Partial Closure Of Stomata Research Articles

Impact of root architecture and transpiration rate on drought tolerance in stay‐green sorghum

AbstractSorghum [Sorghum bicolor (L.) Moench] yield loss due to terminal drought stress is common in semiarid regions. Stay‐green is a drought adaptation trait, and a deeper understanding of stay‐green‐associated traits is necessary for sorghum breeding. We hypothesize that the stay‐green trait in sorghum may be associated with the root architecture and transpiration rate under drought stress. The objectives were to (i) identify the relationship among stay‐green‐associated traits, (ii) compare the root system architecture and transpiration rate of stay‐green (B35 and 296B) and senescent (BTx623 and R16) genotypes under drought stress, and (iii) quantify the impacts of reproductive stage drought stress on gas exchange and grain yield of stay‐green and senescent genotypes. A series of drought experiments were conducted with these genotypes. Under drought stress, the stay‐green genotypes had an increased total root length in the top 30–60 cm (18%) and 60–90 cm of soil (45%) than the senescent genotypes. In contrast, under progressive soil drying, stay‐green genotypes had a decreased transpiration rate (9%) than senescent genotypes by an early (∼1 h) partial closure of stomata under high vapor pressure deficit conditions. The increased seed yield (43%) in stay‐green genotypes is due to an increased photosynthetic rate (30%) and individual seed size (35%) than senescent genotypes. Overall, it is concluded that stay‐green phenotypes had two distinct drought adaptive mechanisms: (i) increased root length for increased soil exploration for water and (ii) an early decrease in the transpiration rate to conserve soil moisture. Identifying genomic markers for these traits would accelerate drought‐tolerant sorghum breeding.

Realistic Physiological Options to Increase Grain Legume Yield under Drought.

Increasing yield resiliency under water deficits remains a high priority for crop improvement. In considering the yield benefit of a plant trait modification, two facts are often overlooked: (1) the total amount of water available to a crop through a growing season ultimately constrains growth and yield cannot exceed what is possible with the limited amount of available water, and (2) soil water content always changes over time, so plant response needs to be considered within a temporally dynamic context of day-to-day variation in soil water status. Many previous evaluations of drought traits have implicitly considered water deficit from a "static" perspective, but while the static approach of stable water deficit treatments is experimentally congruous, the results are not realistic representations of real-world drought conditions, where soil water levels are always changing. No trait always results in a positive response under all drought scenarios. In this paper, we suggest two key traits for improving grain legume yield under water deficit conditions: (1) partial stomata closure at elevated atmospheric vapor pressure deficit that results in soil water conservation, and (2) lessening of the high sensitivity of nitrogen fixation activity to soil drying.

Rice CO 53: A high yielding drought tolerant rice variety for drought prone districts of Tamil Nadu

Early duration drought tolerant rice culture CB 06803, a derivative of the cross PMK (R ) 3 x Norungan was released as Rice CO 53 during the year 2020 as an alternate variety for Anna(R)4, with 115 -120 days duration. It possesses desirable features like high yield, drought tolerance and better physiological efficiency. This culture with semi dwarf stature has efficient tillering capacity, long droopy panicles with a highly acceptable plant characters and is a replacement for the rice variety Anna (R) 4 due to its grain yield and pest and disease resistance. In the overall yield analysis, the culture CB06803 recorded an overall mean grain yield of 3718 kg/ha which was 12.19 per cent increase over TKM (R) 12 and 14.08 per cent over Anna (R) 4 under dry condition. Under semi dry condition, it recorded 3866 kg/ha with 18.40 per cent increase over TKM (R) 12 and 8.67 per cent over Anna (R) 4. It is moderately resistant to WBPH, leaf blast, neck blast, sheath rot, brown spot and RTD under artificial condition. It has white short bold rice with high milling percentage (69.6), head rice recovery (59.6%) and suitable for idly making. The culture, CB 06803 had better  agronomic efficiency than TKM (R)12 and Anna (R) 4 by recording more number of productive tillers per plant and grain yield under control and drought. The culture CB 06803 had better physiological efficiency under drought than TKM (R) 12 and Anna (R) 4  having higher RWC, total chlorophyll content, chlorophyll stability index, photosynthetic rate, stomatal conductance and photo chemical efficiency. The culture had higher proline content than TKM (R) 12 and was on par with Anna (R) 4. It had higher catalase activity than Anna (R) 4. CB 06803 had better physiological efficiency under drought by showing partial closure of stomata as that of Sahbagidhan. CO 53 with higher grain yield under drought, disease resistance and short bold grains and suitable for cultivation in drought prone districts of Tamil Nadu as direct seeded rainfed or semi dry rice ecosystem. Keywords: Rice CO 53, bold grain rice, short duration, drought tolerance

