Influence of Temperature On Tall Fescue Response to Atmospheric Vapor Pressure Deficit.

The effects of temperature and atmospheric vapor pressure deficit (VPD) on water use and growth of tall fescue were investigated, first in a group of eight controlled-environment greenhouses and then in growth chambers in the NCSU Phytotron.  Previous studies have suggested that VPD is the main controlling factor for plant water use regardless of temperature, but our results indicate that there may be a significant interaction between the two variables. In the greenhouses, four environmental conditions were imposed: high and low temperature at a controlled high and low VPD.  In humid conditions (the low VPD), plants exposed to cool and warm temperatures used virtually the same amount of water, supporting the hypothesis that VPD rather than temperature is the main controller; however, in dry conditions (high VPD) this did not hold true. Plants in cool/dry conditions used less water than those in cool/humid conditions, indicating a strong stomatal sensitivity which would result in water conservation during periods of atmospheric dryness.  In warm conditions, however, the stomatal sensitivity to high atmospheric VPD was not as apparent, with plants in dry greenhouses using more water than those in humid ones.  Growth chamber studies utilizing a range of VPD at high and low temperatures indicate that adjustments in stomatal response to temperature are not instantaneous, but require several days to occur. Results thus far demonstrate that high temperature may impair the ability of tall fescue to conserve water during times of atmospheric dryness.