Apoplastic and symplastic solute concentrations contribute to osmotic adjustment in bean genotypes during drought stress
The present study investigates changes in the inorganic ions, proline, and endogenous abscisic acid (ABA) contents of the apoplastic and symplastic compartments of leaves from drought-tolerant (Yakutiye) and drought-sensitive (Zulbiye) cultivars of the common bean (Phaseolus vulgaris L.). Drought stress caused a decrease in leaf water potential and stomatal conductance in both cultivars. Concentrations of proline in the drought-tolerant and drought-sensitive cultivars increased in response to drought stress in both compartments. The symplastic K+ concentration decreased in both cultivars. However, the opposite trend was observed concerning K+ concentrations in the apoplastic areas. While the symplastic Na+ concentrations significantly decreased in the drought-tolerant cultivar, the apoplastic Na+ concentrations increased during drought stress. However, Na+ concentrations did not significantly change in either of the compartments in the drought-sensitive cultivar. The Ca2+ concentrations in the sensitive cultivar significantly decreased in both compartments during drought stress. In the tolerant cultivar, the Ca2+ concentration significantly increased in the symplast but decreased in the apoplast. Cl- concentrations in the tolerant cultivar did not significantly change in either compartment. In the sensitive cultivar, the Cl- concentration increased in the apoplastic area but decreased in the symplastic area. In addition, while the symplastic sap of the leaves exhibited a constant pH value, it diminished in the apoplast during drought stress. Symplastic and apoplastic ABA concentrations significantly increased in both cultivars. It might be said that inorganic ions (especially Na+, K+, and Ca2+) and ABA concentrations changed between the apoplastic and symplastic spaces to contribute to osmotic adjustment under drought stress. In addition, the drought-tolerant cultivar showed a much higher capacity to maintain osmotic adjustment between the symplast and the apoplast.
Key words: Apoplast, symplast, drought stress, tolerance, inorganic ions, abscisic acid