Within Plant Resistance to Water Flow in Tomato and Sweet Melons

Efficient water resource management in relation to water use and crop yields is premised on the knowledge of plant resistance to water flow. However, such studies are limited and for most crops, the within plant resistance to water flow remains largely unknown. In this study, within plant resistance to water transport (hydraulic conductance) was monitored in tomato (Lycopersicum esculuntum) and sweet melon (Citrullus lanatus) using the high pressure flow meter (HPFM) and evaporative flux (EF) methods. In the evaporative flux method, measurements of transpiration flux and leaf water potential were used to calculate the total resistance to water flow using Ohm’s law analogy. Measurements of tranpiration flux (Q) relationship, plant resistance calculated from the slope of their relationship, ranged from 6.57x10-01 to 2.27x10-03 Mpa m-2s-1 for tomato and sweet melon, respectively. The magnitude of whole plant hydraulic conductance calculated by the evaporative flux method and measured on the HPFM were not significantly different. This is probably due to that fact that Kp includes the hydraulic conductance of the root system, which offers the highest resistance to water flow in a plant, and the frictional resistance of the proximal part of the crown. Day time course of water relation parameters were monitored in melon and tomato (predawn, 1100 to 1400 h). The effect of time of day was pronounced on the dynamics of water relations, particularly around mid-day when sharp differences in the magnitudes of the measured water relation parameters were obtained. Minimum (predawn) leaf water potential (ψl) ranged from 0.135 to 0.207 MPa for both crops. The greatest conductance was found in the leaf and the lowest in the root in both crops. While the highest within plant resistance to the flow is contained in the root system, the stem component constitutes the least resistance (greatest conductance) to within plant flow of water in tomato. Repeated measurement analysis showed the existence of significant species effects on plant water relations (leaf water potential, evaporative/transpiration flux, xylem hydraulic conductance). Superiority within plant xylem transport and evaporative losses were obtained in tomato, from the relations of EF and Kh; this crop showed a higher hydraulic sufficiency than melon. The expression of hydraulic conductance of the root and shoot system relative to plant attributes did not eliminate differences in the magnitudes of conductance elements in tomato and melon. Differences obtained between melon and tomato in whole plant leaf and stem area specific hydraulic conductance (KI) indicate the carbon efficiency and, hence, the cost of resource allocation to areas of root surface and leaves for enhanced scavenging for water and mineral nutrients. Thus, application of mineral nutrients to enhance crop productivity should take into consideration the hydraulic conductance for specific plants.