Lower Leaf Water Potential Research Articles

Zoysiagrass Rooting and Drought Tolerance are Reduced by Frequent Irrigation

Textbook recommendations suggest that turf should be watered deeply and infrequently to encourage drought resistance. Data supporting this recommendation are lacking, however. Studies were done to determine the influence of irrigation frequency on `Meyer' zoysiagrass (Zoysia japonica Steud.) rooting and drought resistance. Turf was established on a silt loam soil in 27-cm-diameter by 92-cm-deep containers in the greenhouse. Irrigation was performed daily or at the onset of wilt with a water volume equal to daily or cumulative evapotranspiration of well-watered turf in small weighing lysimeters. After 90 days of irrigation treatments, a dry-down was imposed during which no additional water was applied for >50 days. Compared to turf irrigated daily, turf watered at the onset of wilt exhibited: i) lower (more-negative) leaf water and osmotic potentials prior to the onset of drought; ii) higher leaf water potential and better turf quality at the end of dry-down; and iii) deeper rooting as indicated by lower soil moisture content at 50- and 70-cm depths at the end of dry down.

The Water Relations of Carex stans in Wet Sedge-Moss Tundra at a High Arctic Oasis, Devon Island, N.W.T., Canada

In the eastern Canadian High Arctic, wet sedge-moss tundras, dominated by Carex stans, are usually associated with oases where ameliorated physical conditions allow relatively high plant production. Although Carex stans occurs in water-saturated soils, low transpiration rates, stomatal conductances and leaf water potentials indicated that mild water stress was common in this species. Low transpiration rates under conditions of relatively high solar irradiance and air temperature resulted in leaf temperatures up to 10'C higher than that of the air. Trends in transpiration rate closely followed those in solar irradiance, air and leaf temperature and the leaf-to-air vapor pressure difference. Transpiration also exhibited significant positive correlations with soil temperature and wind speed. The maximum stomatal conductance in Carex stans (0.16 mol m-2 S-1) was considerably lower than maxima reported for polar semidesert graminoids; furthermore, the maximum rate in Carex was seldom achieved. Stomatal conductance showed strongest positive correlations with solar irradiance, leaf temperature, and leaf-toair temperature and vapor pressure differences, and showed a strong negative correlation to soil temperature at -10 cm. Carex leaf water potentials ranged from -0.65 to -2.26 MPa, suggesting a water status more characteristic of polar semidesert graminoids than those of wet tundra. Leaf water potential was most highly correlated with air temperature and air vapor pressure deficit. Several anomalous relationships between stomatal conductance and other variables were observed. For example, stomatal conductance did not respond to changes in leaf water potential, was insensitive to changes in air vapor pressure deficit and was positively correlated with the leaf-to-air vapor pressure difference when a negative correlation was expected. Water stress in Carex stans is hypothesized to result from low soil temperatures and poor soil aeration, characteristics of sedge-moss meadows which result from their waterlogged condition. Several studies have reported that productivity of wet graminoid tundra remains constant from year to year despite considerable annual variation in thermal inputs. We propose that in relatively warm summers, high arctic tundras dominated by Carex stans show no increased productivity due to greater instances of water stress and stomatal closure.

Influence of Variable Amounts of Irrigation Water and Nitrogen Fertilizer on Growth, Yield and Water Use of Grain Sorghum

