Irrigation frequency transiently alters whole plant gas exchange, water and hormone status, but irrigation volume determines cumulative growth in two herbaceous crops

Physiological effects of irrigation frequency, at the same irrigation volume, have received little attention but might determine crop yield and water use efficiency. Potted plants of two species, tomato and basil, received two irrigation treatments that both supplied the same irrigation volume (75% of that received by a well-watered treatment—WW), but either frequently (once or twice per day-FDI) or infrequently (every three days-IDI). Stem diameter variations, whole-plant gas exchange, root and leaf water potential, and foliar hormones were monitored for 11 days after applying the treatments, and whole-plant biomass accumulation determined at the end of that period. Treatments showed temporal and spatial differences in soil moisture, with FDI resulting in a wet upper layer and dry lower layer. In both species, water stress integral in IDI was three-fold higher than in FDI, and gas exchange lower than FDI plants. Despite these differences, both treatments accumulated biomass and stem diameter growth similarly. In tomato, IDI induced compensatory stem growth (higher than WW plants) after re-watering, and attenuated hormone accumulation (lower jasmonic, gibberellic, and salicylic acid concentrations than FDI plants) that maintained growth. In basil, stem growth of IDI plants only recovered to WW and FDI levels upon re-watering, but lower sensitivity of stem growth to water deficits explained similar final biomass accumulation to FDI plants. Although both deficit irrigation treatments showed similar cumulative growth, temporal differences in physiological responses suggest that irrigation frequency could be tailored to specific crop species depending on their sensitivity to soil water deficits and re-hydration.