RESPONSES TO WATER-STRESS OF APOPLASTIC WATER FRACTION AND BULK MODULUS OF ELASTICITY IN SUNFLOWER (HELIANTHUS-ANNUUS L) GENOTYPES OF CONTRASTING CAPACITY FOR OSMOTIC ADJUSTMENT

The responses to water stress of the bulk modulus of elasticity (epsil on) and the apoplastic water fraction were examined using six sunflowe r cultivars of differing capacity for osmotic adjustment (OA). Water s tress did not affect the partitioning of water between apoplastic (ca. 20%) and symplastic fractions in leaves which expanded during the exp osure to stress in any genotype. Hence, no genotype-linked effects on either the buffering of cell water status during stress or on the esti mates of bulk leaf osmotic potential could be expected. Genotypes diff ered in the degree of change in epsilon (estimated from pressure/volum e [PN] curves) and OA (estimated using both In RWC/ In Psi o plots and PN curves) induced by exposure to stress. In three genotypes epsilon increased significantly (p = 0.05) as a consequence of stress, in anot her three change were small. OA was the only attribute of the three ex amined that could have contributed to turgor maintenance under stress. There was a strong negative association between leaf expansion and de gree of OA across genotypes (r = -0.91) and a strong positive one betw een OA and E (r = 0.94). However all genotypes evidenced some degree o f OA. These results are consistent with part of the genotype differenc es in OA being attributable to variations in leaf expansion during exp osure to stress.