AbstractSorghum hybrid CSH‐6 was grown in fields in Delhi, India between July–November 1986 in order to study the effect of nitrogen nutrition and irrigation on dry matter accumulation, grain yield and water use. The treatments included 40 Kg Nha−1 combined with two irrigations (30 DAS, 60 DAS), one irrigation (60 DAS) and no irrigation respectively. Rainfall during the crop season was only 17 cm. The unirrigated plants were considerably water stressed and exhibited very low leaf water potential, less leaf area, delayed anthesis, longer crop duration but shorter grain filling duration. The ears showed sterility and yield was only 0.41 t ha−1 without nitrogen fertilization. Addition of nitrogen fertilizer had no significant effect on yield in unirrigated plants. A single irrigation 60 DAS increased yield due to increase in both grain number and grain weight per ear in fertilized and unfertilized crop respectively. Two irrigations in the unfertilized crop increased the yield to 2.2 t ha−1 while similar treatment in the fertilized crop did not increase the yield significantly. Irrigation increased the WUE for grain yield. The results indicate that nitrogen stress and water stress reduced grain yield primarily through grain number rather than grain weight. Irrigation relieved both water stress and nutrient stress. Nitrogen nutrition was not beneficial under severe water stress conditions but was considerably helpful under mild stress. Biomass, grain yield and harvest index show significant correlation with preanthesis water use.

Photosynthetic potential of Thymus vulgaris selections under two light regimes and three soil water levels

A greenhouse experiment was conducted to measure the effects of two light regimes (LR) (supplemental, SL of 200 μmol m −2 s −1 and natural, NL) and three different soil water levels (SWL) (50, 70 and 90% field capacity) on biomass production, relative growth rate (RGR), net CO 2 assimilation ( Pn), stomatal ( g s ) and mesophyll ( g m ) conductance, chlorophyll a and b (chl a and chl b) content and leaf water potential (LWP) on two Thymus vulgaris L. clonal selections. There was a significant increase in total dry matter (DM) production and RGR due to SL and increased SWL. The highest Pn, 24.6 μmol CO 2 m −2 s −1, was obtained from Selection 1, grown under SL at 70% SWL. The lowest Pn, 6.3 μmol CO 2 m −2 s −1, was obtained from Selection 2, grown under NL at 50% SWL. Selection 1 had higher Pn, g s and g m than Selection 2 under all growing conditions. Thymus vulgaris leaves developed under SL were more quantum-use efficient than those grown under NL. Total leaf chlorophyll concentration per leaf area basis was significantly ( P ⩽ 0.01) higher in plants grown under SL than NL. The lowest LWP was recorded for Selection 2 grown under SL at 50% SWL.

Seasonal variations of leaf water potential and growth in fertigated carob-trees (Ceratonia siliqua L.)

Variations of predawn and midday leaf water potential and relative growth rates were studied in mature carob trees (Ceratonia siliqua L. cv “Mulata”) submitted to a fertigation experiment. Three levels of irrigation were tested: 0%, 50% and 100%, based on daily standard evaporation values. For each irrigation level two nitrogen amounts were applied −21 and 63 kg N ha-1 year-1 as ammonium nitrate. The experiment was run between July 91 and August 1993. Measurements of leaf water potential and absolute branch length increments were made at monthly intervals, during the entire experimental period or during seasonal growth, respectively. Leaf water potential was related to soil volumetric water content, maximum and minimum air temperature and daily evaporation. Predawn leaf water potentials were always higher than −1.1 MPa. Midday leaf water potential values presented very large seasonal variations and very low values independent of treatments. The low leaf water potentials observed for the fertigated trees during summer, suggest that this parameter may be related not only to the evaporative demand but also to growth investment. The amount of fertigation was positively correlated with vegetative growth increment and fruit production. Practical implications for irrigation schedules of leaf water potential patterns together with drought adaptation mechanisms of carob tree are discussed.

Growth rate, plant development and water relations of the ABA‐deficient tomato mutant sitiens

Given the close relationship between a plant's growth rate and its pattern of biomass allocation and the effects of abscisic acid (ABA) on biomass allocation, we studied the influence of ABA on biomass allocation and growth rate of wildtype tomato (Lycopersicon esculentum Mill. cv. Moneymaker) plants and their strongly ABA‐deficient mutant sitiens. The relative growth rate of sitiens was 22% lower than that of the wildtype, as the result of a decreased specific leaf area. The net assimilation rate and the leaf weight ratio were not affected. The mutant showed a much higher transpiration rate and lower hydraulic conductance of the roots. These two factors resulted in sitiens having a significantly lower leaf water potential and turgor. resulting in reduced leaf expansion and, consequently, a lower specific leaf area relative to the wildtype. Addition of ABA to the sitiens roots resulted in phenotypic reversion to the wildtype. We conclude that the influence of ABA‐deficiency on biomass allocation and relative growth rate is the result of altered water relations in the plants, rather than of a direct effect on sink strength of different plant organs.

