Significance of Accelerated Leaf Senescence at Low Water Potentials for Water Loss and Grain Yield in Maize1

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Abstract
Productivity is lost when plants are subjected to dry soil conditions and this loss is often accompanied by the senescence of leaf tissue. The role of leaf senescence in water conservation and grain production was therefore evaluated in maize (Zea mays L.) subjected to low water potentials for most of the grain filling period. The plants were grown in soil in controlled environment chambers and water was withheld from the soil after pollination had occurred. Low leaf water potentials were maintained for 30 days (until the grain matured) by daily addition of small amounts of water to the soil. The low water potentials (−1.8 to −2.0 MPa) inhibited dry matter accumulation and transpiration by the plants. However, in addition to the initial inhibition of transpiration by stomatal closure, there was a further inhibition (about 11% of the control rate) associated with accelerated senescence of the leaves. Excision of senesced leaf tissue showed that, despite this inhibition of water loss attributable to senescence, the dead tissue itself continued to lose a small amount of water while attached to the plants. Although the low water potentials were of long enough duration to allow significant acclimation of the plants, the senescing tissue contributed only a small amount of dry matter to the grain by mobilizing leaf constituents. Since the amount of water conserved by the senescing tissue was also small, it is likely that accelerated leaf senescence contributes little to grain production at low water potentials and has the disadvantage that it sacrifices potential photosynthetic surface. Therefore, selection against accelerated senescence of leaves at low water potentials may be desirable for crops.