Effect Of Nitrogen On The Kernel Sink Development In Summer Maize (Zea Mays L.)

Nitrogen supply affects the carbon-nitrogen metabolism during plant growth in summer maize (Zea mays L.). Dry matter production, kernel number determination and kernel development were investigated under different rates and times of nitrogen application. The influence of nitrogen availability on kernel growth in vitro was also studied. The mechanism of nitrogen affecting kernel development was discussed by comparing the results of in vitro culture and field trials.The growth of upper and middle kernels was quite different in summer maize. The upper kernels were very important to increase ear weight although they were only a small part of the ear. Increasing the nitrogen supply appropriately during early and middle growth stage of summer maize promoted kernels growth (especially for upper kernels), which led to high kernel number (KN), kernel weight (KW) and grain yield significantly. Nitrogen deficiency during early growth stage resulted in reduction of KN, KW and grain yield. Excess nitrogen supply.in early growth or silking stage had no obvious effects on KN, KW and grain yield.The increased KN by nitrogen was mainly due to the reduced kernel abortion while spikelet number was not influenced. Increasing the nitrogen supply during early and middle growth stage improved the assimilate and nitrogen level of plant, so as to enhance dry matter production and plant growth rate, which was helpful to kernel growth. Increasing the nitrogen supply in the silking stage had no effect on the dry matter production, but the assimilate supply for kernel growth was reduced due to the competition between carbon and nitrogen metabolism.Increasing the nitrogen supply during early and middle growth stage enhanced carbon-nitrogen metabolism and the assimilate supply for kernel, so as to promote the endosperm cell division and to enlarge the sink capacity of kernel and ear. Meanwhile, the activity of enzymes related sucrose metabolism (sucrose invertase, sucrose synthase, ADPG-ppase and starch synthase) and starch deposition were also increased, which led to high KW. Nitrogen had more obvious effect on the growth of upper kernels than that of middle kernels. Increasing nitrogen supply reduced the difference between upper and middle kernels growth, and improved the ear weight. Nitrogen deficiency in the early and middle growth stage reduced the sink capacity and activity, which resulted in lower KW. Increasing nitrogen supply in silking stage had little effect on the kernel development.The growth difference of upper and middle kernels was still obvious in vitro culture, and upper kernels were more sensitive to nitrogen stress. Sufficient nitrogen supply during grain filling period promoted kernel growth by improving carbon-nitrogen metabolism and sink activity of kernels. Nitrogen shortage during grain filling period decreased the sink metabolism and activity, which hindered the sucrose decomposition and starch synthesis, and finally resulted in small KW. Nitrogen stress during endosperm cell division or linear filling stage reduced the kernel sink capacity and influenced grain filling rate separately, and all of them was unfavorable to kernel weight increase. The impact of nitrogen stress at different stage on kernel growth was: stress during grain filling period > during linear filling period > during endosperm cell division.Nitrogen supply had both indirect and direct effects on the kernel development. On the one hand,nitrogen influenced the plant dry matter production so as to change the assimilate supply for kernel growth. On the other hand, nitrogen affected the activity of enzymes related to kernel growth. Within the range of 90~180kg N/hm2, kernel weight and grain yield could be improved significantly by the fertilizer application with the ratio of 1:3 for sowing stage and spikelet differentiation stage.