Effect of salinity on emergence and on water stress and early seedling growth of sunflower and maize

To study the effect of salinity on the emergence and early seedling growth of sunflower and maize, experiments were conducted in pots filled with sandy clay and sandy loam. The experiments consisted of a control and four saline water qualities with chloride concentrations of 15, 30, 45, and 60 meq/l. The emergence of sunflower and maize were affected by salinity. Soil texture did not affect the emergence. During early seedling growth, salinity and soil texture affected the development of the seedlings that showed symptoms of water stress in the form of lower leaf water potential, stomatal conductance, and evapotranspiration. The higher the salinity, the lower the leaf area and the dry matter production. Leaf, stem, and root showed an almost similar growth reduction due to salinity. Seedlings on sandy loam were more affected by water stress than those on sandy clay.

Ecophysiological analysis of woody species in contrasting temperate communities during wet and dry years.

This study employed an intensive sampling regime in which leaf gas exchange and tissue-water relations were measured simultaneously on the same leaf at midday on 19 tree species from three distinct forest communities during wet (1990) and dry (1991) growing seasons. The study sites were located on a xeric barrens, a misic valley floor, and a wet-mesic floodplain in central Pennsylvania, United States. The xeric, mesic, and wetmesic sties had drought-related decreases in gravimetric soil moisture of 53, 34 and 27%, respectively. During the wet year, xeric and mesic communities had high seasonal mean photosynthetic rates (A) and stomatal conductance of water vapor (g wv) and low midday leaf water potential (ψ), whereas the wet-mesic community had low A and g wv and high midday ψ. The mesic and wet-mesic communities had dry year decreases in predawn ψ, g wv and A with the greatest drought effect occurring in the mesic community. Regression analysis indicated that species from each site that exhibited high wet-year A and g wv tended to have low midday ψ. This trend was reversed only in the mesic community in the drought year. Despite differences in midday ψ, all three communities had similar midday leaf turgor pressure (ψp) in the wet year attributable to lower osmotic potential at zero turgor (ψ π0 ) with increasing site droughtiness. Lower wet year ψ π0 in the xeric community was due to low symplast volume rather than high solute content. Species with the lowest ψ π0 in the wet year often did not have the lowest ψ π100 possibly related to differences in tissue elasticity. Moreover, increased elasticity during drought may have masked osmotic adjustment in ψ π100 but not in ψ π0 , via dilution of solutes at full hydration in some species. Despite the sampling regime used, there were no relationships between gas exchange and osmotic and elastic parameters that were consistently significant among communities or years. This result questions the universal, direct effect of osmotic and elastic adjustments in the maintenance of photosynthesis during drought. By including a large number of species, this study provided new insight to the ecophysiology of contrasting forest communities, and the community-wide impact of drought on contrasting sites.

Plant Spacing, Density, and Yield of Wheat Subjected to Postanthesis Water Deficits

The optimum planting density for Gutha wheat (Triticum aestivum L. emend. Thell.) grown in mediterranean‐type environments varies with site and season. This study investigated the influence of postanthesis water deficits on the water relations and yield of wheat sown at different plant densities with even and uneven plant densities along the row. Unevenness of machine planting resulted in a yield reduction of 20% compared with evenly spaced plants in plants subjected to postanthesis water deficits, but no yield reduction was observed in irrigated wheat. Wheat at high densities (100 and 200 plants m‐2) developed lower leaf and ear water potentials than plants at low densities (25 and 50 plants m‐2) during postanthesis water deficits. The greater deficits, however, did not reduce yields. Grain yields were generally lower at densities of 25 and 50 plants per square meter than at 200 plants per square meter when postanthesis water deficits developed, but not when irrigated. The later‐formed tillers (Tillers 2, 3 and 4), which were only produced at low densities, had smaller thousand‐grain weights compared to those of the mainstem and the first tiller when water deficits were induced during the postanthesis period. The delay in anthesis in the later‐formed tillers was up to 20 d and this resulted in grain fill occurring at low leaf and ear water potentials. We conclude that reducing the density of palnts to reduce water use may decrease rather than increase wheat yield under water limited conditions.

