Abscisic Acid Inhibits Leaf Expansion by Limiting Cell Expansion But Not Cell Division in Arabidopsis

Abscisic acid (ABA) accumulation during water stress inhibits leaf expansion to limit plant water loss. When this acclimation is induced by exogenous ABA, we have previously shown that it is followed by rapid leaf expansion, with leaf area eventually recovering to the control level. We therefore hypothesize that ABA inhibits cell expansion but not cell division, and the maintenance of cell division enables such recovery of leaf expansion after ABA degradation. To test this hypothesis, we treated Arabidopsis (Arabidopsis thaliana) plants with 0 or 1 mM ABA at the rosette stage with 7–8 leaves. During 6 days following the treatment, ABA inhibited expansion of 5th and 7th leaves by 10% and 53%, respectively, while it had no effect on older leaves. Regardless of leaf age, epidermal cell number per leaf was unaffected by ABA, suggesting that ABA inhibits leaf expansion solely by limiting cell expansion. In addition, ABA affected neither number of stomata per leaf nor guard cell length, which regulate the rate of gas exchange and transpiration. These results suggest that ABA-induced inhibition of leaf expansion is a mechanism to conserve water by limiting increases in non-stomatal evaporative area, as opposed to stomatal closure that reduces transpiration. This mechanism may not limit plant growth and photosynthetic capacity, as leaves maintain both cell division and stomatal formation.