Differential Responses of Hybrid Bluegrass and Kentucky Bluegrass to Drought and Heat Stress

This study was designed to investigate differential responses of hybrids from Texas bluegrass (Poa arachnifera Torr.) x Kentucky bluegrass (Poa pratensis L.) (KBG) and KBG genotypes to drought and heat stress. Plants of two hybrids, '845' and 'BDF', and two KBG genotypes ('Midnight' and 'C-74') were grown under optimal temperature (22/18 degrees C) and well-watered (control) or unwatered (drought) or superoptimal temperatures (35/30 degrees C; heat stress) conditions for 35 days in growth chambers. Under optimal conditions, the two hybrids and two KBG genotypes were not significantly different in turf quality, leaf photochemical efficiency expressed as chlorophyll fluorescence ratio (Fv/Fm), leaf net photosynthetic rate (Pn), transpiration rate, water use efficiency (WUE), root dry matter, or root viability. The results suggest that the interspecific hybridization resulted in similar growth and physiological traits in the hybrid bluegrass as in a turf-type species under optimal temperature and irrigation regimes. Under drought stress, all these parameters were comparable to those for KBG 'Midnight', but significantly higher than the corresponding parameters for KBG 'C-74'. Under heat stress, both hybrids had significantly higher turf quality, Fv/Fm, Pn, transpiration rate, WUE, root dry weight in deeper soil depth (40 to 60 cm), and root viability in the upper 40-cm layer compared with both KBG genotypes. These results suggested that hybrid bluegrass exhibited improvement in drought and heat tolerance, particularly in comparison with KBG 'C-74', but to a great extent for heat tolerance. The maintenance of higher transpiration and photosynthesis, WUE, and root viability was associated with the improvement in heat tolerance in hybrid bluegrass.