Effect of snow removal on leaf water potential, soil moisture, leaf and soil nutrient status and leaf peroxidase activity of sugar maple

Effect of removal of snow cover in winter was investigated in an 80-year-old sugar maple (Acer saccharum Marsh.) stand in southern Quebec. We hypothesized that winter soil frost would induce some of the decline symptoms observed in sugar maple stands in southern Quebec in the early 1980's. Snow was continuously removed from around trees for a one week (partial removal) or for a four-month period (complete removal) during the 1990–1991 winter. Foliage and soils were sampled periodically during the summer of 1991. The complete snow removal treated trees showed decreased leaf water potential and increased peroxidase activity over most of the growing season. Foliar Ca was reduced in both snow removal treatments early in the growing season while foliar N was reduced in the complete snow removal trees late in the growing season. Soil NO 3 − and K+ were elevated in both snow removal treatments at various times throughout the growing season. Prolonged soil frost in a sugar maple stand can induce lower leaf water potential, higher leaf peroxidase activity and early leaf senescence during the following growing season. Soil frost may have reduced nutrient uptake without affecting significantly the leaf nutrient status.

Physiological Mechanisms of Drought Resistance in Four Native Ornamental Perennials

Understanding physiological drought resistance mechanisms in ornamentals may help growers and landscapers minimize plant water stress after wholesale production. We characterized the drought resistance of four potted, native, ornamental perennials: purple coneflower [Echinacea purpurea (L.) Moench], orange coneflower [Rudbeckia fulgida var. Sullivantii (Beadle & Boynt.) Cronq.], beebalm (Monarda didyma L.), and swamp sunflower (Helianthus angustifolius L.). We measured a) stomatal conductance of leaves of drying plants, b) lethal water potential and relative water content, and c) leaf osmotic adjustment during the lethal drying period. Maintenance of stomatal opening as leaves dry, low lethal water status values, and ability to osmotically adjust indicate relative drought tolerance, with the reverse indicating drought avoidance. Echinacea purpurea had low leaf water potential (ψL) and relative water content (RWC) at stomatal closure and low lethal ψL and RWC, results indicating high dehydration tolerance, relative to the other three species. Rudbeckia fulgida var. Sullivantii had a similar low ψL at stomatal closure and low lethal ψL and displayed relatively large osmotic adjustment. Monarda didyma had the highest ψL and RWC at stomatal closure and an intermediate lethal ψL, yet displayed a relatively large osmotic adjustment. Helianthus angustifolius became desiccated more rapidly than the other species, despite having a high ψL at stomatal closure; it had a high lethal ψL and displayed very little osmotic adjustment, results indicating relatively low dehydration tolerance. Despite differences in stomatal sensitivity, dehydration tolerance, and osmotic adjustment, all four perennials fall predominantly in the drought-avoidance category, relative to the dehydration tolerance previously reported for a wide range of plant species.

