Developmental Responses to Drought and Abscisic Acid in Sunflower Roots

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Abstract
Aeroponically grown sunflower seedlings (Helianthus annuus L. cv. Russian Giant) were droughted or treated with abscisic acid (ABA) for 7 d. Drought stress prompted a three-phase growth response in sunflower roots: an initial phase of increased root elongation was followed by a period of almost complete inhibition between about 6 h and 72 h; this was followed, in turn, by a phase of partial recovery in the rate of root elongation. Drought decreased the size of the apical meristem as cells in the proximal region of the meristem vacuolated and elongated. Root-to-shoot biomass ratios (R:S) increased initially but declined after 72 h. Drought stress decreased water potential (Φ) and osmotic potential (Φл and increased turgor pressure Φp in the apical 30 mm of the roots. These initial changes were transitory, lasting about 3 h. Thereafter, Φ and Φл began to rise; Φp fell back to control levels. In the later stages of treatment, Φ fell as the stress grew more severe, but fp was maintained by osmotic adjustment. Desiccation for 1 h increased turgor pressures in excised 30 mm apical segments. The transitory increase in root elongation was contemporary with the initial rise in Φp in the root apices, while the periods of greatest inhibition and partial recovery in root elongation were contemporary with the periods of decline and partial recovery in the length of the apical meristem respectively. The inhibition of root elongation and the anatomical changes in the root apices were not determined by loss of turgor or lack of photosynthate, but rather appeared to be an active response by the meristem to a drop in external Φ. Treatment with ABA triggered many of the same changes as drought stress: ABA promoted a three-phase growth response, increased R:S, triggered the same initial changes in Φ, Φл, and Φp, increased Φp in excised 3.0 mm apical segments, and induced the same pattern of anatomical changes in the root apices as drought stress. It is proposed that ABA mediates drought-induced changes in the primary development of sunflower roots.