Effects of Water Stress on the Indoleacetic Acid Oxidase Activity in Wheat Leaves

Abstract

The indoleacetic acid oxidase system is thought to influence plant growth by regulating the concentration of endogenous IAA. Plant tissues capable of rapid growth have been found to exhibit low total IAA oxidase activity, whereas in slower grow-ing tissues higher levels of activity were observed (4). However, there may be a high content of specific types of IAA oxidase in some rapidly growing cells (6). Direct evidence for an in-verse relationship between IAA oxidase activity and endoge-nous IAA content has been reported (5, 10). Recent studies have shown that the level of IAA oxidase activity may be affected by environmental stress in the form of low temperatures (1) or water stress (2, 3). The latter experi-ments indicated that IAA oxidase activity in etiolated pea seedlings and tomato leaves increased following water stress, suggesting that reduced growth in droughted plants may be due in part to accelerated auxin degradation. Accumulation of abscisic acid during drought would also inhibit growth (8). The data presented here show that indoleacetic acid oxidase in light-grown wheat leaves does not increase with water stress but, in fact, declines markedly. MATERIALS AND METHODS Two cultivars of hard red winter wheat (Triticumn aestivumt L.) that differ in drought hardiness (15) were used in this study: "KanKing" (CI 12719) and "Ponca" (CI 12128). Seeds were planted in 15-cm clay pots with a soil, sand, peat moss mixture (2:1: 2), and grown in a controlled environment chamber. Temperatures in the chamber were 24 C day and 16 C night, with a 16-hr photoperiod (Gro-Lux fluorescent lamps giving an intensity of 20,000 lux). All plants were watered daily for 7 days after planting, then water was withheld from experimen-tal plants for the duration of the experiment, while control plants received water daily. Beginning with the 4th day after the last watering, samples of water-stressed and nonstressed leaf tissue were taken for enzyme analysis at daily intervals until the relative water content of the stressed leaves had dropped to 30 to 50%, as determined by the method of Todd etal. (14). Enzyme Extraction. Enzyme preparations were carried out at 2 C. Each sample, consisting of the leaves of five plants. was ground in a mortar along with 3 ml of cold 0.05 M phosphate