Effects of Water Stress on the Content of IAA and the Activities of IAA Oxidase and Peroxidase in Cotton Leaves

Cotton (Gossypium hirsutum var Deltapine 15 cv Yuauman No.46) was stressed at the beginning of blossoming and wilted for a day. Samples of leaves on main stem at various levels were collected with non-stressed leaves as controls. In addition, more non-stressed mature leaves were excised and stressed in -1.7 MPa PEG and dry air for 2,6 h and 12 h. Sample extracts were analyzed for IAA by GLC. The activity of IAA was assayed by the straight growth of wheat coleoptile. After water stress of the whole plants, water potential of leaves decreased about -1.87 MPa compared with that of their controls (Fig. 1). Water content of detached mature leaves dropped about 11% during the first 6h. Rewatering the 12h stressed leaves could partially recover their water content (Fig. 2). Water stress could decreases the total content of IAA in intact cotton leaves (Fig. 3). The changes of total content of IAA depended on the age of leaf, being more pronounced in younger leaves. Fig. 3 showed that the active transformation of conjugated IAA into free IAA occurred in young leaves and indicated that conjugated IAA may play an important role in homeostasis and adaptation of young leaves to water stress. In detached mature leaves, water stress quickly reduced IAA level which showed a linear correlation with the water content of the leaves (r=0.905) (Fig. 4,5). In intact and detached leaves, water-stress increased the activity of peroxidase (Fig.6,8). Water-stressed old leaf lacked one band in the isozymogram of peroxidase (Fig. 7). During water stress, the activity of IAA oxidase kept constant (Fig. 8), so the declining of IAA content could be related to the increasing activity of peroxidase instead of that of IAA oxidase. All samples examined showed the presence of active IAA.