Analysis of xylem water as an indicator of current chloride uptake status in citrus trees

The potential use of stem xylem chloride (Cl−) analysis as an indicator for sudden variation in Cl− uptake by the tree was examined. Three extraction techniques (xylem sap extracted by centrifuge, and water extraction of fresh or dry ground xylem) were linearly correlated to each other as well as to soil salinity. Chloride values in xylem sap extracted by centrifugation were about half (3-34 mM) of those found in water extraction of fresh or dry ground xylem (9-84 mM), reflecting some Cl− accumulation by existing living cells. Based on dry ground xylem, xylem Cl− concentration was not affected by time of day; however, when centrifuge extraction was used, daily values were highest at the beginning and end of the day, and lowest during mid-day. Additionally, Cl− concentration was not affected by stem thickness (5-25 mm) using the dry ground extraction method. Rootstock affected xylem Cl− content in the same way it affected leaf Cl− content; xylem Cl− content was highest for trees grafted on salt-sensitive rootstocks and vice versa. As opposed to leaf analysis in which Cl− concentration can only increase with time, xylem Cl− content always followed soil salinity variation, either increasing or decreasing. Thus, analysis of stem xylem water can be a valuable tool for detecting short-term variation in Cl− uptake. This method is important in trees since leaf analysis reflects the cumulative Cl− content and therefore does not always give the current transitory mineral uptake status.