Physiological Mechanism Of Leaf Senescence And Nitrogen Remobilization In Maize (Zea Mays L.) As Affected By Source And Sink Manipulation

During leaf senescence, nitrogen in vegetative organs begins to remobilize to grain. Leaf senescence affected by various factors, such as source and sink relationship, hormones, the condition of moisture in the soil and plant nutrition state and so on. As two sources of grain nitrogen, pre-silking nitrogen remobilization and post-silking nitrogen uptake were closely related with leaf senescence. To understand the mechanism of leaf senescence and nitrogen remobilization in maize as affected by source and sink manipulation, we investigated two years of field experiment and three years greenhouse experiment. The main findings present as follows:1) 15N labeling was used to analyze the dynamic behavior of both pre-and post-silking N in relation to source and sink manipulation in maize plants. The results showed that remobilization N had a greater contribution to grain N during early grain filling with post-silking N importance increasing during the later filling stage. Pre-silking N remobilization efficiency in the leaves was 33% even though grain yield was zero (no pollination treatments). Grains gained 62% and 76% of pre- and post-silking 15N, respectively, suggesting that enhancing post-silking N uptake rather than N remobilization is more likely to increase grain N accumulation.2) In order to understand the regulation of prevention pollination on leaf senescence in maize, we investigated the effect of pollination prevention on leaf senescence, post-silking DM and N accumulation and remobilization in different vegetative organs in one landrace and ten representative maize hybrids released between 1973 and 2000 in China. When pollination was prevented by covering silk with paper bags at silking stage, leaf senescence of ND60, YD 13, SD7 and ZD958 was delayed as shown by the remaining GLA per plant at maturity. However, in BMY, DY6, YD2, and XY335, prevention of pollination did not have an effect on leaf senescence. Even more, in hybrids ZD2, DY13, and YD4, prevention of pollination resulted in faster leaf senescence as indicated by reduced GLA at maturity. Physiologically, the response of leaf senescence to pollination prevention can be at least partially explained by the change of leaf NRE.3) By the greenhouse pot experiments in two years, we investigated the effect of source and sink manipulation on nitrogen remobilization and leaf senescence in ZD958 and ZD2. The results showed that with pollination prevention manipulation, the relative expression level of ZmSee2p which was related to leaf senescence was lower in ZD958 compared to control. The reason for leaf senescence of ZD958 delayed in prevention pollination treatment was that, leaf nitrogen import was bigger than export after silking. On the other hand, post-silking nitrogen uptake in ZD2 was reduced in prevention pollination treatment, and the rate of leaf protein degradation was much rapidly with higher sugar in no pollination plant. The reason for leaf senescence of ZD2 accelerated was probably that, there was a certain gene which related to the leaf senescence and nitrogen movements in leaf.In conclusion, during grain filling stage, source and sink relationship could regulate dry matter and nitrogen accumulation. Genotypical difference existed in the response of leaf senescence and nitrogen remobilization to source and sink manipulation. Based on normal growth condition, delay nitrogen supply time appropriately is beneficial for grain nitrogen accumulation. Defoliation and partial pollination reduced grain nitrogen accumulation, therefore, good source and sink ratio is favorable in obtaining a high grain yield.