Botany Department, Washington State University, Pullman, Washington 99164-4238.
Plant Physiol. 1992 Aug;99(4):1426-34. doi: 10.1104/pp.99.4.1426.
Castor bean (Ricinus communis L.) has a high photosynthetic capacity under high humidity and a pronounced sensitivity of photosynthesis to high water vapor pressure deficit (VPD). The sensitivity of photosynthesis to varying VPD was analyzed by measuring CO(2) assimilation, stomatal conductance (g(s)), quantum yield of photosystem II (phi(II)), and nonphotochemical quenching of chlorophyll fluorescence (q(N)) under different VPD. Under both medium (1000) and high (1800 micromoles quanta per square meter per second) light intensities, CO(2) assimilation decreased as the VPD between the leaf and the air around the leaf increased. The g(s) initially dropped rapidly with increasing VPD and then showed a slower decrease above a VPD of 10 to 20 millibars. Over a temperature range from 20 to 40 degrees C, CO(2) assimilation and g(s) were inhibited by high VPD (20 millibars). However, the rate of transpiration increased with increasing temperature at either low or high VPD due to an increase in g(s). The relative inhibition of photosynthesis under photorespiring (atmospheric levels of CO(2) and O(2)) versus nonphotorespiring (700 microbars CO(2) and 2% O(2)) conditions was greater under high VPD (30 millibars) than under low VPD (3 millibars). Also, with increasing light intensity the relative inhibition of photosynthesis by O(2) increased under high VPD, but decreased under low VPD. The effect of high VPD on photosynthesis under various conditions could not be totally accounted for by the decrease in the intercellular CO(2) in the leaf (C(i)) where C(i) was estimated from gas exchange measurements. However, estimates of C(i) from measurements of phi(II) and q(N) suggest that the decrease in photosynthesis and increase in photorespiration under high VPD can be totally accounted for by stomatal closure and a decrease in C(i). The results also suggest that nonuniform closure of stomata may occur in well-watered plants under high VPD, causing overestimates in the calculation of C(i) from gas exchange measurements. Under low VPD, 30 degrees C, high light, and saturating CO(2), castor bean (C(3) tropical shrub) has a rate of photosynthesis (61 micromoles CO(2) per square meter per second) that is about 50% higher than that of tobacco (C(3)) or maize (C(4)) under the same conditions. The chlorophyll content, total soluble protein, and ribulose-1,5-bisphosphate carboxylase/oxygenase level on a leaf area basis were much higher in castor bean than in maize or tobacco, which accounts for its high rates of photosynthesis under low VPD.
蓖麻（Ricinus communis L.）在高湿度下具有很高的光合作用能力，并且对高水汽压亏缺（VPD）的光合作用敏感性非常显著。通过在不同 VPD 下测量 CO2 同化、气孔导度（gs）、光合作用系统 II 的量子产率（phi（II））和叶绿素荧光非光化学猝灭（q（N））来分析光合作用对不同 VPD 的敏感性。在中等（1000 毫摩尔量子/平方米/秒）和高光强（1800 毫摩尔量子/平方米/秒）下，随着叶片与叶片周围空气之间的 VPD 增加，CO2 同化减少。gs 最初随 VPD 的增加而迅速下降，然后在 VPD 超过 10 至 20 毫巴时下降速度较慢。在 20 至 40 摄氏度的温度范围内，高 VPD（20 毫巴）抑制 CO2 同化和 gs。然而，由于 gs 的增加，在低或高光 VPD 下，蒸腾速率随着温度的升高而增加。在光呼吸（大气水平的 CO2 和 O2）与非光呼吸（700 微巴 CO2 和 2% O2）条件下，高 VPD（30 毫巴）下光合作用的相对抑制比低 VPD（3 毫巴）下更大。此外，随着光强的增加，高 VPD 下 O2 对光合作用的相对抑制增加，而低 VPD 下则减少。高 VPD 对各种条件下光合作用的影响不能完全用叶片内 CO2（C(i)）的减少来解释，其中 C(i)是根据气体交换测量值估计的。然而，phi(II)和 q（N）测量值对 C(i)的估计表明，高 VPD 下光合作用的降低和光呼吸的增加可以完全归因于气孔关闭和 C(i)的降低。结果还表明，在高 VPD 下，水分充足的植物中可能会发生不均匀的气孔关闭，从而导致从气体交换测量值计算 C(i)时出现高估。在低 VPD、30 摄氏度、高光强和饱和 CO2 下，蓖麻（C3 热带灌木）的光合作用速率（61 微摩尔 CO2/平方米/秒）比相同条件下的烟草（C3）或玉米（C4）高约 50%。与玉米或烟草相比，蓖麻叶片的叶绿素含量、总可溶性蛋白和核酮糖-1,5-二磷酸羧化酶/加氧酶水平要高得多，这解释了其在低 VPD 下高光合作用率的原因。
