Physiological changes associated with performance of Kentucky bluegrass cultivars during summer stress

Understanding the factors influencing the performance of Kentucky bluegrass (Pea pratensis L.) cultivars under summer stress is necessary for developing criteria for identifying resistant germplasm, The objectives of this study were to evaluate two Kentucky bluegrass cultivars for leaf water (psi(1)) and osmotic potential (psi(pi)), stomatal resistance (R(s)), leaf:air temperature differential (Delta T) and determine the relationship of these parameters to drought and heat tolerance, Stress-resistant ('Midnight') and susceptible ('Nugget') cultivars were evaluated in a field study during 1993 and 1994 under moisture-limiting conditions. Leaf water potential for 'Nugget' was higher than for 'Midnight' in 1993 and similar in 1994. 'Midnight' had lower psi(pi) than 'Nugget' during the evaluation period in 1994. 'Midnight' maintained more open stomata (lower R(s)) and lower Delta T than 'Nugget' at the end of the dry down period when 'Nugget' was showing visual signs of stress. 'Midnight' and 'Nugget' had similar root weight at the 0- to 45-cm depth zone in 1994. Lower basal osmotic potential (i.e., higher solute concentration) may be the physiological mechanism allowing larger stomatal aperture in 'Midnight'. Greater transpirational cooling in 'Midnight' relative to 'Nugget' was correlated with higher turf quality for 'Midnight'.