The Response of The Young Tomato Plant to a Brief Period of Water Shortage. IV. Effects of Water Stress on the Ribonucleic Acid Metabolism of Tomato Leaves.

The growth response was determined of young tomato plants subjected to wilting treatments of short duration. The experiments were conducted in pots in the glasshouse using Jondaryan loam, and the wilting treatments were at a "moderate" and a "severe" level. Even with the severe treatment, soil water did not fall below the permanent wilting percentage. Both wilting treatments reduced growth during the period of wilting,· but growth rates upon re-watering were greater than for the control plants. There was no indication that this recovery effect was complete at the final harvest. During wilting, a higher stem weight ratio and a lower lamina weight ratio were developed than in the control, and r~lative growth rates and net assimilation rates were depressed in both treatments. After wilting, lamina weight ratios became higher than control, stem weight ratios became lower, and net assimilation rates and relative growth rates rose above control values. These treatment effects may be interpreted as a tendency towards senescence during wilting and a return to a more juvenile condition upon re-watering. It was concluded that the changes in weight ratios were due to modifications of the normal pattern of translocation between plant parts. It was also concluded that the changes occurring in response to the water shortage were initiated relatively early in the drying cycle. The effects of water shortage on the water economy of the plant were also considered. It was found that, whilst there was a high effiCiency of transpiration during wilting, the drastic effects of the water shortage on growth made the greater efficiency of doubtful merit. After wilting, there was no economy in water usage by plants of either of the wilt treatments.