partial pressure of CO2 were unaffected by salinity stress. Results indicated that substrate salinity inhibited photosynthesis of A. souamosa via limitations on mesophyll capacity for CO2 assimilation and had little effect on gas phase limitations.

Papaya Ringspot Virus Influences Net Gas Exchange of Papaya Leaves

Leaves of container-grown papaya (Carica papaya L.) plants were inoculated with papaya ringspot virus (PRV) to determine its influence on dark respiration and photosynthesis. Photosynthetic capacity, apparent quantum yield, ratio of variable to maximum fluorescence from dark-adapted leaves, and photosynthetic CO2-use efficiency were reduced by PRV infection. Internal CO2 partial pressure at ambient external CO2 was not affected, but leaf dark respiration was increased by PRV infection. These results suggest that reduced growth, yield, and fruit quality common in PRV-infected papaya plants is caused, at least partially, by reduced photosynthesis and increased respiration.

Humidity Pretreatment Affects the Responses of Stomata and CO&lt;sub&gt;2&lt;/sub&gt; Assimilation to Vapor Pressure Difference in C&lt;sub&gt;3&lt;/sub&gt; and C&lt;sub&gt;4&lt;/sub&gt; Plants

Experiments were carried out to investigate the long-term influence of humidity on the short-term responses of stomata and CO2 assimilation to vapor pressure difference in Oryza sativa (rice, C3 species) and Panicum maximum (green panic, C4 species). Plants were grown for four weeks in growth chambers set at 35% and 85% relative humidity at 25°C air temperature, 38+2 Pa CO2 partial pressure and 1,700μmol m-2s-1 photon flux density. Soil was saturated with water in both humidity treatments. Low humidity pretreatments caused low leaf conductance and low rates of transpiration and CO2 assimilation in O. sativa, but small changes in stomatal responses to humidity and in CO2 assimilation were found in P. maximum. From the short-term gas exchange experiments, it was noted that the responsiveness of leaf conductance to vapor pressure difference were affected by humidity pretreatments in O. sativa, whereas unaffected in P. maximum. In O. sativa measurements of CO2 assimilation as a function of internal CO2 partial pressure (A-Ci curve) indicated that low humidity pretreatments reduced the CO2 assimilation at high internal CO2 partial pressure, but the initial slope of the A-Ci curve was unaffected. Furthermore, plant characteristics such as total dry weight and leaf area of plants subjected to low umidity were lower than plants subjected to high humidity. The reductions in O. sativa, however, were larger than in P. maximum. Stomatal frequency from low humidity grown plant was higher than that from high humidity grown plants in both species although there is no significant difference. The data indicated that if the short term inhibition of net CO2 assimilation at a high vapor pressure difference was imposed during vegetative growth, the photosynthetic biochemistry and the resultant plant growth were largely depressed in O. sativa, a C3 species.

Transient Depression of Photosynthesis in Bean Leaves During Rapid Water Loss

The response of the photosynthetic capacity of mesophyll to a rapid loss of water from the leaf was evaluated by measuring transpiration, net CO2 uptake and temperature in dwarf bean leaves exposed to saturating CO2 partial pressures in a leaf chamber before and after cutting the petiole in air. Some plants were pre-exposed to low or high photon flux density, or to water deficit before measurements. During 3-5 min after cutting, when the turgorpassive opening of the stomata accelerated the water loss from the leaf, the rate of CO2 uptake was depressed; the depression was more pronounced in young leaves, under higher CO2 partial pressure, and in leaves pre-exposed to low photon flux density or to slight water deficit. During subsequent closure of the stomata, the CO2 uptake rate accelerated, although the leaf water content and the internal CO2 partial pressure declined. The depression in CO2 uptake was poorly correlated with the water loss rate after cutting, but it was positively correlated with the increase in water loss rate relative to the transpiration rate before the cutting. It is speculated that sharp perturbation in leaf water regime may induce a transient leakage of cellular membranes and redistribution of metabolites between cell compartments, resulting in a depression of photosynthesis. Subsequent restoration of the concentrations of metabolites in compartments tends to restore the rate of photosynthesis, which then declines due to substantial stomatal closure.

Leaf Gas Exchange in Water‐Stressed Common Bean and Tepary Bean

Tepary bean, Phaseolus acutifolius A. Gray var. latifolius G. Freeman, is adapted to hot arid conditions and might be a valuable source of genes to improve the drought tolerance of P. vulgaris L. We tested the hypothesis that P. acutifolius adaptation to arid environments relies on differences in gas exchange characteristics leading to greater water‐use efficiency. Both species were subjected to controlled levels of water stress by enclosing the root systems in semipermeable membranes immersed in nutrient solutions osmotically adjusted with polyethylene glycol. Phaseolus acutifolius had higher net photosynthetic rates (Pn) than P. vulgaris at high to moderately low water potentials. The Pn rates of P. acutifolius declined more rapidly with lower water potential than those of P. vulgaris. This response was related to increased stomatal closure with decreased water potential. Higher Pn, rates at any given internal CO2 partial pressure (C1) led to higher water‐use efficiency in P. acutifolius than in P. vulgaris. Analysis of the initial slope of Pn vs. C1, curves indicated that P. acutifolius bad higher carboxylation efficiency (CE) at low C1. In both species, water stress significantly reduced light‐ and CO2‐saturated Pn rates and only slightly lowered the CE. Similar mesophyll palisade surface area in P. acutifolius and P. vulgaris suggested that differences in Pn rates at low C1 might be related to biochemical rather than anatomical leaf characteristics. Higher water‐use efficiency at moderate water deficits may contribute to P. acutifolius adaptation to arid environments.

Changes in water relations and in some physiological functions of bean under very light osmotic shock induced by polyethylene glycol

Bean plantlets (Phaseolus vulgaris L. cv. Topcrop) were stressed at the age of 16–18 days by gradual (2–8%) or abrupt addition of 6% (w/v) polyethylene glycol Mw 6000 (PEG 6000) to Hoagland solution. Leaf conductance, photosynthesis, internal CO2 partial pressure (Ci), relative water content (RWC), water content/dry weight (H2O/DW), apoplastic PEG concentrations and weight of leaves, stems and roots were determined. Leaf conductance, photosynthesis and Ci were determined on non‐detached primary leaves, and leaf potentials (water, osmotic and turgor potentials) were investigated in freshly detached (non‐rehydrated) primary leaves, both in treated and control plants; RWC and osmotic potential were also assessed at the null turgor point. Low PEG 6000 concentrations induced early and evident decrease in leaf conductance and photosynthesis, whereas Ci decreased only moderately and tended to recover during advanced stress. There were moderate though significant decreases in RWC and H2O/DW, no change or increases in water potential, no significant changes in osmotic potential and a moderate but significant increase in turgor potential. Even when referred to null turgor point, RWC significantly decreased and osmotic potential was unchanged. It was concluded that apoplastic PEG 6000 accumulation at evaporating sites would account for the early decrease in conductance which would also justify the unchanged or the prevalent increase in water potential and turgor potential. The subsequent PEG diffusion and concentration in the leaf apoplastic water would have induced the RWC and H2O/DW decrease and the final turgor flexion documented.