Abscisic Acid and Water Relations of Rice (Oryza sativa L.): Sequential Responses to Water Stress in the Leaf

Publisher Website
Google Scholar
Abstract
Water stress was imposed by withholding water at an early vegetative stage from plants of two rice cultivars (IR20 and 63–83) grown in pots. As stress intensified the following sequence of responses of the leaves was observed: (i) rise in abscisic acid (ABA) content, (ii) closure of stomata, (iii) initiation of leaf rolling. In both cultivars, turgor (ψp) declined linearly with total water potential (ψ) of the leaf. Bulk leaf ABA content increased linearly as ψp declined, and attained twice the control (unstressed) level following a reduction in ψp of about 0.12 MPa. Stomatal conductance exhibited a sigmoidal relationship to ψp, declining abruptly when a particular ‘critical’ ψp was reached (threshold response). The critical potentials varied considerably between experiments, but were closely correlated with control potentials and with the potentials at which ABA concentration doubled relative to controls. Leaf rolling was initiated at ψs near to zero ψp. Increases in the ratio of adaxial to abaxial conductance were associated with rolling. Variations in the above responses could be accounted for by variations in the rate of stress development, which in terms of ψ reduction ranged from 0.38 to 0.86 MPa day−1. Fast drying rates resulted in: (a) reduced osmotic adjustment, (b) increased amounts of ABA in the leaf at a given level of ψ or ψp, (c) an increase in the ABA concentration present at 50 per cent stomatal closure, and (d) initiation of leaf rolling at a higher ψ.