Hexokinase (HXK) is a sugar-phosphorylating enzyme involved in sugar-sensing. It has recently been shown that HXK in guard cells mediates stomatal closure and coordinates photosynthesis with transpiration in the annual species tomato and Arabidopsis. To examine the role of HXK in the control of the stomatal movement of perennial plants, we generated citrus plants that express Arabidopsis HXK1 (AtHXK1) under KST1, a guard cell-specific promoter. The expression of KST1 in the guard cells of citrus plants has been verified using GFP as a reporter gene. The expression of AtHXK1 in the guard cells of citrus reduced stomatal conductance and transpiration with no negative effect on the rate of photosynthesis, leading to increased water-use efficiency. The effects of light intensity and humidity on stomatal behavior were examined in rooted leaves of the citrus plants. The optimal intensity of photosynthetically active radiation and lower humidity enhanced stomatal closure of AtHXK1-expressing leaves, supporting the role of sugar in the regulation of citrus stomata. These results suggest that HXK coordinates photosynthesis and transpiration and stimulates stomatal closure not only in annual species, but also in perennial species.

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A methodology to estimate water status of palm trees from aerial thermal images was developed. Deficit irrigation of 80% in three drip-irrigated date-palm plots in the northern Dead Sea region was manipulated during the winter of 2007 and 2008. An uncooled thermal camera was used for extensive aerial imaging to detect palm trees and pure-canopy pixels by using only aerial thermal images. An automatic procedure, based on watershed segmentation analysis, was developed which enabled detection of all palm trees in the thermal images. Two new methods were developed to select palm trees and pure pixels within them: basin-based and pixel-based. From the temperatures of pure-canopy pixels, significant differences were found between palm trees under commercial and deficit irrigation regimes, in all three plots. Automated detection of canopy, based on aerial thermal images, is a key step towards commercial mapping of within-plot water-status variability. A protocol, based on the developed methodology, was suggested for mapping water status variability in a palm plot, and for irrigation scheduling.