Effects of Cytokinin and Potassium on Stomatal and Photosynthetic Recovery of Kentucky Bluegrass from Drought Stress

Drought-induced stomatal closure may limit resumption of photosynthesis and plant growth if stomata do not quickly reopened when plants are later rehydrated. Cytokinins (CK) and K are known to regulate stomatal opening and closure. The objective of this study was to investigate whether foliar application of CK and K would enhance stomatal reopening and photosynthetic recovery in leaves of perennial grass after water deficit is alleviated. Kentucky bluegrass (Poa pratensis L. ‘Brilliant’) plants were subjected to drought stress by withholding irrigation for 15 d and then rewatered for 6 d in growth chambers. A synthetic CK (6-benzylaminopurine [6-BA]) and KCl with the concentration of 10 μM and 50 mM, respectively, were foliar sprayed on drought-stressed plants. The experiment consisted of well-watered control, drought stress followed by rewatering, drought stress followed by rewatering and foliar spray of 6-BA (10 μM), and drought stress followed by rewatering and foliar spray of KCl (50 mM). Treatments were arranged as a completely randomized block design with four replicates for each treatment. Soil volumetric water content (SWC), leaf relative water content (RWC), net photosynthetic rate (Pₙ), stomatal conductance (gₛ), transpiration rate (Tr), and stomatal aperture decreased with the progression of drought stress. The RWC, Pₙ, gₛ, Tr, and stomatal aperture increased after rewatering, to a greater extent in 6-BA- or KCl-treated plants than plants sprayed with water only. A positive correlation between stomatal aperture and Pₙ (r² = 0.79) and between stomatal aperture and gₛ (r² = 0.87) were detected under drought stress and during rewatering. Our results indicate that exogenous application of 6-BA and KCl promoted stomatal reopening following drought-induced closure, leading to enhanced gₛ and rapid postdrought resumption of photosynthesis on rewatering.