Effects of water supply and vapor pressure deficit on leaf stomatal conductance and photosynthetic water use efficiency in alternative aridland crop species

From a plant physiology perspective, low soil water availability and high vapor pressure deficits (VPDs) are two major challenges in aridland agriculture. Agriculture in Nevada where most of the precipitation occurs as snowfall in the winter depends upon irrigation during the summer growing season. However, withdrawals of irrigation water from the Walker River have been one of the primary factors leading to the severely decreased size of the terminal Walker Lake and impaired water quality in the lake. High VPDs in desert environments could potentially reduce crop yields by closing stomata which could decrease stomatal conductance, diffusion of CO2 into the leaf for photosynthesis, and photosynthetic water use efficiency. A range of alternative crop species that might require less irrigation water and be able to tolerate high VPDs were studied to determine their suitability for aridland agriculture. Chapter 1 describes how soil properties affect low soil water availability and how weather affects VPD in arid environments. It also describes the effects of low soil water availability and high VPD on plant physiology such as stomatal closure, decreased stomatal conductance, and decreased plant growth.;Chapter 2 presents the article containing the results of my research. When evaluating the potential for growing alternative crop species in arid environments, high VPDs that could potentially inhibit crop productivity by limiting stomatal conductance (gs) and CO2 uptake must be considered. The primary objective of this study was to quantify the effects of VPD and irrigation levels on leaf stomatal conductance and photosynthetic water use efficiency (PWUE) for a range of alternative crop species for aridland agriculture. We also sought to assess whether PWUE characteristics of test species could be used to predict biomass yield. We evaluated fourteen species in a field trial in the Walker River Basin of northern Nevada, U.S.A. Plots (9 x 7 m) of each species were subjected to two irrigation treatments, 4 or 2 acre-feet per growing season, to simulate normal-year and dry-year irrigation levels. We quantified gs and photosynthesis (A) under decreasing relative humidity (RH) (increasing VPDs) during measurement of leaf gas exchange throughout the 2010 growing season. We calculated PWUE as A divided by transpiration. Vertical profiles of canopy relative humidity, temperature, VPD, and leaf area index (LAI) were quantified on two days in 2011 to assess the effects of crop canopy LAIs on VPD.