Carbon Isotope Composition Indicates Improved Photosynthetic Water Use Efficiency of Strawberry Plants under Deficit Irrigation

During photosynthesis, the extent of discrimination against the heavier isotope of carbon, C-13, in relation to the lighter isotope, C-12, is related to the ratio of internal to external partial pressure of CO2. This is influenced by both stomatal conductance and photosynthetic rate, and therefore is indirectly related to water use efficiency, with greater C-13 enrichment i.e. a higher carbon isotope composition (delta C-13) being associated with greater water use efficiency. The shallow root system, large leaf area, and high water content of fruits of the commercial strawberry, Fragaria x ananassa Duch., means that strawberry production uses large quantities of water. To save water via deficit irrigation requires greater understanding of the response of strawberry plants to limited water availability. We explored the impact of deficit irrigation on delta C-13 of the cultivar 'Elsanta' in a series of experiments in which plants in pots were given either full or deficit irrigation (80% ETp or lower). In three separate experiments when 60-day 'Elsanta' was grown under plastic tunnels, delta C-13 was significantly increased under deficit compared with full irrigation. Similarly when 60-day 'Elsanta' was grown in a controlled environment unit, delta C-13 was significantly increased under deficit irrigation. The only case where there was no significant effect of irrigation on delta C-13 of 60-day plants was when they were grown in freely-draining perlite rather than in a peat-based growing substrate. When 'Elsanta' was grown as a main season crop, delta C-13 was not significantly affected by irrigation, but was higher than in the other experiments. The lowest delta C-13 (poorest water use efficiency) was found in leaves of fully-irrigated plants grown in the controlled environment unit. The largest differences in delta C-13 between fully- and deficit-irrigated plants also occurred in this environment. This suggests that where water use efficiency is poorest, the potential to improve it using deficit irrigation may be greatest.