The Effects of Water-Stress on Leaf H2180 Enrichment
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Summary. Water-stress experiments with Phaseolus vulgar& L. were undertaken to determine the transpiration rate dependency of the naturally occurring leaf H2~80 fractionation process. Water-stress leaf H2~sO levels were observed to be unexpectedly higher than controls. Speculations on the cause of this phenomenon are discussed. Since transpiration rate variations should theoretically affect only the rate and not the extent of leaf Hz180 fractionation, the respective time courses for water-stressed and control leaf H2180 accumulations were compared. Water-stressed leaves displayed a slower rate of isotopic enrichment relative to controls, as was predicted from their reduced transpiration rates. In an absolute sense, however, both control and water-stress leaf H2180 fractionation rates were markedly greater than projected values from the existing model. Consequently, transpiration rates cannot be derived accurately at present from the observed rates of leaf Hzl80 discrimination. Several modifications of the theory are also considered.Keywords:
Water Stress
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Water Stress
δ18O
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Stomatal Conductance
Water potential
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The relationship between water stress and rates of net photosynthesis, respiration, and transpiration was determined for four Abies species native to Canada. Net photosynthesis, after an initial optimum rate, declined in three phases as water stress increased, viz. a rapid linear rate of decline, a second more gradual reduction and, finally, a steady rate of zero net photosynthesis. Of the four species, photosynthesis of A. grandis was affected the least by water stress, whereas that of A. balsamea was affected the most. Respiration declined at about the same water stress as photosynthesis, but was only reduced to between 40–75% of the maximum rate, depending on the species. Transpiration declined at similar water stresses to those of photosynthesis and after an initial decline, continued at between 10–30% of the maximum rate at water stresses up to 35 bars.
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It was clarified in the previous papers that leaf water potential, stomatal aperture and photosynthetic rate in rice plants decreased with increase in solar radiation and vapor pressure deficit even though rice plants grew under the condition where sufficient water was supplied to roots in submerged paddy field, and that the decrease in leaf water potential, stomatal aperture and photosynthetic rate were more remarkable in rice plants with low root activity or with poor root system. The present study was conducted to investigate the relationship between water uptake and transpiration rates and the effects of this relation on leaf water potential and stomatal aperture through their diurnal changes, and to discuss the characteristics for maintaining water balance in rice plants. Transpiration rate was higher than water uptake rate in the morning when transpiration was increasing rapidly with rapid increase in solar radiation and vapor pressure deficit. Both rates were practically the same in the midday and then transpiration rate was lower than water uptake rate in the evening when transpiration rate was decreasing rapidly with rapid decrease in solar radiation and vapor pressure deficit (Fig. 2 A). The difference between water uptake and transpiration rates was very small even when transpiration rate was changing rapidly (Fig. 2 B). In case of reduced water uptake due to low water potential of culture solution or NaN3 treatment to roots, transpiration rate decreased remarkably due to increase of stomatal closure in the daytime (Figs. 3 and 4). Therefore, the difference between water uptake and transpiration rates did not increase so much and leaf water potential decreased a very little even in the midday with high transpiration demand compared with decrease of water uptake rate and stomatal aperture (Figs. 3 and 4). These results suggested that water balance in rice plants was maintained by the process as follows: There is too much transpiration in the daytime with high solar radiation and valor pressure deficit, so water uptake could not overtake the transpiration, and leaf water potential decreased to a certain extent. As stomata in rice plants were very sensitive to change of leaf water potential compared with those of other plants, stomata closed very rapidly with response to the decrease of water potential, so that transpiration rate decreased to almost the same as water uptake. Therefore, the difference between water uptake and transpiration rates was very small, and decrease of leaf water potential was prevented. Futhermore, in case of rice plants with reduced water uptake due to low water potential of culture solution or NaN3 treatment to roots, all were the same as in the process of maintaining water balance. From these results and the high correlation between stomatal aperture and photosynthetic rate, it was considered that water uptake ability directly affected photosynthetic rate under sufficient solar radiation in rice plants with stomata responding very sensitively to change of leaf water potential. Futhermore, it was suggested that rapid wilting often observed in rice, soybean and cucumber under very large vapor pressure deficit or on water saturated soil in rainy season, baiu, could arise from both decrease of water uptake ablility and loss of sensitivity of stomata to decrease of leaf water potential.
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Photosynthetic responses of intact leaves of the desert shrub Encelia farinosa were measured during a long term drought cycle in order to understand the responses of stomatal and nonstomatal components to water stress. Photosynthetic rate at high irradiance and leaf conductance to water vapor both decreased linearly with declining leaf water potential. The intercellular CO(2) concentration (c(i)) remained fairly constant as a function of leaf water potential in plants subjected to a slow drought cycle of 25 days, but decreased in plants exposed to a 12-day drought cycle. With increasing water stress, the slope of the dependence of photosynthesis on c(i) (carboxylation efficiency) decreased, the maximum photosynthetic rates at high c(i) became saturated at lower values, and water use efficiency increased. Both the carboxylation efficiency and photosynthetic rates were positively correlated with leaf nitrogen content. Associated with lower leaf conductances, the calculated stomatal limitation to photosynthesis increased with water stress. However, because of simultaneous changes in the dependence of photosynthesis on c(i) with water stress, increased leaf conductance alone in water-stressed leaves would not result in an increase in photosynthetic rates to prestressed levels. Both active osmotic adjustment and changes in specific leaf mass occurred during the drought cycle. In response to increased water stress, leaf specific mass increased. However, the increases in specific leaf mass were associated with the production of a reflective pubescence and there were no changes in specific mass of the photosynthetic tissues. The significance of these responses for carbon gain and water loss under arid conditions are discussed.
