Physiology of grapefruit (Citrus paradisi Macf.) cultivars as affected by rootstock

'Marsh Seedless' and 'Redblush' cultivars of grapefruit (Citrus paradisi Macf.) were budded on nine different rootstocks under the typical sub-tropical growing conditions of North India. Significant variations in tree physiology were observed due to rootstock effects. Trees of both scions had the highest photosynthetic rate (A) and stomatal conductance (g(s)) on 'Rough Lemon' rootstock. Rates of transpiration (E) in 'Marsh Seedless' were higher on 'Sour Orange', 'Rough Lemon', 'KarnaKhatta', 'Billikhichli', 'RLC-4', 'Troyer Citrange', 'Jatti Khatti', or 'Attani-2' rootstocks than on 'Attani-1'. In 'Redblush', values of E were significantly higher on 'Troyer Citrange', 'Attani-2', 'Jatti Khatti', 'Billikhichli', 'Sour Orange', and 'Karna Khatta' than on 'Attani-1' or 'RLC-4'. In 'Marsh Seedless', higher relative water content (RWC) values and lower excised leaf water loss (ELWL) values were observed on the rootstocks 'Billikhichli' and 'RLC-4', but the RWC was highest in 'Redblush' on 'Rough Lemon' rootstock, and the lowest ELWL value was recorded on 'Troyer Citrange'. 'Redblush' had the highest superoxide dismutase (SOD), peroxidase (POD), and glutathione reductase (GR) activities on 'Attani-2' rootstock, whereas catalase (CAT) was most active in the leaves of 'Redblush' trees budded on 'RLC-4'. The highest activities of CAT and GR in 'Marsh Seedless' were on 'Billikhichli, whereas SOD and POD activities were highest in the leaves of 'Marsh Seedless' on 'Rough Lemon' and on 'Attani-2' rootstocks, respectively. 'Marsh Seedless' on 'Troyer' or 'Karna Khatta', and 'Redblush' on 'Billikhichli' or 'Rough Lemon', had the highest total soluble protein concentrations in their leaves. Overall, differential responses to rootstock were observed in both grapefruit scion cultivars. 'Rough Lemon' was the best rootstock to induce high metabolic activity in both scion cultivars under normal growing conditions. However, high RWC values and antioxidant enzyme activities in 'Marsh Seedless' on 'Billikhichli' and in 'Redblush' on 'Troyer Citrange', 'Attani-2', or 'RLC-4', indicated their utility as rootstocks under stressful growing conditions.