Water status and development of tropical trees during seasonal drought

Bud break, shoot growth and flowering of trees involve cell expansion, known to be inhibited by moderate water deficits. In apparent contradiction to physiological theory, many trees flower or exchange leaves during the 6 month-long, severe dry season in the tropical dry forest of Guanacaste, Costa Rica. To explore this paradox, changes in tree water status during the dry season were monitored in numerous trees. Water potential of stem tissues (Ψstem) was obtained by a modification of the pressure chamber technique, in which xylem tension was released by cutting defoliated branch samples at both ends. During the early dry season twigs bearing old, senescent leaves generally had a low leaf water potential (Ψleaf), while Ψstem varied with water availability. At dry sites, Ψstem was very low in hardwood trees ( −0.2 MPa) in lightwood trees storing water with osmotic potentials between −0.8 and −2.1 MPa. At moist sites trees bearing old leaves rehydrated during drought; their Ψstem increased from low values (<−3 MPa) to near saturation, resulting in differences of 3–4 MPa between Ψstem and Ψleaf. Indirect evidence indicates that rehydration resulted from osmotic adjustment of stem tissues and improved water availability due to extension of roots into moist subsoil layers. In confirmation of physiological theory, elimination of xylem tension by leaf shedding and establishment of a high solute content and high Ψstem were prerequisites for flowering and bud break during drought.

Mechanisms of Different Responses of Leaf Photosynthesis in African Rice(Oryza glaberrima Steud.) and Rice(Oryza sativa L.) to Low Leaf Water Potential.

This research was conducted to elucidate the mechanisms in water stress tolerance of Oryza glaberrima Steud. with special reference to stomatal conductance. Leaf photosynthesis (LPS) of O. glaberrima Steud. with developing soil water deficiency, was still maintained above zero when the leaf water potential decreased to as low as -2.0 MPa, while in O. sativa L. LPS became zero at -1.6 MPa. Stomatal conductances also changed in parallel with LPS in both species, suggesting that the different response of LPS to reduced leaf water potential is due to the different stomatal response between two species. In the relationship between the stomatal conductance and abscisic acid (ABA) content in the leaves, Saka, a line of O. glaberrima Steud., showed a slower decrease of stomatal conductance with increase of ABA than Nipponbare, a cultivar of O. sativa L. This attenuated response of stomatal conductance to leaf ABA content in O. glaberrima Steud. was supposed to be the cause of maintenance of higher LPS than in O. sativa L. under low leaf water potential conditions. The Rubisco content showed no significant difference in the response to developing low leaf water potential between the two species.

Wild and cultivated barley genotypes demonstrate varying ability to acclimate to plant water deficits

Wild barley ( Hordeum spontaneum Koech.) crosses easily with cultivated barley ( Hordeum vulgare L.) and so can be used as novel germplasm in breeding programs. In order to examine the potential usefulness of wild barley as a source of germplasm with characteristics beneficial to crop plants exposed to plant water deficits, wild barley accessions were collected from a range of habitats in Israel. Photosynthesis capacity at low leaf water potential (Ψ w), and cellular-level stress acclimation capability were evaluated in these lines. Genotypes identified as having the capacity to maintain higher photosynthesis at low leaf Ψ w (i.e.‘high acclimation lines’), as a group, demonstrated several related cellular-level physiological acclimation mechanisms when exposed to water deficits. These genotypes underwent relatively greater osmotic adjustment under stress. Osmotic adjustment in these lines was associated with the capacity to shift the protoplast volume/leaf Ψ w relationship upon exposure to stress such that greater cell volume was maintained at low Ψ w. Wild barley accessions which showed low photosynthesis under stress did not demonstrate this acclimation mechanism. Some of the wild barley accessions demonstrated greater osmotic adjustment in response to plan water deficits than the two varieties of cultivated barley which were also evaluated in this study. It is concluded that wild barley may be a good source of germplasm which shows resistance to water stress, specifically regarding the capacity to acclimate to plant water deficits such that photosynthesis is not as sensitive to low leaf Ψ w.

