HORMONAL RESPONSES TO PARTIAL DRYING OF THE ROOT-SYSTEM OF HELIANTHUS-ANNUUS

Plants of Helianthus annuus were pot-grown in soil, with approximately 30% of the root system protruding through the base. After 7 d, the up per part of the root system of half of the plants was exposed to droug ht (internal roots) while the lower part was kept in aerated nutrient solution (protruding root). The treatment rapidly reduced the internal roots' water content from 26.1 to 21.9 g g(-1) dry weight (DW), white in protruding roots of stressed plants it slowly and continuously dec reased from 31.9 to 25.29 g(-1) DW. Leaf water content rapidly decreas ed in treated plants from 7.4 to 6.4 g g(-1) DW in the first 2 d and t hen reached a plateau. In stressed plants leaf stomatal resistance was significantly higher in the first 3 d while leaf water potential was lower only on the last day. Abscisic acid (ABA) concentration in treat ed plants increased significantly compared to the controls. In treated internal roots, ABA rose from the first day, reaching a maximum of 1. 48+/-0.49 nmol g (-1) DW after 36. In treated protruding roots a maxim um of 0.99+/-0.09 nmol g(-1) DW was reached after 1 d. ABA concentrati on in the xylem sap increased 2 d and 3 d after the start of soil dryi ng, with a maximum of 113+/-12 nmol I-1 during the third day. The ABA rise in the leaves of treated plants was less significant. Indol-3yl-a cetic acid (IAA) concentration in internal roots of treated plants rea ched a maximum of 22.54+/-3.34 nmol g(-1) DW on the third day, then de creased dramatically. The protruding root system of control plants sho wed a maximum value of 16.05+/-1.77 nmol g(-1) DW on the sixth day. Li ttle difference in cytokinin content of xylem sap was noted between co ntrol and treated plants. Hormonal variations in different parts of th e plant are discussed in relation to drought stress.