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Explanation:

A.

The transpiration stream that passes through a plant may follow an apoplastic route, with low resistance to flow, or a cell-to-cell route, in which cellular membranes impede water flow. However, passage of water through membranes can be facilitated by aquaporins thereby decreasing resistance. We investigated the relationship between transpiration, which can be down-regulated by abscisic acid (ABA) or by high humidity, and the osmotic water permeability (Pos) of protoplasts. By using leaf protoplasts of wild-type (wt) Arabidopsis thaliana plants and of mutants that are low in ABA (aba1) or insensitive to ABA (abi1 and abi2), we found that protoplasts from aba1 and abi mutants have very low Pos values compared with those from wt plants when the plants are grown at 45% relative humidity. High values of Pos were found 3 h after the addition of ABA to the culture medium of aba1 plants; addition of ABA to abi plants did not restore the Pos to wt levels. There was no such increase in Pos when excised leaves of aba1 plants were treated with ABA. When the transpiration stream was attenuated by growing the plants at 85% relative humidity, the Pos of protoplasts from all plants (wt and mutants) was higher. We suggest that attenuation of the transpiration stream in whole plants is required for the up-regulation of the Pos of the membranes, and that this up-regulation, which does not require ABA, is mediated by the activation of aquaporins in the plasma membrane.

B.

The influence of abscisic acid (ABA) on carbon metabolism, rate of photorespiration, and the activity of the photorespiratory enzymes ribulose bisphosphate oxygenase and glycolate oxidase in 7-day-old barley seedlings (Hordeum vulgare L. var. Alfa) was investigated. Plants treated with ABA had enhanced incorporation of labeled carbon from 14CO2 into glycolic acid, glycine, and serine, while 14C incorporation into 3-phosphoglyceric acid and sugarphosphate esters was depressed. Parallel with this effect, treated plants showed a rise in activity of RuBP oxygenase and glycolic acid oxidase. The rate of photorespiration was increased twofold by ABA treatment at molar while the CO2-compensation point increased 46% and stomatal resistance increased more than twofold over control plants.

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