Gas Exchange and Water Relations of Field-Grown Concord (Vitis labruscanaBailey) Grapevines

Gas exchange and water relations of irrigated and non-irrigated vines and the effect of the duration of leaf exposure to direct sunlight on gas exchange parameters of irrigated vines were studied on field-grown Concord (<i>Vitis labrusca</i>) grapevines. Leaf water potential of the non-irrigated vines decreased from -1.3 MPa (900 h) to -1.6 MPa (1600 h), while that of the irrigated vines decreased from -1.0 MPa to -1.6 MPa during the same time period. Differences in xylem water potential between the two treatments changed from -0.45 MPa (900 h) to -0.37 MPa (1600h). Xylem water potential was found to be more indicative of vine water status than that of leaf water potential. Shoot hydraulic resistance of irrigated vines remained unchanged in spite of low afternoon leaf water potential. Assimilation rate and stomatal conductance of non-irrigated vines were significantly reduced as leaf water potential decreased, where only a slight reduction was measured in the irrigated vines. These results may indicate the existence of a root signal controlling stomatal aperture in Concord. Lower afternoon assimilation rates and stomatal conductances were found in continuously exposed leaves compared to leaves exposed to direct sunlight only in the afternoon. However, the relationship between assimilation rate and stomatal conductance remained the same regardless of the timing or duration of exposure of the leaves.

Leaf Water Relations and Gas Exchange in Two Grain Sorghum Genotypes Differing in Their Pre- and Post-Flowering Drought Tolerance

Summary Two grain sorghum [ Sorghum bicolor (L.) Moench.] lines (TX7078 and B35) of similar maturity but known to exhibit differences in sensitivity to drought in pre- and post-flowering growth stages were evaluated in order to quantify the physiological bases for differences in drought tolerance between lines and groWth stages. The relationships among leaf water status, osmotic adjustment, leaf gas-exchange characteristics, and epicuticular wax load were characterized for irrigated and non-irrigated plants during preand post-flowering growth stages. Osmotic adjustment during pre-flowering stress did not differ between lines, and average adjustment was approximately 0.42 MPa. In contrast, osmotic adjustment occurred only in TX7078 during post-flowering stress, and the extent of adjustment relative to irrigated plants was only 0.16 MPa. Neither osmotic adjustment nor epicuticular wax load appeared to be related to the relative drought tolerance of these genotypes during pre- or post-flowering growth stages. A reversal in the ranking of water-use efficiency (WUE) of TX7078 and B35 occurred between pre- and post-flowering growth stages. Non-irrigated plants of TX7078 exhibited significantly higher WUE than B35 at the pre-flowering stage, but the opposite was true in the post-flowering stage. The reason for the reversal in WUE is not known, but there was no significant relation between osmotic adjustment and WUE. TX7078 and B35 showed a contrasting response for intercellular CO 2 concentration (C i ) in both pre- and post-flowering experiments; C i was strongly reduced by water deficit in TX7078, but it was not significantly affected in B35, suggesting that these genotypes may differ fundamentally in their photosynthetic response to low leaf water potential.

Metabolic changes in barley seedlings at different aphid infestation levels

Different infestation levels with the greenbug ( Schizaphis graminum Rondani) were correlated with physiological parameters and several metabolite concentrations in young barley plants ( Hordeum vulgare cv Aramir). Leaf water potential, chlorophyll content, carbon dioxide assimilation and total soluble carbohydrates decreased significantly with more than 18 aphids per plant. Conversely, proline concentration increased with more than 18 aphids. With 100 individuals per plant, sugar concentrations decreased five times with respect to the non-infested control. Similar changes were observed in plants subjected to drought stress. These results suggest that several metabolic changes that occur in barley infested by a large number of aphids may be due in part to the lower leaf water potential caused by the insect.

