Genotypic Difference Of Maize Roots In Response To Localized Nitrate Supply And The Possible Physiological Mechanisms

Nitrogen (N) distribution in soils is highly heterogeneous. One way for plants to adapt to this soil character is proliferating more laterals around the N-rich sites so as to acquire N efficiently. It has been well documented that localized supply of nitrate (NO3-) can stimulate lateral root growth. The physiological mechanism, however, is less understood. In the current study, the response of lateral roots (LR) to local nitrate supply was compared in two maize (Zea mays L.) inbred lines (478 and Wu312) by adopting an agar culture system. The possible involvement of sucrose and auxin in the local nitrate -induced LR growth were investigated. The main results and conclusions are as follows.(1) On the basis of supplying basic nutrient solution with low nitrate (0.01 mmol/L), local supply of 0.2 to 1.0 mmol/L nitrate stimulated the elongation, but not the number, of lateral roots of both maize inbred lines, with significantly stronger effect in 478 than in Wu312.The optimum concentration for the stimulation effect in both the genotypes was 1.0 mmol/L. Further increase in nitrate concentration resulted in weaker stimulation effect. The inhibitory effect of high NO3- on root growth was weaker in 478 than in Wu312. The stimulation effect disappeared in Wu312 when local nitrate concentration was as high as 7.5 mmol/L, and the lateral elongation was completely inhibited when local nitrate was 15 mmol/L. In 478, however, the stimulation effect could be observed even the local nitrate supply is as high as 25 mmol/L.(2) Solely supply of sucrose did not affect LR growth. At the condition of localized 0.5 mmol/L nitrate, sucrose enhanced the stimulation effect of local NO3" on LR elongation. This effect was not found when local nitrate is 1.0 mmol/L. When local nitrate was 7.5 mmol/L, elongation of LR in Wu312 was inhibited. Sucrose supply could alleviate this inhibition, but not completely. It is supposed that sucrose might act as the carbon supply in the process of local nitrate induced LR growth.(3) Localized 100 nmol/L NAA increased LR number regardless of local nitrate supply. At localized 0.5 mmol/L NO3-, NAA supply increase LR elongation of Wu312 further so that the LR length was equal to that of 1.0 mmol/L, NO3- treatment. When local nitrate was 7.5 mmol/L, elongation of LR in Wu312 was inhibited, NAA could incresae the root growth and restore LR growth to the same level as in 1.0 mmol/L NO3- supply.TIBA treatment at hypocotyl or the root at A segment (0-5 cm) could completely inhibit the stimulation effect of localize nitrate on LR growth in B segment (5-10 cm). No LR was found if TIBA was applied in B segment. Supply of TIBA on C segment (10-15 cm) rarely affects the stimulation of local nitrate on LR growth in B segment.These results suggested that the stimulation effect of local nitrate on LR growth is dependent on auxin. And the auxin mainly comes from the basipetal transport from shoot to roots, but not from the seed or the basipetal transport of auxin from root tip to LR growth sites.