Plant waterflow restrictions among sweet corn lines related to limited‐transpiration trait

AbstractWhile partial stomata closure under elevated vapor pressure deficit (VPD), that is, a limited‐transpiration trait resulting in soil water conservation and crop drought resiliency, has been identified in a few lines in all major crop species, the basis for the trait is not resolved. Since changes in stomatal aperture are associated with hydraulic processes, properties influencing waterflow in the plant are candidates associated with expression of limited‐transpiration. In a previous study with leaves of sweet corn (Zea mays L.), limited‐transpiration was only weakly associated with the effect of silver ion, an aquaporin inhibitor, on leaf transpiration rate. The hypothesis was explored that total aquaporin activity rather than only silver‐sensitive aquaporins may be more relevant in the expression of limited transpiration. Leaves were fed potent aquaporin inhibitors of mercury and peroxide which both resulted in greater transpiration inhibition than silver, but there were no differences in response among lines. A second hypothesis was that there may be a major limitation of waterflow in the root system accounting for genotypic variation in expression of the limited‐transpiration trait. No differences in root hydraulic conductance were found among sweet corn lines differing in the limited‐transpiration trait. However, it was discovered that the root conductance decreased during the photoperiod in all tested lines. Measurements of root hydraulic conductance in response to a silver treatment showed differences among lines with an association between the VPD breakpoint for limited transpiration and transpiration sensitivity to the silver treatment.

Comparison of water flow capacity in leaves among sweet corn genotypes as basis for plant transpiration rate sensitivity to vapor pressure deficit

AbstractGenotypes in crop species have been identified that initiate partial stomata closure at elevated atmospheric vapor pressure deficit (VPD), which results in conserved soil water for crop use during subsequent water‐deficit episodes and thereby allowing for possible yield increase. In sweet corn (Zea mays L), 17 genotypes have been previously identified with the VPD‐responsive trait, although the VPD value at the initiation of stomata closure varied among genotypes. A hypothesis to explain variation in transpiration response to VPD is that water flow capacity in the leaves differs among genotypes. To gauge water flow capacity in leaves, the rate of stomata opening was observed visually after stomata closure was induced by 3 kPa VPD. The stomata opening time was rapid and varied among genotypes from 90 to 179 s. However, there was no correlation between opening time and the VPD at which partial stomata closure was initiated in intact plants. An additional set of experiments was done to examine whether genotypic differences in a subpopulation of silver‐inhibited aquaporins might contribute to differences in leaf water flow. There was a correlation among genotypes between slow opening time of the stomata and greater inhibition of transpiration rate following feeding leaves with silver ion. However, the response to the silver treatment did not correlate with the VPD at which transpiration decrease of intact plants was initiated. These results indicate that the differences observed in the water flow capacity in sweet corn leaves were not major factors accounting for the genotypic differences in whole‐plant transpiration response to elevated VPD.