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In Pistacia vera L., the average diurnal patterns of net CO2 assimilation, stomatal conductance and transpiration rates were assessed at the leaf surface during a clear summer day, with reference to the diurnal fluctuations of leaf temperature and photosynthetic photon flux. The rate of leaf gas exchange was highest at 9:00. The subsequent decline in net carbon assimilation did not appear to be related to changes in stomatal conductance (gs). The latter seemed to be primarily limited by vapour pressure deficit (VPD) of 3.8 kPa and leaf temperature (Tl) of 36 degrees C. Further, stomatal conductance declined at 13:00, at a time when VPD slightly increased to 4.2 kPa and Tl reached 38 degrees C. The partial gs recovery, two hours later, did not seem to be related to changes in the ambient conditions. The midday transpiration rate did not show any significant decline, hence leaf temperature did not change markedly. The photosynthetic efficiency declined at leaf temperatures higher than 31 degrees C.
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Cotton plants, Gossypium hirsutum L. were grown in a growth room under incident radiation levels of 65, 35, and 17 Langleys per hour to determine the effects of vapor pressure deficits (VPD's) of 2, 9, and 17 mm Hg at high soil water potential, and the effects of decreasing soil water potential and reirrigation on transpiration, leaf temperature, stomatal activity, photosynthesis, and respiration at a VPD of 9 mm Hg.Transpiration was positively correlated with radiation level, air VPD and soil water potential. Reirrigation following stress led to slow recovery, which may be related to root damage occurring during stress. Leaf water potential decreased with, but not as fast as, soil water potential.Leaf temperature was usually positively correlated with light intensity and negatively correlated with transpiration, air VPD, and soil water. At high soil water, leaf temperatures ranged from a fraction of 1 to a few degrees above ambient, except at medium and low light and a VPD of 19 mm Hg when they were slightly below ambient, probably because of increased transpirational cooling. During low soil water leaf temperatures as high as 3.4 degrees above ambient were recorded. Reirrigation reduced leaf temperature before appreciably increasing transpiration. The upper leaf surface tended to be warmer than the lower at the beginning of the day and when soil water was adequate; otherwise there was little difference or the lower surface was warmer. This pattern seemed to reflect transpiration cooling and leaf position effects.Although stomata were more numerous in the lower than the upper epidermis, most of the time a greater percentage of the upper were open. With sufficient soil water present, stomata opened with light and closed with darkness. Fewer stomata opened under low than high light intensity and under even moderate, as compared with high soil water. It required several days following reirrigation for stomata to regain original activity levels.Apparent photosynthesis of cotton leaves occasionally oscillated with variable amplitude and frequency. When soil water was adequate, photosynthesis was nearly proportional to light intensity, with some indication of higher rates at higher VPD's. As soil water decreased, photosynthesis first increased and then markedly decreased. Following reirrigation, photosynthesis rapidly recovered.Respiration was slowed moderately by decreasing soil water but increased before watering. Respiration slowed with increasing leaf age only on leaves that were previously under high light intensity.
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Water scarcity is often the most common limiting factor to sugarcane production. With the on-going climate change, the occurrence and duration of drought periods is expected to increase in the future. Water stress affects many processes linked to growth and development, among which is photosynthesis. If such effects were to be incorporated into models, it would lead to more accurate cane and sugar yield prediction. In this context, a trial was established under a rainshelter facility at Mount Edgecombe with well-watered and water-stressed sugarcane variety NCo376. The rate of photosynthesis (PN), light interception (LI), plant extension rate (PER), leaf temperature (ΦT) and leaf water potential (ΨL) were measured, together with the soil water potential. Mild water stress affected PER to a relatively greater extent than LI and PN. Difference in PN between the well-watered and water-stressed sugarcane were observed when the ΨL of the latter reached –0.7 MPa. At this stage the PN which was at 20.0 µmol/m 2 /s started to decline, reaching the lowest level of 2.2 µmol/m 2 /s at a ΨL of –1.6 MPa. A strong correlation (R 2 =0.97) was obtained when regressing PN with ΨL so that for every 0.1 MPa decrease in ΨL there was a linear reduction in PN of 1.6 µmol/m 2 /s. The ΦT of the stressed cane was higher due to the poorer cooling effect from higher stomatal resistance and accompanying reduction in transpiration rate. Regressing the values of PN against that of ΦT in the range of 25 to 40oC showed that PN in the stressed crop decreased linearly by 0.4 µmol/m 2 /s for every one-degree rise in temperature. The diurnal pattern of PN in the well-watered crop followed closely the daily trend of incoming solar radiation, whereas in the stressed cane (–1.6 Mpa ΨL) PN was reduced to almost zero after midday. Hence, the photosynthetic efficiency of the stressed cane at –1.6 MPa was 0.22% compared with 1.09% for the unstressed crop. The lower biomass accumulation in the water stressed cane was attributed to reduced LI, PER and PN.
Cane
Water Stress
Water Use Efficiency
Interception
Water use
Environmental factor
Water potential
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