Cytokinin effects on creeping bentgrass responses to heat stress: I. Shoot and root growth

Heat stress injury may involve inhibition of cytokinin biosynthesis in roots. The objective of this study was to examine whether application of a synthetic cytokinin, zeatin riboside (ZR), to the root zone would enhance tolerance of creeping bentgrass (Agrostis palustris L.) to high soil temperature or in combination with high air temperature. Grasses were exposed to three air and soil temperature regimes for 56 d in growth chambers: (i) optimum air and soil temperature (control), 20/20°C; (ii) optimum air but high soil temperature (20/35°C); and (iii) high air and soil temperatures (35/35°C). Four concentrations (0.01, 0.1, 1, and 10 μmol) of ZR or water (control) were injected into the 0-5 cm root zone on the day before heat stress (0 d) and 14 d after. Turf visual quality, canopy net photosynthetic rate (P_n), leaf photochemical efficiency (Fv/Fm), and vertical shoot extension rate decreased, whereas root mortality and root electrolyte leakage increased at 20/35 and 35/35°C, and to a greater extent at 35/35°C. Applications of 1 and 10 μmol ZR mitigated heat stress injury to shoots and roots during most of the experimental period, with 10 μmol ZR being more effective when applied at either 0 or 14 d of heat stress. Application of 0.1 μmol ZR was less effective than 1 and 10 μmol ZR. Application of 0.01 μmol ZR had no effects on shoot and root responses to high soil temperature alone or combined with high air temperature. Endogenous cytokinin content in both shoots and roots increased with the application of 1 and 10 μmol ZR. These results demonstrated that applying ZR at 1 or 10 μmol concentration to the root zone could alleviate heat stress injury of creeping bentgrass.