Review of quantitative sensitivity of faba bean physiology to temperature and soil-water deficit

Faba bean (Vicia faba L.) is an important component of cropping systems in cool, arid environments. However, no review has specifically focused on the quantitative sensitivity of physiological processes in faba bean to low temperature and water deficits. The objective of this review was to examine published functional relationships between physiological activity and these environmental variables. Among faba bean genotypes, temperature generally resulted in a consistent linear response in plant ontogeny and leaf area development. By contrast, nitrogen fixation exhibited a sharp threshold response to temperature such that at temperatures below ~13.5–15°C faba bean had virtually no nitrogen fixation activity. This inability to fix nitrogen under cool temperatures is likely to be a major weakness for faba bean in cool-season production systems. Water deficit also had a large impact on the physiology of faba bean. Ontogeny was generally shortened when plants were subjected to drought, resulting in major yield decreases. Genotypic differences within faba bean have been identified for initiation of partial stomata closure at high soil-water content, resulting in possible soil-water conservation in the field. Also, differences among genotypes have been identified in the sensitivity of nitrogen fixation activity to water deficits. Finally, collectively the reviewed functional relationships have been applied to simulation analysis of the geospatial impact of irrigation regimes and of sowing date for faba bean production. These geospatial studies offered insights on options to improve faba bean management.

Atmospheric CO2 enrichment effect on water use efficiency in chickpea (Cicer arietinum L.)

AbstractEffect of atmospheric CO2 enrichment on crop water relations was evaluated in an open‐top chamber experiment on kabuli chickpea (Cicer arietinum L.; Pusa 1,105). Increase in atmospheric CO2 concentration (e[CO2], 580 ± 20 μmol/mol) resulted in 21%–46% increase in leaf water potential, and 7%–11% in relative leaf water content, indicating higher water content in leaves, compared to the ambient level (a[CO2], 384 ± 12 μmol/mol). Higher leaf water status was connoted through spectral indices over the growing period. A rise in CO2, however, brought increase in leaf diffusive resistance and canopy temperature by 15%–35% and 3%–11%, respectively, potentially indicating partial closure of stomata. Increase in leaf area index (13%–73%), and higher spectral vegetation index suggested improved growth of chickpea under e[CO2]. An increase in leaf area compensated the decrease in diffusive resistance, and therefore seasonal total water use was similar between enriched and ambient conditions. Although water use efficiency (WUE) increased by 30% (p < .05) due to an increase in crop biomass under e[CO2], the canopy conductance remained unaffected. Results suggest a possibility of improved plant water status, enhanced biomass production and increased WUE for chickpea, with similar crop water use under future e[CO2] levels.

Abscisic acid phytohormone estimation in tubers and shoots of Ipomoea batatas subjected to long drought stress using competitive immunological assay.

Sweet potato (Ipomoea batatas L.), typically cultivated in temperate climates under low inputs, is one of the most important crops worldwide. Abscisic acid (ABA) is an important plant stress-induced phytohormone. Hitherto, few works analyzed the ABA function in sweet potato tissue growth. Very scarce information is available concerning the ABA role in sweet potato response to water scarcity conditions. Here, we show the ABA content variation in shoots and tubers of eight sweet potato accessions subjected to drought stress. ABA was also related to other resistance traits, such as chlorophyll content index (CCI), carbon isotopic discrimination (Δ13 C), oxalic acid (OA) and water use efficiency (WUE), to assess stress response mechanisms to water deficit between their organs. The most resilient drought-stressed sweet potato plants accumulated ABA-shoot, and significantly decreased the ABA-tuber content. ABA signaling was related to Δ13 C and CCI decrease and WUE increment, as an attempt to cope with water stress by partially closing the stomata. The partial closure of stomata could be in part due to the presence of OA-shoots, known to affect the intensity of the ABA-shoot signal in stomatal closure. Higher CCI content and minimal Δ13 C-shoot differences indicated good carboxylation fractionation, with higher Δ13 C-tuber content as an indicator of efficient tuber 13 C fixation and growth. Our work demonstrated that ABA could be used in conjunction with the other traits studied for the assessment of sweet potato whole-plant responses to environmental stresses, and thus aid the selection of the best drought tolerant genotypes for breeding programs.

