Carbon isotope discrimination and surrogates of water use efficiency for tomato under various soil moistures

Carbon isotope discrimination (Δ‰) is negatively correlated with water use efficiency (WUE) in several C3 species and was proposed as a selection criterion for improved WUE. Tomato (Lycopersion esculentum L.) cv. Improved Apollo, was exposed to soil moisture levels, saturated, field capacity (FC) and deficit soil moisture conditions, in a container experiment in a screenhouse. The objective was to test for: (1) difference between soil moisture levels as they affect water use efficiency, boimass production, and distribution; (2) evaluate the relationship between various measures of WUE and carbon isotope discrimination (Δ‰), between biomass production and Δ‰, and between stem diameter (SD) and WUE and, the use of SD as an alternate surrogate; and (3) discern the physiological processes associated with changes in WUE and Δ‰ with respect to different soil moisture levels in a heavy clay soil. Tomato plants responses were quantified in terms of biomass and fresh fruit yield, plant morphology, leaf gas exchange properties, WUE of biomass, and fruit, as well as Δ‰ by leaves. While number of leaves and nodes, and leaf area were least in the saturated treatment and greatest at FC; stem diameter at harvest was greatest under saturated conditions. Stem diameter was positively correlated with Δ‰, indicating the possibility of using stem diameter as a surrogate of WUE based upon dry weight of biomass or fruit, and for negative relationships between Δ‰. Midday leaf water potential and relative water content decreased with increasing moisture stress (deficit or saturated conditions). However, leaf chlorophyll concentration decreased as soil moisture content increased from deficit to FC to saturated. Fresh fruit yield was significantly greater at FC (4.476 kg/plant) compared with saturated (2.617 kg) and deficit (1.985 kg) treatments, as a result of greater mean weight/fruit rather than more fruit/plant. As fresh fruit yield and dry matter content of fruit were negatively related, the WUE of fresh fruit was not inversely related to Δ‰. The data indicate that in tomato, where marketable yield consists of fresh produce, the established negative relationship between WUE and Δ‰ does not hold true, and benefits of using Δ‰ as a surrogate for fresh yield WUE may not be effective while the strong negative correlation between SD and WUE suggests possibility of employing SD as surrogate of WUE