GENETIC-VARIATION AND CHANGES WITH ONTOGENY OF OSMOTIC ADJUSTMENT IN SUNFLOWER (HELIANTHUS-ANNUUS L)

Intraspecific variation for osmotic adjustment in sunflower was examined using a collection of 33 genotypes of different origin which were exposed to water stress at the 8-leaf stage. Changes in osmotic adjustment with ontogeny were also evaluated in the pre- and post-anthesis phases using seven genotypes drawn from this collection. Estimates of osmotic adjustment were derived from measurements of leaf relative water content (RWC) and osmotic potential (pi) during a period in which the soil was allowed to dry out gradually. The degree of osmotic adjustment, expressed as the value of RWC for a pi of - 1.7 MPa (RWCe), was derived from the In RWC/ln pi relationship. Both monophasic and biphasic In RWC/ln pi relationships were found. Irrespective of the form of the relationship, all genotypes at the 8-leaf stage showed some degree of osmotic adjustment. This was also true for the cultivars included in the subset examined in pre- and post-anthesis phases. Significant differences (P = 0.05) in RWCe were found between extreme genotypes in all three phases. Significant (P = 0.05) linear relationships were found between RWCe measured in the 8-leaf stage and that measured in the pre- and post-anthesis phases, establishing the viability of measurements in the 8-leaf stage as a means of selection for osmotic adjustment in later developmental stages. Genotype rank order was stable (P = 0.01) across the three ontogenetic phases examined.