Growth responses and performance of Kentucky bluegrass under summer stress

Cultivars and selections of Kentucky bluegrass (Poa pratensis L,) exhibit varying degrees of summer stress tolerance. Understanding the factors associated with performance under summer stress is important for identifying stress tolerant germplasm. The objective of this field study was to evaluate shoot and root growth responses of 10 Kentucky bluegrass genotypes subjected to high temperature and drought stress conditions in New Jersey. Turf canopy characteristics, shoot and root growth responses, and soil water depletion patterns were evaluated on a Nixon Loam (fine-loamy, mixed mesic Typic Hapludult) and used to characterize Kentucky bluegrass selections having variable summer stress tolerance. Tolerant entries maintained 19% more roots at the 15- to 30-cm depth in 1995 and 65% more roots at the 30- to 45-cm depth than intolerant entries. Gravimetric soil water contents of tolerant plots were significantly lower at the 15- to 30-cm depth than intolerant entries for both years. Tolerant Kentucky bluegrass, able to exploit deep soil moisture under heat and drought conditions, exhibited significantly fewer summer stress symptoms, had significantly lower stomatal resistance (based on subset of cultivars), and maintained canopy temperature 5 degrees C cooler than intolerant entries at the end of stress periods. This indicated that maintenance of transpirational cooling was an important factor associated with better summer stress performance of turf plots under high temperature and drought conditions in New Jersey.