Eddy covariance flux measurements of gaseous elemental mercury over a grassland

Abstract. Direct measurements of the net ecosystem exchange (NEE) of gaseous elemental mercury (Hg0) are important to improve our understanding of global Hg cycling and, ultimately, human and wildlife Hg exposure. The lack of long-term, ecosystem-scale measurements causes large uncertainties in Hg0 flux estimates. It currently remains unclear whether terrestrial ecosystems are net sinks or sources of atmospheric Hg0. Here, we show a detailed validation of direct Hg0 flux measurements based on the eddy covariance technique (Eddy Mercury) using a Lumex RA-915 AM mercury monitor. The flux detection limit derived from a zero-flux experiment in the laboratory was 0.22 ng m−2 h−1 (maximum) with a 50 % cutoff at 0.074 ng m−2 h−1. We present eddy covariance NEE measurements of Hg0 over a low-Hg soil (41–75 ng Hg g−1 in the topsoil, referring to a depth of 0–10 cm), conducted in summer 2018 at a managed grassland at the Swiss FluxNet site in Chamau, Switzerland (CH-Cha). The statistical estimate of the Hg0 flux detection limit under outdoor conditions at the site was 5.9 ng m−2 h−1 (50 % cutoff). We measured a net summertime emission over a period of 34 d with a median Hg0 flux of 2.5 ng m−2 h−1 (with a −0.6 to 7.4 ng m−2 h−1 range between the 25th and 75th percentiles). We observed a distinct diel cycle with higher median daytime fluxes (8.4 ng m−2 h−1) than nighttime fluxes (1.0 ng m−2 h−1). Drought stress during the measurement campaign in summer 2018 induced partial stomata closure of vegetation. Partial stomata closure led to a midday depression in CO2 uptake, which did not recover during the afternoon. The median CO2 flux was only 24 % of the median CO2 flux measured during the same period in the previous year (2017). We suggest that partial stomata closure also dampened Hg0 uptake by vegetation, resulting in a NEE of Hg0 that was dominated by soil emission. Finally, we provide suggestions to further improve the precision and handling of the “Eddy Mercury” system in order to assure its suitability for long-term NEE measurements of Hg0 over natural background surfaces with low soil Hg concentrations (< 100 ng g−1). With these improvements, Eddy Mercury has the potential to be integrated into global networks of micrometeorological tower sites (FluxNet) and to provide the long-term observations on terrestrial atmosphere Hg0 exchange necessary to validate regional and global mercury models.

Response of Physiological Indicators to Environmental Factors under Water Level Regulation of Paddy Fields in Southern China

Agricultural water level control is the key to the combined control of water-saving irrigation and controlled drainage. It is easy to observe and master in farmland practice, and has important theoretical and practical significance. In this paper, a systematic study on stomatal regulation and photosynthetic-evapotranspiration coupling mechanism, as well as the effect of meteorological environmental factors on photosynthetic-evapotranspiration coupling mechanism of paddy rice in drought and flooding conditions was conducted by taking paddy field’s water level as a regulation indicator of paddy rice’s irrigation and drainage. Results showed that net photosynthetic rate (Pn) and evapotranspiration rate (Tr) had quadratic and linear relationship with stomatal conductance (Gs), respectively. The responding range of Gs in drought condition was lower than that in flooding condition. Drought stress aggravated the photosynthetic stomatal limitation of rice leaves at noon, and partial closure of stomata was the main reason resulting in the reduction of Pn suffering from drought at noon. Change curve of paddy rice’s leaf temperature difference (ΔT) showed a W or V type daily change curve and its change rule was rightly contrary to that of Tr. Pn had quadratic curve relationships with photosynthetically active radiation (PAR) and surrounding CO2 concentration (Cs) both in the morning and in the afternoon, Pn had quadratic curve relationships in the morning and linear relationships in the afternoon with air temperature (Ta), respectively. Tr showed linear relationships with PAR, Ta, relative air humidity (RH) and barometric pressure saturation deficit (VPD).

