Determining transpiration coefficients of ‘Rojo Brillante’ persimmon trees under Mediterranean climatic conditions

There is scarce knowledge on what are the actual water requirements of persimmon trees (Diospyros kaki). Research on this topic is required to better adjust irrigation water supplies to the actual crop water needs. This study aimed to determine the transpiration coefficients (Kₚ) of persimmon ‘Rojo Brillante’ trees and assess how transpiration and Kₚ are influenced by environmental factors and the atmospheric demand under Mediterranean climatic conditions. This was done over the course of three growing seasons by means of sap flow measurements calibrated with whole canopy gas exchange chambers. Irrigation was applied aiming to avoid water limiting conditions for trees. This was verified by monitoring the trees’ water status across the growing seasons. Sap flow measurements were linearly and highly correlated (p < 0.001) with canopy transpiration (Eₚ) obtained with the whole canopy chambers, but either overestimated or underestimated it. Eₚ increased from April until June-July in response to canopy development and increased midday canopy light interception. There was a positive linear relationship between daily Eₚ, reference evapotranspiration (ETₒ) and solar radiation. However, a non-linear relationship was obtained between Eₚ and vapor pressure deficit (VPD). A positive non-linear relationship (p < 0.01) was observed between Kₚ (Eₚ / ETₒ in non-limited water conditions) and VPD, illustrating the low stomatal regulation that persimmon trees exhibit in response to air dryness. Kₚ varied over the growing seasons following the ETₒ pattern although showing comparatively a slighter decrease during the last part of the growing season. From June to September, Kₚ of persimmon ‘Rojo Brillante’ trees with a midday photosynthetically active radiation (PAR) intercepted of ~30 % ranged from 0.45 to 0.56. A strong relationship was obtained between Kₚ and percentage of midday intercepted PAR that can be used to estimate Kₚ of persimmon ‘Rojo Brillante’ trees with a range of ground canopy covers. This information is useful for deriving specific water use models for persimmon trees to be implemented in decision support systems. The monthly transpiration coefficients here provided will be useful for optimizing irrigation scheduling decisions in persimmon orchards and therefore irrigation water management.