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Revised sap flow driven stem diameter model for tomatoes grown under assimilation lighting

A comprehensive plant model that was originally developed to predict shortand long-term variations in tomato stem diameter stopped working properly when used for tomato plants grown under assimilation lighting during winter. Under these prolonged day lengths in winter, an atypical diel pattern emerged showing an increase in stem diameter 1-2 hours after the lights were turned on. In order to explain this specific plant behavior, sap flow sensors and linear variable displacement transducers were installed on tomato plants (Solanum lycopersicum L. ‘Foundation’). Plants were lighted for 18 hours per day with a combination of natural sunlight and HPS lamps (169 μmol m-2 s-1). Two hypotheses to explain this atypical plant behavior were formulated, and subsequently examined: (i) transport of sugars from leaves to roots decreases phloem water potential, which imports water from the xylem and causes an increase in stem diameter; and (ii) the sudden increase in light intensity causes a drop in xylem water potential, resulting in changes in hydraulic conductance due to cavitation, which in turn affects the water exchange between xylem and storage and, hence, the variation in stem diameter.