Morphological and physiological plasticity of Catalpa bungei roots under partial root-zone drought as affected by nitrogen forms

A greenhouse experiment was conducted to evaluate root plasticity and drought responses of Catalpa bungei under partial root-zone drought as affected by nitrogen (N) forms. A two-factor experimental design was used in which two nitrogen forms (ammonium nitrogen (AN) and nitrate nitrogen (NN)) were combined with four water treatments (well-watered condition (WW), vertical partial root-zone drought (V-PRD), horizontal partial root-zone drought (H-PRD), full root-zone drought (FRD)). Root parameters were interactively affected by water conditions and N forms. Root development was sharply inhibited by FRD, and nitrate (NN) was more beneficial than ammonium (AN) for lateral root elongation under FRD. Root responded to PRDs with changing of their morphological plasticity, depending on the supplied N forms. Moreover, root system exhibited contrasting strategy for water foraging under V-PRD and H-PRD. Under V-PRD, root forage for water by hydrotropism, which was indicated by active root proliferation in moist soil zone. This strategy contributed to the efficient carbohydrate production and dominant xylem development under V-PRD. In contrast, root forage for water under H-PRD by significant root proliferation in the drought soil zone. The absence of horizontal hydrotropism under H-PRD condition and the presence of vertical hydrotropism under V-PRD was related to the gravitropism action. Physiological plasticity under PRDs might be preliminary disclosed by the specific activation of antioxidant defense in the drought root zone. Specifically, APX, CAT and SOD were induced in the drought root zone (top zone) of V-PRD, and APX was induced in the drought root zone (right zone) of H-PRD. Root physiological plasticity under PRDs was related to N forms and it displayed contrasting patterns in NN and AN treatment.