Optimizing Dosages of Seaweed Extract-based Cytokinins and Zeatin Riboside for Improving Creeping Bentgrass Heat Tolerance.

Heat stress is the primary factor limiting summer performance of creeping bentgrass (Agrostis stolonifera L.) in many temperate to subtropical regions.  Seaweed extract (SWE)-based cytokinins have been used to improve heat stress tolerance, but effective dosage ranges of exogenous natural cytokinins from SWE and synthetic cytokinins for improving heat stress tolerance are not well documented.  This study was designed to determine optimum dosages of SWE-based cytokinins and synthetic cytokinin (trans-zeatin riboside; ZR) for improving creeping bentgrass heat tolerance.  ‘L-93’ creeping bentgrass was established in pots filled with calcined clay, and treated with SWE-based cytokinins at various concentrations (0.1, 1, 10, 100, and 200 µM) and ZR (0.1, 1, 10, 100, 1000 µM) biweekly at 38 mL m^-2 for 4 applications.  After the second treatment, the grass was transferred to a growth chamber and subjected to heat stress (35/25^oC day/night, 450 µmol m^-2 s^-1 light intensity, 10-h photoperiod) for 7 weeks.  A randomized block design was used with 11 treatments and 8 replications.  Applying SWE-based cytokinins at 10, 100 and 200 µM and ZR at 10, 100 µM significantly improved visual quality, photochemical efficiency, and endogenous ZR content, when compared to the control.  These treatments also tended to improve leaf nitrate reductase activity, protein content, and root growth.  Applying ZR at a higher dosage (1000 µM) caused phytotoxcity.  The data suggest optimum dosages of SWE-based cytokinin and ZR range from 10 to 100 µM and exogenous cytokinin may increase endogenous cytokinins, improving nitrogen metabolism of creeping bentgrass subjected to heat stress.