Timing of water limitations affect source to sink differences in δ¹³C composition in apple

Deficit irrigation is used to reduce vegetative vigor, increase fruit quality, and conserve water resources. However, physiological responses to deficit irrigation can vary depending on soil and environmental conditions. Although physiological measurements are often made at single points in time, responses are often longer lasting and a measurement that integrates responses over time would have greater value in assessing the effectiveness of deficit irrigation practices. Carbon isotope composition has long been used as a proxy measurement for water-use efficiency, stomatal conductance, and carbon dioxide exchange with the atmosphere and is heavily influenced by water status. Potentially, fruit, leaves, or other tissues could be used as samples for carbon isotope measurements. However, it is not well known how irrigation practices can influence both source and sink tissue carbon isotope composition in perennial systems. Here, we used two experiments to determine how irrigation timing affects both source and sink δ¹³C at the end of the season. Irrigation limitations were initiated after bloom for either the whole season or for early, middle, or late season and compared to a well-watered control. For both experiments, leaves were poor indicators of irrigation deficit treatments that were applied during the season. There were no significant differences in leaf δ¹³C between deficit treatments and the control for both experiments. However, all sink tissues including roots and stems in experiment one for both years and for fruits in experiment two for both years were significantly more enriched compared to the well-watered control. Environmental conditions during the season also appeared to influence the magnitude of difference in δ¹³C between deficit irrigation treatments and the control. These results indicate that the use of sink tissues are more sensitive for measuring signals associated with in-season water deficits. Carbon isotope composition can be an effective proxy to measure efficacy of irrigation treatments at the physiological level.