Daily and seasonal changes of sap flow in Gamhong apple cultivar and estimate the tree-level transpiration using Penman-Monteith reference evapotranspiration

초록

Understanding the environmental drivers and hydraulic dynamics of plants is pivotal for elucidating future water use strategies and refining precise irrigation techniques. In our investigation, we focused on the daily and seasonal fluctuations in sap flow (SF) using compensation heat-pulse techniques on 7-year-old 'Gamhong' apple trees. Predictions of stand transpiration were integrated using the Penman-Monteith grass reference evapotranspiration. Additionally, we measured leaf-level diurnal and seasonal stomatal conductance (g(s)), leaf water potential (Psi(Leaf)), and transpiration rate (E) to explore the relationship between plant-level transpiration and orchard microclimate. Diurnal SF exhibited a pattern similar to global radiation (R-g) and vapor pressure deficit (VPD), increasing in the morning with a peak in the afternoon and decreasing towards evening. Seasonal water use varied, with rates of 10.62 +/- 0.282 L day(-1) in early (May-June), 8.40 +/- 0.222 L day(-1) in mid- (July-August), and 5.96 +/- 0.154 L day(-1) in late season (September-October). Plant water consumption was influenced by weather conditions and evaporative demand driven by atmospheric VPD and R-g. Clear and sunny days resulted in higher water use (10.23 +/- 0.291 L day(-1)) compared to partially sunny but windy days (9.85 +/- 0.32 L day(-1)) and cloudy/rainy days (5.37 +/- 0.176 L day(-1)). Plant transpiration strongly correlated with VPD (R-2 = 0.565), R-g (R-2 = 0.616), relative humidity (R-2 = 0.623), and wind velocity (u, R-2 = 0.684), while no significant correlation was observed with air temperature (T-a, R-2 = 0.19) or soil moisture status (Psi(soil), R-2 = 0.19). VPD and R-g emerged as the primary drivers, with Psi(soil) playing a minor role and not limiting tree-level transpiration. Leaf-level g(s), Psi(Leaf), and E displayed similar diurnal trends. In our study, the integrated daily sap flow provided valuable insights into plant-water relations differently across growing seasons and its relationship with reference evapotranspiration (E-o). The Penman-Monteith equation, using grass reference evapotranspiration indicated a positive correlation (R-2 = 0.764, P < 0.001) between actual and estimated transpiration. However, the slope of the relationship differed significantly from 1.0, suggesting that E-o per unit leaf area of grass corresponds to E-288 = E-o/2.88 (L m(-2) of leaf area d(-1)), which underestimated evapotranspiration.

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