Wise R R, Ortiz-Lopez A, Ort D R
Department of Plant Biology, University of Illinois, Urbana, Illinois 61801.
Plant Physiol. 1992 Sep;100(1):26-32. doi: 10.1104/pp.100.1.26.
Inhomogeneous photosynthetic activity has been reported to occur in drought-stressed leaves. In addition, it has been suggested that these water stress-induced nonuniformities in photosynthesis are caused by "patchy" stomatal closure and that the phenomenon may have created the illusion of a nonstomatal component to the inhibition of photosynthesis. Because these earlier studies were performed with nonacclimated growth chamber-grown plants, we sought to determine whether such "patches" existed in drought-treated, field-grown plants or in chamber-grown plants that had been acclimated to low leaf water potentials (psi(leaf)). Cotton (Gossypium hirsutum L.) was grown in the field and subjected to drought by withholding irrigation and rain from 24 d after planting. The distribution of photosynthesis, which may reflect the stomatal aperture distribution in a heterobaric species such as cotton, was assayed by autoradiography after briefly exposing attached leaves of field-grown plants to (14)CO(2). A homogeneous distribution of radioactive photosynthate was evident even at the lowest psi(leaf) of -1.34 MPa. "Patchiness" could, however, be induced by uprooting the plant and allowing the shoot to air dry for 6 to 8 min. In parallel studies, growth chamber-grown plants were acclimated to drought by withholding irrigation for three 5-d drought cycles interspersed with irrigation. This drought acclimation lowered the psi(leaf) value at which control rates of photosynthesis could be sustained by approximately 0.7 MPa and was accompanied by a similar decline in the psi(leaf) at which patchiness first appeared. Photosynthetic inhomogeneities in chamber-grown plants that were visible during moderate water stress and ambient levels of CO(2) could be largely removed with elevated CO(2) levels (3000 muL L(-1)), suggesting that they were stomatal in nature. However, advanced dehydration (less than approximately 2.0 MPa) resulted in "patches" that could not be so removed and were probably caused by nonstomatal factors. The demonstration that patches do not exist in drought-treated, field-grown cotton and that the presence of patches in chamber-grown plants can be altered by treatments that cause an acclimation of photosynthesis leads us to conclude that spatial heterogeneities in photosynthesis probably do not occur frequently under natural drought conditions.
据报道，干旱胁迫的叶片中会出现不均匀的光合活性。此外，有人提出，这些水分胁迫诱导的光合作用不均匀性是由“斑块状”气孔关闭引起的，并且这种现象可能造成了光合作用抑制存在非气孔成分的假象。由于这些早期研究是用未适应环境的生长室种植的植物进行的，我们试图确定在干旱处理的田间种植植物或已适应低叶水势（ψ(leaf)）的生长室种植植物中是否存在这种“斑块”。棉花（陆地棉）在田间种植，从种植后24天起停止灌溉和降雨，使其遭受干旱。在将田间种植植物的附着叶片短暂暴露于(14)CO(2)后，通过放射自显影测定光合作用的分布，这可能反映了棉花等异压物种中的气孔孔径分布。即使在最低ψ(leaf)为-1.34 MPa时，放射性光合产物的分布也是均匀的。然而，通过拔起植株并让地上部分风干6至8分钟，可以诱导出“斑块状”。在平行研究中，生长室种植的植物通过进行三个5天的干旱周期（其间穿插灌溉）来停止灌溉，从而适应干旱。这种干旱适应使光合作用的对照速率能够维持的ψ(leaf)值降低了约0.7 MPa，并且在斑块首次出现时的ψ(leaf)也有类似程度的下降。在中度水分胁迫和环境CO(2)水平下，生长室种植植物中可见的光合不均匀性在CO(2)水平升高（3000 μL L(-1)）时可基本消除，这表明它们本质上是气孔性的。然而，严重脱水（低于约2.0 MPa）会导致无法通过这种方式消除的“斑块”，这可能是由非气孔因素引起的。干旱处理的田间种植棉花中不存在斑块，以及生长室种植植物中斑块的存在可通过导致光合作用适应的处理而改变，这一事实使我们得出结论，在自然干旱条件下，光合作用的空间异质性可能并不经常发生。
