Protein Changes during Heat Stress in Three Kentucky Bluegrass Cultivars Differing in Heat Tolerance

Supraoptimal temperature limits growth and metabolic activities of cool-season turfgrasses. Understanding genetic variations and mechanisms in turfgrass heat tolerance would facilitate breeding and management programs to improve turf quality under summer stress. The objective of this study was to investigate protein changes associated with heat tolerance in three Kentucky bluegrass (Poa pratensis L.) cultivars. Plants of 'Brilliant', 'Midnight', and 'Eagleton' were subjected to 20°C (day/night, control) or 40°C (day/night, heat stress) in growth chambers. Eagleton maintained higher chlorophyll content and fewer yellow leaves than Brilliant, while Midnight was intermediate for both parameters at 28 d of heat stress. The content of cytoplasmic and membrane proteins declined during heat stress, to a greater extent in Brilliant than in Midnight and Eagleton. The content of both types of proteins in Brilliant were significantly lower than in Eagleton, but not different from that in Midnight. The sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) analysis revealed that Brilliant exhibited more severe protein degradation than other two cultivars. Heat stress induced expression of several heat shock proteins (HSPs) in the cytoplasm (64, 78, and 85 kDa) and membranes (39, 45, and 66 kDa) in all three cultivars, but the induction occurred 7 to 14 d earlier in Eagleton or Midnight than in Brilliant. An additional membrane protein of 68 kDa was induced in Midnight under heat stress. The results suggested that better heat tolerance in the Kentucky bluegrass was associated with induction of HSPs during the early phase of heat stress and maintenance of higher protein content and less severe protein degradation during prolonged periods of heat stress.