Interactive effect of drought and nitrogen on growth, some key physiological attributes and oxidative defense system in carrot (Daucus carota L.) plants

Exogenous application of nitrogen is an efficient means of enhancing plant stress tolerance through modulation of a number of physio-biochemical processes. Thus, the present investigation was carried out to assess the effectiveness of foliar applied nitrogen (100, 200 and 300 mg L-1) on growth and some key physiological attributes in carrot plants under drought stress [50% field capacity (FC)]. Drought-induced increase was observed in leaf free proline and glycinebetaine (GB), as well as total phenolics. However, there was no significant effect on shoot and root dry weights, ascorbic acid, and MDA, but chlorophyll a and ratio of chlorophyll a/b significantly decreased under water deficit conditions. Foliar-applied urea as a source of nitrogen significantly increased the shoot and root dry weights, root length, chlorophyll b, total chlorophyll, leaf free proline, GB, total phenolics, activities of catalase (CAT) and peroxidase (POD) enzymes and contents of total soluble proteins, but it decreased chlorophyll a/b ratio. So overall, exogenously applied nitrogen particularly 300 mg L-1 was very effective in improving the drought tolerance of carrot plants by enhancing the levels of chlorophyll pigments and up-regulating the oxidative defense system.