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 upper part of the root system of half of the plants was exposed to drought (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), while in protruding roots of stressed plants it slowly and continuously decreased from 31.9 to 25.2 g g-1 DW. Leaf water content rapidly decreased in treated plants from 7.4 to 6.4 g g-1 DW in the first 2 d and then 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 treated 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 3 d. In treated protruding roots a maximum of 0.99 +/- 0.09 nmol g-1 DW was reached after 1 d. ABA concentration in the xylem sap increased 2 d and 3 d after the start of soil drying, with a maximum of 113 +/- 12 nmol l-1 during the third day. The ABA rise in the leaves of treated plants was less significant. Indol-3yl-acetic acid (IAA) concentration in internal roots of treated plants reached a maximum of 22.54 +/- 3.34 nmol g-1 DW on the third day, then decreased dramatically. The protruding root system of control plants showed a maximum value of 16.05 +/- 1.77 nmol g-1 DW on the sixth day. Little difference in cytokinin content of xylem sap was noted between control and treated plants. Hormonal variations in different parts of the plant are discussed in relation to drought stress.