ECOPHYSIOLOGICAL RESPONSES OF CITRUS TREES AND SUGAR ACCUMULATION OF FRUIT IN RESPONSE TO ALTERED PLANT WATER RELATIONS

This study was undertaken to quantify the effects of daily fertigation on ecophysiological responses in citrus trees (Citrus spp.) as part of a larger study. Initial research was conducted to optimise and standardise the sampling procedure to quantify stem water potential (ψstem) in citrus trees. To reliably determine the plant water status of citrus trees, the following conditions are required to minimise unwanted variation in ψstem measurements. Bagging of leaves with black polyethylene envelopes covered with aluminium foil 3 to 4 hours prior to measuring ψstem allows the plant water status in those leaves to equilibrate with whole-tree plant water status, thereby providing a realistic measurement of the current water status. The use of aluminium foil to cover the bagged leaves, reduces unwanted heat stress by reflecting sunlight, and dramatically reduced variation in ψstem. On average, the difference between measurements of bagged leaves without aluminium foil and bagged leaves covered with aluminium foil was 0.19 MPa in the 2002-03 season and 0.13 MPa in the 2003-04 season. The time of day at which ψstem measurements are made is important to ensure consistency in comparisons among treatments and interpretation of irrigation treatment effects. “Physiological midday” is the preferred time of day to measure ψstem, i.e. 1100 HR. Transpiring leaves with open stomata would be in sun-exposed positions on the east side of trees and should be used for making ψstem measurements. On average, the difference in ψstem on the east and west side of trees was 0.17 MPa and 0.22 MPa in the 2002-03 and 2003-04 seasons, respectively. Under similar experimental conditions as those used here, only three leaves per replicate are required to detect a difference of 0.05 MPa in ψstem between treatment

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