Assessing transpiration estimates in tall fescue: The relationship among transpiration, growth, and vapor pressure deficits

Limitations in water availability for irrigation due to drought and water-use regulations necessitate accurate approaches to estimate water use. An energy balance approach is commonly used that is inherently empirical and requires an ill-defined coefficient. An alternative is to use a relationship based on vapor pressure deficit (VPD) and plant growth to predict plant transpiration rate. This study was undertaken to evaluate these approaches for tall fescue (Festuca arundinacea Schreb.). Experiments examined differences in water loss of tall fescue plants when grown in three temperatures with varying vapor pressure deficit (VPD), and with treatments of low nutrition and of growth regulator trinexapac-ethyl, which depressed growth. Within a temperature, the low-nutrition and growth-regulator treatments greatly affected clipping mass, however water loss remained similar. In hydroponic experiments, treatments altering clipping mass did not necessarily change total plant growth. Hence, a challenge to using whole-plant growth for estimating transpiration of this grass is to accurately determine growth only from clipping data. Transpiration was positively correlated with VPD, especially within each temperature, but there were indications that the higher temperature treatments caused decreased plant control over transpiration. The instability of physiological control over transpiration highlights the potential limitations of both equations in estimating transpiration rates.