Phosphorus nutritional effects on root hydraulic conductance, xylem water flow and flux of magnesium and calcium in squash plants

Abstract

Previous studies have found that P nutrition of plants is an important factor in the uptake and translocation of Mg and Ca, and increasing root osmotic hydraulic conductance (Lo) and osmotically driven xylem exudate flow (Jv). Experiments were designed to determine if the observed changes in Mg and Ca uptake and translocation, Jv, and Lo from altered P nutrition are related or are separate functions. When six-week old squash (Cucurbita pepo L.) plants grown in perlite were treated with P levels ranging from 50 to 400 μM P for seven days, Jv and Lo increased as P treatment level increased. Xylem exudate concentrations of Mg and Ca were maintained as Jv increased, resulting in an increase in total flux of these mineral elements. The increase in Mg and Ca flux in the xylem exudate correlated with increased shoot Mg and Ca levels as P nutritional level was raised. Further studies with greenhouse grown plants indicated that the increases in Jv, Lo, and Mg and Ca flux were more responsive to changes in P nutritional level than to similar changes in levels of other anions. In hydroponically grown squash plants, xylem exudate was collected for a 20 min period after 0, 2 and 4 h in treatments of 50 and 500 μM P or after P treatment was increased from 50 to 500 μM. Immediately after nutrient solution P was increased (time 0), there was a 33% increase in Jv and a 22% increase in Lo when compared to the 50 μM P treatment. The Jv and Lo of the 50–500 μM P treatment did not equal levels of the continuous 500 μM control at time 0, but were similar after 2 and 4 h. Flux of Mg and Ca did not increase as rapidly as Jv in the 50–500 treatment indicating that regulation of Mg and Ca uptake and xylem loading by P may lag behind that of water movement.