Proline and Oxidative Metabolism in Young Pecan Trees Associated with Sulphate Accumulation

Pecan Carya illinoinensis (Wangenh.) K. Koch.] is a deciduous tree whose fruits (nuts) are of high economic value and offer excellent nutritional benefits. However, soils high in sulphates can limit its growth and development. Working with 5-year-old trees of ‘Western Schley’ pecan grown in soils high in sulphates, the levels of proline and oxidative metabolism were recorded in the leaflets. Results showed that different levels of visible leaflet damage (‘sufficiency’, ‘low’, ‘moderate’ or ‘severe’) were associated with different levels of leaflet sulphates (mg kg^?1): ‘sufficiency’ (≤40), ‘low’ (41–60), ‘moderate’ (61–80) and ‘severe’ (80–100). ‘Severe’ sulphate damage was associated with significant reductions in chlorophyll (TChl) (17.04?μg g^?1), relative water content (RWC) (50%) and leaf area (LA), and with increases in the concentrations of total carotenoids (TC) and proline (Prl). Increases were also observed in the activities of the oxidative metabolism enzymes: superoxide dismutase (SOD) (1.82 units min^?1 g^?1), catalase (CAT) (2.86 μmol H₂O₂ min^?1 g^?1) and antioxidant capacity (AC) (87% DPPH inhibition). However, guaiacol peroxidase (GP) showed a reduction (2.97?nmol GSH min^?1 g^?1). An inverse relationship was found between the sulphate concentration in the leaflets with respect to the evaluated parameters of TChl, TC, RWC, LA, AC, and GP. Proline synthesis and antioxidant enzymatic activity indicate salt stress in pecan leaflets in orchards irrigated with deep-well water high in sulphates.