Water use by turfgrass is an issue of increasing concern because nonsaline water resources are becoming more expensive and scarce. A key question is how dry can soil become before it limits physiological activity and growth of grasses? Although it is understood that water deficits in the plant result in loss of cell turgor and wilting, stomata closure, decreased photosynthetic rates, cessation of cell enlargement, and interruption of many metabolic processes (Kramer and Boyer, 1995), these results offer little concrete guidance about overall plant response to soil water content.
An increasing number of studies, primarily with field crops, have shown that there is a close correlation between plant activity and soil water content. Sadras and Milroy (1996) reviewed this literature and found that plant gas exchange, including photosynthesis rate, did not decrease for most situations until the fraction of extractable soil water content had decreased to a range of 0.25 to 0.40. Variation in the soil water content at which the decline in gas exchange occurred was attributed to soil characteristics and species differences. Although Sadras and Milroy identified no studies with turfgrasses, Miller (2000) has since reported a study with ‘Tifdwarf’ bermudagrass (Cynodon dactylon L. Pers. × C. transvaalensis Burtt Davy) grown on either U.S. Golf Association specification sand or fine sand. The results showed the same two-segment response observed in other plant species. The average fraction of transpirable soil water (FTSW) for two soils and two experiments at which bermudagrass water loss rate began to decrease was 0.30.
This current study was undertaken to extend studies on relative rate of water loss of turfgrasses in response to drying soils to six cultivars of seashore paspalum (Paspalum vaginatum Swartz). Seashore paspalum is a highly salt-tolerant, warm-season species (Duncan, 1998) that is adapted to tropical and subtropical regions of the world (Trenholm and Unruh, 2002). Interest and use of seashore paspalum on golf courses has increased in the last 10 years as a result of the availability of several new cultivars with high turf quality. Tolerance to saline environments has become the definitive trait for this species; however, it is also known to grow well in a wide variety of soil textures and pH levels and to have a comparatively lower nitrogen requirement (Duncan, 1998; Trenholm and Unruh, 2002).
Possible genotypic differences in response to drying soil could be critical in developing turf with greater tolerance to drying soil. Huang et al. (1997) ranked the drought resistance of four seashore paspalum genotypes in comparison with three other warm-season species. They reported that variation existed among the tested seashore paspalum genotypes and that breeding for related characteristics could further improve drought resistance. However, breeding would be facilitated by identifying the specific physiological trait that contributes to variation in drought tolerance. A high volumetric soil water content at which transpiration rate begins to decrease would be a possible mechanism to conserve soil water and maintain turf quality for a longer time as drought conditions persist. Identification of possible genotypic differences is a critical first step in breeding for enhanced performance with soil drying.
To fully understand the influence of turfgrass response to soil drying, it may be necessary to compare the response of grasses to drying soil when grown on divergent soil types. Sand is used to construct green surfaces on golf courses, whereas other turf areas typically use native soils (sand, loam, or clay). In 2002, ≈73% of the turf grown in Florida was grown in a sand-based soil and 23% grown in muck soils (Haydu et al., 2003). Sinclair et al. (1998) found in their experiments that sandy soils gave a response different from usually observed with most other soils. The soil water content (FTSW) at which plant transpiration rate began to decline was quite low. In their study, the breakpoint for the decline in soybean gas exchange among five sandy soils ranged from 0.11 to 0.18 in FTSW. They concluded that the lower breakpoint for the sandy soils was the result of difficulty in defining the upper drained limit for sandy soils in their pot study. In pot studies, zero potential may exist at the bottom of the pot so that the amount of water held by sand in these pots was greatly exaggerated relative to field situations.
The objective of this study was to compare the relative water loss rates of six cultivars of seashore paspalum as soil was allowed to dry. The experiment was performed using two soils: an organic soil and sand, which are relevant to turf growth in Florida. The data were analyzed to determine the breakpoint at which water loss rate declined relative to well-watered controls. Radiation reflectance data were also recorded in this study with seashore paspalum cultivars as a possible means for monitoring changes in grass quality associated with soil drying.
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