Enzymatic and non-enzymatic response of grafted and ungrafted young European pear (Pyrus communis L.) trees to drought stress

The optimization and selection of the scion-rootstock combination are commonly based on its physiological and biochemical alterations capacity, especially in the rootstock, which is significant to elucidate the mechanisms associated with water stress tolerance. To verify the antioxidant defense systems under drought stress, two ungrafted rootstocks, 'Dargazi' seedling (D) and 'Pyrodwarf' clonal (P), and two pear scion cultivars including 'Dargazi' (D) and 'Louise Bonne' (L), that were grafted on the studied rootstocks were examined. We considered growth and chlorophyll fluorescence traits, some oxidative stress markers, and non-enzymatic and enzymatic antioxidant parameters in plants subjected to different soil water regimes. Treatments were, well-watered as control (100% FC), moderate (60% FC), and severe drought (30% FC) under greenhouse conditions. Drought stress influenced chlorophyll fluorescence contents and limited the quantum yield of PSII (Fv/Fm), mainly in P rootstock. In contrast, the highest specific leaf area (SLA), and lowest leaf mass area (LMA) was observed in the P rootstock, which indicated that the P rootstock was more sensitive to drought stress. Physiological and biochemical alterations and the main components of multivariate analysis showed that hydrogen peroxide (H2O2) and malondialdehyde (MDA) were accumulated under water deficit conditions, and P and D rootstocks showed a higher level of H2O2. We found a reduction in chlorophyll pigments, carotenoid (CAR), and ascorbic acid (AA), by drought stress, which had the highest amount in P and D rootstocks. Water stress directly influ-enced anthocyanin (ANT), total phenolic content (TPC), total flavonoid content (TFC), catalase (CAT), guaiacol peroxidase (GPOX), and ascorbate peroxidase (APOX) activity, especially under severe stress levels, and there was a high activity of GPOX in grafted combinations. In general, rootstocks showed more oxidative stress than grafted combinations because of the high increase in H2O2 and MDA, and response to stress by the accumulation of some enzymatic and non-enzymatic factors. However, grafted combinations mostly showed better tolerance with the enzymatic antioxidant defense system, which can be the result of resistance induced by rootstocks and the cultivar's capacity to minimize the negative effects of drought stress.