Cumulative and Legacy Effects of Irrigation on Growth, Physiology, and Gene Expression in Veronica longifolia Under Field Conditions.

Climate change is expected to increase the frequency of fluctuating moisture regimes on the Korean Peninsula, yet plant responses to moderate field moisture variation remain poorly understood. We investigated the effects of irrigation on growth, physiology, and gene expression of the native perennial plant Veronica longifolia under field conditions. Plants were exposed to irrigated and nonirrigated treatments for 20 weeks in 2024, followed by a subsequent growing season in 2025 without irrigation to assess cumulative and carryover effects. During the 2024 treatment period, seasonal mean soil water content ranged from 14.4% to 27.6% in the irrigated plot and from 10.1% to 24.8% in the nonirrigated plot, remaining within or above a generally favorable range for growth and suggesting that severe drought stress was unlikely. Nevertheless, irrigation increased plant height and leaf area, with leaf area 31.1% and 28.7% greater in the irrigated plot during the rainy and postrainy periods, respectively. Irrigation also enhanced reproductive performance, increasing seed mass, seed length, and seed width by 16.7%, 14.4%, and 14.2%, respectively. Seasonal physiological responses showed minimal differences in photosynthetic parameters, except for transient reversals during the rainy season, likely associated with excessive soil moisture. In 2025, biomass differences between treatments disappeared under uniform nonirrigated conditions; however, plants previously subjected to irrigation maintained higher photosynthetic activity and stomatal conductance. Gene expression analyses showed treatment-history-associated differences in transcripts commonly linked to abiotic stress regulation (VlADH, VlDREB1, and VlERF), antioxidant defense (VlPOD), carbon assimilation (VlrbcS), and photosystem II (PSII) repair (VlpsbA). Because gene expression was assessed at a single sampling time, these transcriptional patterns should be interpreted as possible carryover responses associated with prior irrigation history rather than direct evidence of physiological memory mechanisms. These findings suggest that moderate differences in field moisture availability can be associated with cumulative growth responses within the treatment year and with physiological and transcriptional differences in the following year.