Stomatal aperture rather than nitrogen nutrition determined water use efficiency of tomato plants under nitrogen fertigation

Fertigation can improve water use efficiency (WUE) compared with conventional separate supply of water and fertilizers to plants. Yet the mechanisms underlying the improved WUE under fertigation remain largely elusive. Therefore, the impact of water and nitrogen (N) on leaf gas exchange, plant water relations, ABA signaling and WUE as well as leaf δ13C and δ18O were investigated in order to unravel how water and N modulate plant WUE. Results showed that reduced soil water regimes under N fertigation caused partial closure of stomata via decreased plant water status and intensified root-to-shoot ABA signaling, resulting in improved intrinsic WUE (WUEi). Decreased soil water regimes increased plant WUE (WUEp) and leaf δ13C, and the increased leaf δ13C was due to reduced gs and/or higher specific leaf N content enhanced photosynthetic capacity. Leaf δ18O and δ13C further indicated that the significant increase in leaf δ13C under the reduced water regimes was caused primarily by reductions in gs compared with N nutrition. Therefore, gs rather than N nutrition predominated regulation of plant WUE under fertigation. Moderate soil water regimes with sufficient N supply are recommended for fertigation in terms of achieving high fresh fruit yield, WUE and nutrient uptake.

Evaluation of Morpho-Physiological Traits Adjustment of Prosopis tamarugo Under Long-Term Groundwater Depletion in the Hyper-Arid Atacama Desert.

Water extraction from the underground aquifers of the Pampa del Tamarugal (Atacama Desert, Chile) reduced the growing area of Prosopis tamarugo, a strict phreatic species endemic to northern Chile. The objective of this work was to evaluate the effect of various architectural and morpho-physiological traits adjustment of P. tamarugo subjected to three groundwater depletion intervals (GWDr): <1 m (control), 1–4 m and 6–9 m. The traits were evaluated at three levels, plant [height, trunk cross-section area, leaf fraction (fGCC), and crown size], organ [length of internodes, leaf mass per unit area (LMA), leaflet mass and area], and tissue level [wood density (WD), leaf 13C, 18O isotope composition (δ), and intrinsic water use efficiency (iWUE)]. In addition, soil water content (VWC) to 1.3 m soil depth, pre-dawn and midday water potential difference (ΔΨ), and stomatal conductance (gs) were evaluated. At the deeper GWDr, P. tamarugo experienced significant growth restriction and reduced fGCC, the remaining canopy had a significantly higher LMA associated with smaller leaflets. No differences in internode length and WD were observed. Values for δ13C and δ18O indicated that as GWDr increased, iWUE increased as a result of partial stomata closure with no significant effect on net assimilation over time. The morpho-physiological changes experienced by P. tamarugo allowed it to acclimate and survive in a condition of groundwater depletion, keeping a functional but diminished canopy. These adjustments allowed maintenance of a relatively high gs; ΔΨ was not different among GWDrs despite smaller VWC at greater GWDr. Although current conservation initiatives of this species are promising, forest deterioration is expected continue as groundwater depth increases.

THE EFFECT OF SHORT-TERM CHILLING ON THE WATER METABOLISM IN WHEAT AS RELATED TO OPTIMAL AND EXCESSIVE ZINC CONTENT

The effect of short-term chilling (to 4 ° С for 1 day) on the growth and water metabolism in winter wheat ( Triticum aestivum L.) when combined with optimal (2 ﾊM) and excessive (1000 ﾊM) zinc content in the root area was studied in a controlled environment. In the plants growing in a medium with optimal zinc content a 1 day exposure to the 4 ° С temperature caused a reduction of the stomatal pore size, a decline in stomatal conductance and transpiration rate. These modifications supposedly act as defensive adaptations meant to maintain a high water content in the leaves and support the increment of the above-ground biomass. Where the metal concentration in the root area was excessive, there was a heavier decline in stomatal conductance and transpiration rate, and yet the water content in the leaves somewhat decreased. Furthermore, plants in this treatment demonstrated a partial closure of stomata and a considerable decrease in their number. The above changes in the stomatal system together with a slower increment of the root biomass pointing to inhibition of their growth must have been the main reasons for a disruption of the water metabolism in the wheat plants exposed to a low temperature coupled with excessise zinc content in the root area.

