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

Seaweed (Ascophyllum nodosum Jol.) extract-based cytokinins (SWEC) have been used to improve turfgrass stress tolerance, but optimum dosages for improving creeping bentgrass (Agrostis stolonifera L.) heat tolerance are lacking. This study was designed to examine effects of SWEC at various dosages on creeping bentgrass heat tolerance and to compare their effects to a trans-zeatin riboside (t-ZR)-standard in a growth chamber. The SWEC and t-ZR were applied to the foliage of 'L-93' creeping bentgrass four times (14-d interval) at 1, 10, and 100 mu M. After the second application, the grass was subjected to heat stress for 49 d at 35/25 degrees C (day/night) in the initial experiment or 38/28 degrees C (day/night) in the second experiment. The SWEC and t-ZR treatments at 10 and 100 mu M alleviated the decline of turfgrass quality and chlorophyll content, They also increased leaf t-ZR content and nitrate reductase (NR) activity, relative to the control, especially under 38/28 degrees C (day/night). These data indicate that improvements under heat stress may be associated with an increase in plant t-ZR levels and NR activity due to SWEC or t-ZR application. Results suggest that repeated foliar application of SWEC at 10 mu M may be an effective approach for improving turfgrass performance and heat stress tolerance.