Eran Raveh

Expression of Arabidopsis Hexokinase in Citrus Guard Cells Controls Stomatal Aperture and Reduces Transpiration

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.

Chamber response time: A neglected issue in gas exchange measurements

When the dimensions of standard commercial chambers for measuring gas exchange cannot accommodate the object being measured, scientists construct their own chambers. The time needed to reach chamber steady state (chamber response time) depends on net system volume (e.g. chamber and tubing volume) and airflow. Unfortunately, some authors take chamber response time into consideration while others ignore it. We present the formula for calculating chamber response time.

Assessing Salinity Tolerance in Citrus: Latest Developments

Citrus trees are well known as a salt-sensitive crop. Exposing the trees to salt stress usually causes reduction in growth, followed by leaf defoliation and fruit load reduction. Their salt sensitivity reflects their sensitivity to both the osmotic component and toxicity component of salt stress, and it is strongly affected by rootstock/scion interaction. In the following chapter, we will first characterize trees’ salt tolerance as well as the mechanisms behind the osmotic and toxicity components of salt stress. We will describe how the osmotic and toxicity thresholds are characterized, and how they are affected by soil characteristics, irrigation design, and the agrotechniques used for orchard maintenance. We will also explain the dynamic effect of salt stress and how the stress builds up with time. The chapter will end by the description of new potential scientific tools which can help with future understanding of citrus salt tolerance and ideas for future research.