A Role Of Root Morphology In Efficient Nitrogen Acquisition In Maize And Its Physiological Regulation Mechanism

Increasing nitrogen (N) acquisition of plants is a way to enhance N fertilizer use efficiency. The capacity of a plant taking up nitrate is a function of the activity of its N transport system and the size and architecture of its root system. In the present study, two maize (Zea mays L.) inbred lines, 478 (N-efficient) and Wu312 (N-inefficient), were selected to determine the relative importance of N uptake system and root growth in N acquiring ability. Their differences in N assimilation characters were investigated. Finally, the possible involvement of hormones (cytokinin and auxin) in nitrate-mediated root growth was analyzed. The main results were as followed:1. Genotype 478 could take up more N than Wu312 at all nitrate concentrations (0.05 to 20 mM). Its increments of leaf area, root biomass and root length were significantly higher than Wu312 with increasing N accumulation during 7-day interval. The specific nitrogen uptake rate and potential nitrate uptake rate of 478 were lower than that of Wu312. Genotypic difference was not found in photosynthesis rate. It was found that total N uptake increment was significantly and positively correlated with leaf area increment. It was concluded that a larger N demand resulted from greater leaf expansion rate in 478 exerts a driving force for more N uptake. This N demand is met mainly by developing a bigger root system. The activity of nitrate transport system is not the key factor determining the genotypic differences in nitrogen acquisition ability.2. Compared with Wu312, 478 had greater nitrogen assimilation ability, which was shown in higher nitrate reductase (NR) activity, lower NO₃^- concentration in shoots, roots and xylem, and higher amino acids concentration in both shoots and phloem. According to the theories that NO₃^- in shoots may be a negative feedback signal regulating root growth and phloem transport of amino compounds from shoot to roots negatively mediates the activity of nitrate transport system, the greater nitrogen assimilation ability of 478 might, to some degree, explain its vigorous root growth but lower nitrate transport system activity.3. At higher nitrate supply, root elongation of 478 was inhibited, the distance from the site of lateral root elongation to root tip became shorter, the root cap became shorter, and root elongation zone became swollen. These phenomena were identical of those caused by exogenous cytokinin on root growth. Nitrate supply increased endogenous cytokinin (Z+ZR) concentrations in roots of 478. Exogenous cytokinin (6-BA) decreased the stimulation of root elongation caused by lower nitrate, and caused swollen root elongation zone, short root cap, and lateral initiation closer to root tip. In Wu312, however, high nitrate concentration did not affect significantly the morphological of root tip as well as Z+ZR concentrations. It was postulated that the inhibitory effect of high nitrate on root elongation of 478 may be, at least partly, mediated by the increased cytokinin level in roots, which affected the function of root tip and regulated root elongation and lateral root growth finally.4. The function of root tip is related to transport, unloading and distribution of auxin in roots. It was found that IAA concentration was higher in the root sections close to root apex (0-10 cm), and decreased towards the root base. Higher nitrate supply decreased IAA transport in phloem from shoot toroots and IAA concentration gradient from root tip to root base. Exogenous auxin (NAA) reduced the inhibitory effect of high nitrate on root elongation, increased lateral root numbers and stimulated lateral root elongation. It was postulated that IAA transport to and distribution in roots might play a role in nitrate-mediated root morphology.