The Effect of Increasing Irrigation Rates on the Carbon Isotope Discrimination of Apple Leaves

C-13 discrimination (Delta C-13) has been used in research as an indicator of water availability in crops; however, few data are available concerning fruit trees. The aim of this study was to examine the effect of irrigation on the Delta C-13 values of apple leaves. We assumed that Delta C-13 would increase with irrigation intensity. The Delta C-13 of apple trees (Malus domestica) cv. 'Red Jonaprince' was determined in the years 2019-2022. Leaf samples were collected in spring, in June, at the beginning of the irrigation campaign, and in autumn, in September, following the harvest. The irrigation doses were applied to replenish the water consumption, 0% (ET0), 50% (ET50), 75% (ET75), and 100% (ET100), of the calculated evapotranspiration (ET) levels. In November, the leaves collected from different positions on the shoots were sampled, assuming the Delta C-13 signature would reflect the changes occurring in the water supply during their growth. The irrigation rates had a significant effect on the Delta C-13 of the leaves when the data for the spring and summer months were pooled. On average, Delta C-13 increased from 20.77% and 20.73% for ET0 and ET50, respectively, to 20.80% and 20.95% for ET75 and ET100, respectively. When the data obtained for the spring and summer months were analysed separately, the effect of irrigation was weak (p < 0.043). The Delta C-13 value was always higher for treatment ET100 than treatment ET0, for individual experimental years and terms; however, the differences were minor and mostly insignificant. The leaf position had a strong significant effect on Delta C-13; the values gradually decreased from the leaves growing from two-years-old branches (22.50%) to the youngest leaves growing at the top of the current year's shoots (21.07%). This order was similar for all the experimental years. The results of the experiment suggest that C-13 discrimination in apples is affected not only by water availability during growth, but also by the use of C absorbed in previous years.