Stable carbon and nitrogen isotope ratios as indicators of water status and nitrogen effects on peach trees

Carbon and nitrogen isotope ratios are tools that are widely used in ecophysiological studies of forest and herbaceous plants. However, the use of isotope discrimination techniques with fruit trees has been very limited. Three irrigation strategies were applied to peach trees (Prunus persica (L.) Batsch. cv. Andross): full irrigation throughout the growing season; restricted irrigation during stage-II of fruit growth (70% restriction); and restricted irrigation during stage-III (30% restriction). These irrigation treatments were combined with three different nitrogen fertilization treatments: 0, 60 and 120 kg N ha−1. The stable carbon isotope ratio (δ13C) in fruit proved a reliable indicator of peach crop water status and physiological water use efficiency. The leaf carbon isotope ratio was not affected by either the irrigation or N treatments. δ13C in endocarp tissues exhibited a powerful discriminatory capacity for water stress during phase II of fruit growth, whereas δ13C in the mesocarp was an interesting tracer for water stress in phase III of fruit growth. There was an important relationship between leaf N content and photosynthetic efficiency during period GSII, which enhanced leaf WUE. The stable nitrogen isotope ratio (δ15N) in the fruit mesocarp was significantly related to the amount of N applied and the origin of this N. The nitrogen isotope ratio did not, however, exhibit a good discriminative capacity when we studied the influence of water and nitrogen on plant response.