Role of N in Controlling Kernel Growth in Maize

82 p.Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 1997.During the grain-filling period, the supply of N is crucial for kernel growth in maize. Studies were conducted to understand if N has a direct nutritional effect on kernel development or acts indirectly by affecting the supply of photoassimilates. Plants grown in the field with 0 kg N ha$\sp{-1}$ (N-deficient) and 180 kg N ha$\sp{-1}$ (N-sufficient) were stem infused at silking with 240 mg of N, or 15 g of sucrose, or a combination of both (N + sucrose). Control plants were stem-infused with 0.05 mM ethylenediaminetetraacetic acid. To determine the recovery and subsequent distribution of N within the plant, N-deficient plants were also stem-infused with $\sp{15}$N-glycine, or were supplied with $\sp{15}$N-NO$\sb3$ applied to the soil surface. At 3 days after pollination, kernels taken from the plants grown with deficient or sufficient N were cultured in vitro with either a low (0 mM) or a high (30 mM) level of N. Changes in endosperm cell number, starch granule number, invertase activity, and reducing sugar concentration of the endosperm were measured for all treatments. Altering N availability by both stem infusion and kernel culture affected kernel growth through changes in kernel sink capacity, invertase activities, and assimilate flux into the endosperm. One day after stem infusion, all the $\sp{15}$N was recovered in the above-ground portions of the plant. By 6 days after stem infusion, the $\sp{15}$N was distributed in the plant in the same way as if it had been absorbed from the soil. Adding extra N or sucrose by stem infusion increased grain yield of N-deficient plants to a similar extent (49 g plant$\sp{-1}),$ but did not increase grain yield of N-sufficient plants. The infusion treatments also increased grain N content to a similar extent (524 mg plant$\sp{-1}).$ Moreover, plants underwent a temporary osmotic shock indicative of a larger role of stem-infused N and sucrose as osmoticum than as a direct nutritional supplement to initial kernel growth. Supplying both N and sucrose in vitro to N-deficient field-grown kernels maintained the flow of assimilate into the endosperm; thereby establishing a strong kernel sink. However, kernels cultured without N failed to grow. These results suggest that increasing N supply via stem-infusion to N-deficient plants might affect photoassimilate flux into the kernel by regulating water intake, while N supply in vitro controls kernel growth by a nutritional enhancement of key metabolic proteins in the endosperm.U of I OnlyRestricted to the U of I community indefinitely during batch ingest of legacy ETD