Corn compensatory growth upon post-drought rewatering based on the effects of rhizosphere soil nitrification on cytokinin

This study aims to investigate the relationship between rhizosphere soil nitrification and compensatory growth of potted corn (Zea mays L.) seedlings upon post-drought rewatering on the basis of leaf cytokinin induced by roots. The nitrification inhibitor 3, 4-dimethylpyrazole phosphate was added to the soil to restrain nitrification in soil. The experimental design consisted of six treatments: (1) wetness, (2) wetness with the nitrification inhibitor addition, (3) wetness with the nitrification inhibitor and nitrate additions, (4) post-drought rewatering, (5) post-drought rewatering with the nitrification inhibitor addition, and (6) post-drought rewatering with the nitrification inhibitor and nitrate additions. The results showed that compared with wetness rewatering alone increased the aboveground and total biomasses and the net photosynthetic rates of corn, but rewatering had little influence on these parameters following the nitrification inhibitor addition or both the inhibitor and nitrate additions. Without the nitrification inhibitor addition, the increase of carbohydrate content in root exudates of rewatering corn caused by drought stress accounted for rhizosphere soil nitrification rate augment during rewatering that were closely related to the delivery rate of cytokinin from the roots to the leaves; however, increased nitrification rates in non-rhizosphere soil were not recorded in rewatering treatment. Without the addition of the nitrification inhibitor, the net leaf photosynthetic rate of corn increased due to the elevated cytokinin levels in leaves, leading to the occurrence of compensatory growth and high water use efficiency of corn. By contrast, nitrification inhibitor application decreased the rhizosphere soil nitrification rate, followed by the consequent under-compensatory growth and low water use efficiency of corn; and the under-compensatory growth and low water use efficiency of corn also occurred in corns with the addition of both nitrification inhibitor and nitrate. Therefore, rhizosphere nitrification plays a key role in corn compensatory growth.