THE EFFECT OF ZINC ON THE GROWTH AND PHOTOSYNTHETIC APPARATUS OF WHEAT UNDER OPTIMAL AND HYPOTHERMIC CONDITIONS

The effect of zinc in a high concentration (1000 µM) on the growth and the photosynthetic apparatus of winter wheat plants ( Triticum aestivum L.) was investigated in the laboratory under optimal (22 ° C) and low (4 ° C) temperatures. It was shown that irrespective of the temperature, zinc in the stated concentration had a pronounced negative effect on the plants, causing a decrease in the content of the main photosynthetic pigments (chlorophylls and carotenoids) in the leaves, and a decline of the potential quantum efficiency of photosystem II. In addition, the exposure of the experimental plants to this metal caused a partial closure of stomata and a decrease of the stomatal pore size. These changes, along with other possible modifications and/or disruptions of physiological processes, inhibited the photosynthetic activity and growth of the plants. Where the temperature was optimal, however, the detrimental effect of zinc on the plants was attenuated toward the end of the experiment (3 days). Namely, the rate of photosynthesis, as well as the accumulation of shoot biomass reached a level typical of the control plants. Under hypothermic conditions, on the contrary, the inhibitory effect caused by the simultaneous action of these two stress factors, which was manifest in a number of the studied growth indices (shoot height and aboveground biomass) and the state of the photosynthetic apparatus (photosynthetic pigments content, rate of photosynthesis, leaf tissue water content) tended to build up. The approximately equal decrease in the values of the maximum quantum yield of the PS II photochemical activity ( F v / F m ) detected experimentally under temperature optimum and hypothermia is obviously connected with the weak dependence of light-dependent reactions on temperature.

Use of plant woody species electrical potential for irrigation scheduling

The electrical response of plants to environmental stimuli can be measured and quantitatively related to the intensity of several stimulating sources, like temperature, solar radiation, soil water content, evapotranspiration rates, sap flow and dendrometric cycles. These relations can be used to assess the influence of different environmental situations on soil water availability to plants, defined as a steady state condition between leaf transpirative flow and soil water flow to plant roots. A restricted soil water flow due to soil dryness can trigger water stress in plants, if the atmospheric evaporative demand is high, causing partial stomata closure as a physiological response to avoid plant dehydration; water stressed and unstressed plants manifest a differential electrical response. Real time plant electrical response measurements can anticipate actions that prevent the plant reaching actual stress conditions, optimizing stomata gas exchange and photosynthetic rates. An electrophysiological sensor developed in this work, allows remote real-time recording information on plant electrical potential (EP) in the field, which is highly related to EP measurements obtained with a laboratory Keithley voltmeter sensor used in an highly controlled experimental setup. Our electrophysiological sensor is a wireless, autonomous devise, which transmits EP information via Internet to a data server. Using both types of sensors (EP electrodes with a Keithley voltmeter and the electrophysiological sensor), we measured in real time the electrical responses of Persea americana and Prunus domestica plants, to induced water deficits. The differential response for 2 scenarios: irrigation and water restriction is identified by a progressive change in slope on the daily maximal and minimal electric signal values in stressed plants, and a zero-slope for similar signals for well-watered plants. Results show a correspondence between measured signals obtained by our electrophysiological sensor and the EP electrodes connected to the Keithley voltmeter in each irrigation stage. Also, both sensors show a daily cyclical signal (circadian cycle).

Carbon dioxide and water use in forests

Plants are expected to respond to rising levels of atmospheric carbon dioxide by using water more efficiently. Direct evidence of this has been obtained from forests, but the size of the effect will prompt debate. See Letter p.324 Theory suggests that rising atmospheric CO2 concentrations should increase the efficiency with which plants use water, but the actual magnitude of this effect in natural forest ecosystems remains unknown. An analysis of long-term measurements of carbon and water fluxes from forest research sites across the Northern Hemisphere has identified an unexpectedly large increase in water-use efficiency during the past two decades, coinciding with an increase of atmospheric CO2 from 350 to 400 parts per million. This trend is often accompanied by concurrent increases in rates of photosynthetic uptake and carbon sequestration. The authors suggest partial closure of stomata — to maintain constant CO2 concentrations in the plant leaves — as the most likely explanation for the observed trend in water-use efficiency. The results are inconsistent with current theory and terrestrial biosphere models.