Influence of Solar Radiation and Vapour Pressure Deficit on Transpiration Efficiency of Rainfed Sorghum

AbstractBesides assimilation, plant water relations are important aspects of physiological basis of productivity of crops in water limited environment. The relationships of photosynthesis rate, transpiration rate, leaf water potential and stomatal conductance with photosynthetically active radiation (PAR) and vapour pressure deficit (VPD) during pre‐flowering (panicle initiation to ear emergence) and grain filling (from anthesis to maturity) stages of a sorghum hybrid (cv. CSH‐6) grown under rainfed conditions were studied. Photosynthesis rate declined when PAR was above 1300 μmol m−2 s−1. during both the growth stages. Higher transpiration rate during grain filling stage at higher PAR caused the transpiration efficiency to be lower than during pre‐flowering stage when PAR was above 1200 μmol m−2s−1.Leaf water potential and stomatal conductance decreased with increase in PAR. Leaf water potential was higher during pre‐flowering than during grain filling stage but maximum photosynthesis rate was similar during both the growth stages. Changes in VPD did not qualitatively alter the relationships of the physiological variables with PAR.Decreasing photosynthesis rate and LWP at high PAR suggest that photosynthesis rate was limited by low leaf water potential when PAR was optimal, and by low PAR even when leaf water potential was high in rainfed sorghum during rainy season.

Irrigation Cut-off Date Affects Growth, Frost Damage, and Yield of Jojoba

Flower bud injury resulting from freezing temperatures has been a major problem in jojoba [Simmondsia chinensis (Link) C. Schneid.] production. A 3-year field study, which began with 4-year-old plants, evaluated the effect of three irrigation treatments on growth, flower bud survival, seed yield, seed weight, and seed wax concentration of six clones. After 3 years, irrigation cut-off dates of late May (dry treatment) and early September (medium treatment) resulted in reduced plant height and width compared to irrigating through November (wet treatment). Flower bud survival and seed yields were very low in the first year for all treatments. In the second and third years, bud survival for most clones, even at -8C, was greatly improved by withholding water in the fall. In December of the second and third years, plants in the medium and dry plots had lower leaf water potential than those in the wet plot. In the second year, plants in the medium and dry plots had seed yields that were 3.5 times higher and wax yields that were were 2.3 times higher than plants in the wet plot. In the third year, the medium treatment had the highest seed and wax yields. Average seed weight and seed wax concentration were generally highest for plants in the wet plot where seed yields were low. Withholding irrigation from jojoba in the fall appears to improve flower bud survival and seed and wax yields following cold winters.

Use of Transgenic Plants with Ribulose-1,5-Bisphosphate Carboxylase/Oxygenase Antisense DNA to Evaluate the Rate Limitation of Photosynthesis under Water Stress.

The biochemical lesion that causes impaired chloroplast metabolism (and, hence, photosynthetic capacity) in plants exposed to water deficits is still a subject of controversy. In this study we used tobacco (Nicotiana tabacum L.) transformed with "antisense" ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) DNA sequences to evaluate whether Rubisco or some other enzymic step in the photosynthetic carbon reduction cycle pathway rate limits photosynthesis at low leaf water potential ([psi]w). These transformants, along with the wild-type material, provided a novel model system allowing for an evaluation of photosynthetic response to water stress in near-isogenic plants with widely varying levels of functional Rubisco. It was determined that impaired chloroplast metabolism (rather than decreased leaf conductance to CO2) was the major cause of photosynthetic inhibition as leaf [psi]w declined. Significantly, the extent of photosynthetic inhibition at low [psi]w was identical in wild-type and transformed plants. Decreasing Rubisco activity by 68% did not sensitize photosynthetic capacity to water stress. It was hypothesized that, if water stress effects on Rubisco caused photosynthetic inhibition under stress, an increase in the steady-state level of the substrate for this enzyme, ribulose 1,5-bisphosphate (RuBP), would be associated with stress-induced photosynthetic inhibition. Steady-state levels of RuBP were reduced as leaf [psi]w declined, even in transformed plants with low levels of Rubisco. Based on the similarity in photosynthetic response to water stress in wild-type and transformed plants, the reduction in RuBP as stress developed, and studies that demonstrated that ATP supply did not rate limit photosynthesis under stress, we concluded that stress effects on an enzymic step involved in RuBP regeneration caused impaired chloroplast metabolism and photosynthetic inhibition in plants exposed to water deficits.