Nitrate and Cytokinin Interactions in Creeping Bentgrass under Supra-optimal Temperatures.

Heat stress is a major problem affecting creeping bentgrass (Agrostis stolonifera L.) putting green performance in temperate to sub-tropical climates. Exogenous application of cytokinins has been reported to alleviate heat stress in many species, but little information about how cytokinins affect creeping bentgrass grown at variant nitrogen levels under heat is available. The overall objective of this research is to characterize the function and action mechanisms of cytokinins in creeping bentgrass grown at different nitrogen (N) levels under heat. 'L-93' creeping bentgrass, in a 35 C (day)/25 C (night) growth chamber, was treated with two nitrate-nitrogen rates (2.5 and 7.5 kg N ha^-1) and three rates of trans-zeatin riboside (tZR, 0, 10 and 100 μM) every two weeks in a complete factorial arrangement. At d 1, both total leaf nitrate reductase (NR) and active leaf NR showed a diurnal change pattern with lowest activity at 15:00 (light period: 6:00 to 20:00), but there was no difference among treatments. The percentage of active NR was lowest (79.3%) at 21:00. Grass treated with 100 μM tZR had the highest total NR at d1, but grass with 10 μM tZR had highest percent of active NR regardless of N level. At days 15 and 28, N had positive effects on photochemical efficiency, chlorophyll, and percent of active NR, but the cytokinin treatments had no effect. Better quality was observed at d 28 at the higher N level. Based on these data, cytokinin had effects on NR quickly (at d 1 after treatment), with N playing a more important role as stress progressed. These studies will provide insight for developing better N plus cytokinin programs to enhance bentgrass performance under heat.