Basis of Slow‐Wilting Phenotype in Soybean PI 471938

ABSTRACTTo increase soybean [Glycine max (L.) Merr.] productivity, it will be necessary to improve yields in water‐deficit regions. Genotype PI 471938, which exhibits a slow‐wilting phenotype under water‐deficit conditions, has proven to be a good genetic resource in developing drought‐resistant progeny even though the physiological basis for this advantage is not known. The objective of this study was to investigate the involvement of four water‐saving, physiological mechanisms as candidates contributing to drought tolerance of PI 471938. (i) In response to soil drying, the soil water content at which leaf gas exchange began to decrease in PI 471938 was not different from the other tested genotypes. (ii) Measurement of leaf photosynthetic capacity failed to show that PI 471938 had a high capacity allowing high CO2 assimilations even with partial stomata closure. (iii) Plant Introduction 471938 failed to exhibit a limited transpiration rate with increasing vapor pressure deficit (VPD), which would have allowed this genotype to conserve soil water during midday periods of high VPD. (iv) Finally, PI 471938 did not show an ability to maintain high leaf water potential (ΨLeaf) when VPD was increasing. In fact, there was a dramatic decrease in ΨLeaf with increasing VPD, which usually implies a decrease in leaf turgor pressure. Overall this study resulted in the rejection of four major hypotheses to explain the slow‐wilting phenotype exhibited by PI 471938, and the basis for its drought resistance remains unknown.

Predicting the effects of gas diffusivity on photosynthesis and transpiration of plants grown under hypobaria

As part of a Bio-regenerative Life Support System (BLSS) for long-term space missions, plants will likely be grown at reduced pressure. This low pressure will minimize structural requirements for growth chambers on missions to the Moon or Mars. However, at reduced pressures the diffusivity of gases increases. This will affect the rates at which CO 2 is assimilated and water is transpired through stomata. To understand quantitatively the possible effects of reduced pressure on plant growth, CO 2 and H 2O transport were calculated for atmospheres of various total pressures (101, 66, 33, 22, 11 kPa) and CO 2 concentrations (0.04, 0.1 and 0.18 kPa). The diffusivity of a gas is inversely proportional to total pressure and shows dramatic increases at pressures below 33 kPa (1/3 atm). A mathematical relationship based on the principle of thermodynamics was applied to low pressure conditions and can be used for calculating the transpiration and photosynthesis of plants grown in hypobaria. At 33 kPa total pressure, the stomatal conductance increases by a factor of three with the boundary layer conductance increasing by a factor of ∼1.7, since the leaf conductance is a function of both stomatal and the boundary layer conductance, the overall conductance will increase resulting in significantly higher levels of transpiration as the pressure drops. The conductance of gases is also regulated by stomatal aperture in an inverse relationship. The higher CO 2 concentration inside the leaf air space during low pressure treatments may result in higher CO 2 assimilation and partial stomata closure, resulting in a decrease in transpiration rate. The results of this analysis offer guidelines for experiments in pressure and high CO 2 environments to establish ideal conditions for minimizing transpiration and maximizing the plant biomass yield in BLSS.

Hydraulic and chemical signalling in the regulation of stomatal conductance and plant water use in field grapevines growing under deficit irrigation.

