Antecedent soil water content and vapor pressure deficit interactively control water potential in Larrea tridentata

Plant water potential Ψ is regulated by stomatal responses to atmospheric moisture demand D and soil water availability W, but the timescales of influence and interactions between these drivers of plant Ψ are poorly understood. Here, we quantify the effects of antecedent D and W on plant Ψ in the desert shrub Larrea tridentata. Repeated measurements of plant baseline water potential Ψ_B and diurnal water potential Ψ_D were analyzed in a Bayesian framework to evaluate the influence of antecedent D and W at daily and subdaily timescales. Both Ψ_B and Ψ_D exhibited negative, 2- to 4-d lagged responses to daily-scale D; conversely, plant Ψ_D responded almost instantaneously to subdaily D, though the direction of this response depended on antecedent moisture conditions. Plant Ψ_B and Ψ_D responded positively and immediately (no lag) to shallow W, which contrasts the negative, lagged (6-7 d) response to deep W. The changing sensitivity of Ψ_D to subdaily D highlights shifting modes of plant Ψ regulation: D effects on Ψ_D range from negative to neutral to positive depending on past conditions and time of day. Explicit consideration of antecedent conditions across multiple timescales can reveal important complexities in plant responses.