Carbon and nitrogen metabolism related to grain formation in two different senescent types of maize

Field experiments and plant analyses were carried out to investigate the changes of ribulose-1,5-bisphophate carboxyase/oxygenase, phosphoenolpyruvate carboxylase, and chlorophyll in two maize (Zea mays L.) cultivars and their relationship to grain formation as regulated by different nitrogen applications. Two maize hybrids included "Danyu 13," an earlier senescent hybrid, and "Zhongdan 306," a stay-green hybrid. There were four nitrogen (N) treatments: (I)NI: 0 kg N ha(-1), (2) N2: 250 kg N ha(-1) (1/5N as basal application and 4/5N as top-dressing at stalk elongation stage), (3) N3: 250kg N ha(-1) (1/5N as basal, 2/5N as top-dressing at stalk elongation stage and at tasselling stage, respectively), (4) N4: 400kg N ha(-1) (1/5 N as basal and 4/5 N as top-dressing at stalk elongation stage). Results showed that biological yield and grain yield for "Zhongdan 306" were higher than "Danyu 13" in each N treatment, and the maximum grain yield was obtained in N2 and N3 treatment for "Danyu 13" and "Zhongdan 306," separately. The amount of N, chlorophyll (chl), and Rubisco-N in leaves for "Zhongdan 306" was clearly higher than "Danyu 13" in each N treatment during entire growth stage. Synthesis of Rubisco and chl, especially Rubisco, played a very important role in the nitrogen supply for grain formation for two hybrids. Excessive N application (N4 treatment) could not increase grain yield further in both hybrids. For "Zhongdan 306," the carbon sources in grain were 87-96% of total carbon (C) from leaf photosynthesis and 5-14% of total C from mobilized C, while the nitrogen sources in grain were 69-90% of total nitrogen absorbed by roots from soil and 10-31% of total nitrogen from mobilized N. For "Danyu 13," the C from photosynthesis and mobilized C accounted for 66-81% and 19-34% respectively. The mobilized N and N absorbed by roots from soil accounted for 48-68% and 32-53%, separately. That is, for stay-green variety, the C and N in grain was depended mainly on the larger retranslocation of photosynthetic C and N absorbed by roots during maturation, and smaller retranslocation of C and N existed in vegetative mass before silking stage, whereas that for earlier senescent hybrid, relied upon the larger retranslocation of C and N existed in vegetative mass before silking stage, and smaller retranslocation of C from photosynthesis and N absorbed by roots during maturation.