Integrated effect of irrigation frequency and phosphorus level on lettuce: P uptake, root growth and yield

The objective of the present research was to assess the effects of fertigation frequency on plant phosphorus and water uptake. Special attention was given to root measurement in order to elucidate the mechanism that relates the fertigation frequency to P uptake and plant growth. Lettuce (Lactuca sativa L., cv. Iceberg) grown in pots filled with quartz sand was chosen as a test plant. The experiment comprised six treatments, with two concentrations of P in irrigation water (0.2 mM and 1.0 mM), and three daily fertigation frequencies (one, four and ten). It was found that high irrigation frequency induced a significant increase in plant-P concentration at low solution-P concentration, whereas at high P concentration the effect of irrigation frequency was insignificant. Increasing the irrigation frequency significantly enhanced the transpiration flux so that the transpiration flux of plants under low irrigation-P level at 10 daily irrigation events was similar to that of plants under high solution-P. The increases with irrigation frequency of P concentration in lettuce organs and of P influx to the roots, at the low P level, were attributed to the elimination of the depletion zone at the root-soil interface by the supply of fresh nutrient solution, and the enhancement of P uptake. The higher P uptake resulted from higher convective flux of dissolved P from the substrate solution to the root surface owing to the higher average moisture content associated with frequent irrigation. The only significant correlation revealed by multiple stepwise regressions relating nutrient concentrations in the plant to yield was that between plant-P concentration and the yield. On the basis of a quadratic regression, 97% of the dry weight variation could be explained by differences in P uptake, indicating that the main effect of fertigation frequency was related to an improvement in P mobilization and uptake. Thus, frequent irrigations may compensate for P shortage.