Relationship between Water use Efficiency and Cuticular Wax Deposition in Warm Season Forage Crops Grown under Water Deficit Conditions

In order to clarify the relationship between water use efficiency (apparent photosynthetic rate/transpiration rate) and the synthesis and deposition of epicuticular wax and of lipids in leaf internal tissues in various forage crops with different drought tolerance, the contribution of cuticular wax to the leaf cuticular resistance to water loss under water stress was estimated. Under water stress conditions, the cuticular resistance of the drought tolerant plant Chloris gayana Kunth. increased about twofold compared with that of a well-watered plant, whereas the cuticular resistance of the sensitive plant Coix lacryma-jobi L. increased by about 10%. The amount of epicuticular wax deposited on the leaf surface in both plants increased by water stress. However, the amount in the tolerant plant was much higher than those in the sensitive one. Radioactivity of 14C incorporated into the leaf surface wax of stressed C. gayana and C. lacryma-jobi plants was 27 and 50% less than that of well-watered plants, respectively, when they were treated with 14C-malonate and 14C-acetate. Leaves of water-stressed C. gayana assimilated 14C from 14C-malonate and 14C-acetate into the lipids of their internal tissues 5.7 times more than the leaves of well-watered plants, while the leaves of water-stressed C. lacryma-jobi assimilated 78% of the amount recorded in well-watered plants. On the other hand, the cuticular resistance of bloom lines of Sorghum bicolor L. with a high accumulation of wax on the epicuticular surface increased by about 90% by water stress, whereas the cuticular resistance of a bloomless line with a low amount of wax increased by about only 15%. Water use efficiency of bloom lines of sorghum increased remarkably by water stress, whereas that of the bloomless line of sorghum decreased. These data suggest that in the drought-tolerant plants the water retention ability of the leaves increased due to the deposition of a larger amount of wax on their leaf surfaces, and a higher accumulation of lipids in internal tissues. Also these plants produced larger amounts of dry matter by increasing the water use efficiency through the reduction of cuticular evaporation under water stress conditions.