Measuring Xylem Vulnerability in Three Citrus Species

This study documented xylem vulnerability of Citrus x sinensis (Valencia orange), Citrus x paradisi (star ruby red grapefruit), and Citrus x reticulata (tangerine) under drought conditions.  During water shortages, a better understanding of drought tolerance mechanisms in citrus would enable improved water conservation.  Physiological mechanisms of drought resistance were investigated by measuring the formation of xylem embolisms, air bubbles that prevent water flow through vascular tissue. Four methods were compared to determine the best technique for hydrating the stems to achieve maximum hydraulic conductivity through xylem: 1) high pressure flushing for one hour at room temperature, 2) sixteen-hour vacuum treatment of submerged stems at room temperature, 3) low pressure flushing at 12^oC for sixteen hours, and 4) one-hour vacuum treatment of submerged stems followed by one-hour high pressure flushing at room temperature.  Based on previous results, we hypothesized that submergence under vacuum would be the best method for stem hydration because it would effectively remove trapped air molecules from pit membranes.  Based on differences in fruit sizes, we hypothesized that grapefruit would be the most vulnerable to embolism formation.  High-pressure flushing, the widely used standard method produced the lowest hydraulic conductivity, whereas, long-term vacuum and long-term low pressure flushing both caused the highest hydraulic conductivities.  The three citrus species were not significantly different in their vulnerability to embolism formation.  Long-term study would allow for a better understanding of plant hydraulic conductivity in drought-prone regions.