Effects of irrigation strategies on stomata and plant water use were studied in field-grown grapevines (Vitis vinifera L.). We assessed the importance of root-derived chemical signals vs. hydraulic signalling in stomatal regulation. The experiment included two treatments with the same water added to the soil (50% ETc) applied either to the whole root system (DI) or to half of the roots, alternating irrigation side every 15 days (PRD). Well-watered plants (FI) (100% ETc) and non-irrigated grapevines (NI) were also studied. Partial stomata closure occurred in both PRD and DI plants. [ABA] of xylem sap remained constant during the day and was maintained throughout the season, with higher values in NI plants. Xylem sap pH was not affected by soil water availability. A positive correlation between ψpd and maximum g s was found, indicating that grapevine stomata strongly respond to plant water status. In contrast, ABA did not explain stomatal control at veraison. At mid-ripening g s was significantly correlated with ABA, apparently interacting with the rise in xylem sap pH. Therefore, our data suggest that hydraulic feedback and feed-forward root-to-shoot chemical signalling mechanisms might be involved in the control of stomata in response to decreased soil water availability, hydraulic signals playing the dominant role.

Photosynthetic CO 2-Assimilation, Chlorophyll Fluorescence and Zeaxanthin Accumulation in Field Grown Maple Trees in the Course of a Sunny and a Cloudy Day

Changes in photosynthetic CO 2 fixation, variable chlorophyll fluorescence ratios, carotenoid composition, zeaxanthin accumulation and the de-epoxidation state DEPS of the zeaxanthin cycle were simultaneously determined in leaves of field-grown maple trees (Acer platanoides L.) on a sunny and a cloudy day in August. In the course of the sunny day the photosynthetic net CO 2 assimilation rates P N rose with the PPFD in the morning and were then predominantly determined by the stomata opening (partial closure at noon and further closure in the afternoon as seen from the decrease in gH 2 O-values). When the rate of Co 2 -fixation began to decrease at a high sun irradiance (high PPFD) at about 10 :00 am (partial closure of stomata), large zeaxanthin amounts were accumulated in a biphasic process : 1) by a fast phototransformation of existing violaxanthin into zeaxanthin and 2) by a slow but continuous increase of the zeaxanthin level of 50 to 60% via de novo biosynthesis. A strong zeaxanthin accumulation depends on a high proton gradient and only proceeded when P N was light-saturated and was reduced by a partial closure of stomata. The zeaxanthin accumulation in the morning followed the rise in PPFD with some delay as well as the zeaxanthin retransformation to violaxanthin at the PPFD decrease in the late afternoon and evening. On the sunny day the photochemical and non-photochemical quenching coefficients of the chlorophyll fluorescence, qP and qN, showed an inverse relationship. With increasing PPFD qP declined to very low values (below 0.1) and qN increased to high values >0.9. The variable chlorophyll fluorescence ratios Fv'/Fm' and ΔF/Fm', indicators of the quantum efficiency of photosynthetic electron transport of photosystem II, declined parallel to the high PPFD-induced decline of qP and the rise in qN. There was, however, no clear correlation between the changes in chlorophyll fluorescence parameters and the rise and fall of zeaxanthin in the course of the sunny day. In fact, in the afternoon the zeaxanthin level decreased much earlier than the decline in qN and the rise and regeneration of the initial values of qP, Fv'/Fm' or ΔF/Fm'. Despite the strong increase in qN and the large decrease in qP, Fv'/Fm' and ΔF/Fm', the net CO 2 assimilation rates P N of maple leaves only declined by 22 % as compared to the maximum P N values at 10 :15 am indicating that the photosynthetic apparatus was only marginally photoinhibited. It is assumed that only a small fraction of chloroplasts at the upper leaf side became photoinhibited and functioned as a light filter for the other chloroplasts in the lower mesophyll parts which remained photosynthetically active. The results demonstrate that the changes of the chlorophyll fluorescence parameters qP and qN as well as of the ratios Fv'/Fm' or ΔF/Fm', measured with the PAM fluorometer at the upper leaf side, are not representative of the total pool of leaf chloroplasts. For this reason it is suggested to measure the chlorophyll fluorescence parameters not only at the upper but also at the lower leaf side in order to obtain more representative information on the chloroplasts of the whole leaf. In addition, chlorophyll fluorescence measurements should always be complemented by net CO 2 fixation measurements in order to investigate to which degree a presumed photoinhibition really exists at the whole leaf level. On the cloudy day up to 3 pm the PPFD was very low, yet net photosynthesis (P N ) was performed at good rates.