Nitrogen (N) is an essential requirement for kernel growth in maize (Zea mays); however, little is known about how N assimilates are metabolized in young earshoots during seed development. The objective of this study was to assess amino acid metabolism in cob and spikelet tissues during the critical 2 weeks following silking. Two maize hybrids were grown in the field for 2 years at two levels of supplemental N fertilizer (0 and 168 kg N/ha). The effects of the reproductive sink on cob N metabolism were examined by comparing pollinated to unpollinated earshoots. Earshoots were sampled at 2, 8, 14, and 18 d after silking; dissected into cob, spikelet, and/or pedicel and kernel fractions; then analyzed for amino acid profiles and key enzyme activities associated with amino acid metabolism. Major amino acids in the cob were glutamine (Gln), aspartic acid (Asp), asparagine (Asn), glutamate, and alanine. Gln concentrations dropped dramatically from 2 to 14 d after silking in both pollinated and unpollinated cobs, whereas all other measured amino acids accumulated over time in unpollinated spikelets and cobs, especially Asn. N supply had a variable effect on individual amino acid levels in young cobs and spikelets, with Asn being the most notably enhanced. We found that the cob performs significant enzymatic interconversions among Gln, alanine, Asp, and Asn during early reproductive development, which may precondition the N assimilate supply for sustained kernel growth. The measured amino acid profiles and enzymatic activities suggest that the Asn to Gln ratio in cobs may be part of a signal transduction pathway involving aspartate aminotransferase, Gln synthetase, and Asn synthetase to indicate plant N status for kernel development.Though there are numerous studies concerning growth and development of maize (Zea mays) grain, there is a general lack of knowledge concerning the physiology of the cob tissues, to which kernels are attached. This is despite the fact that the cob tissues are the link between vegetative source and reproductive sink tissues. In addition, approximately one-half of the earshoot (minus husk and shank) is cob material at silking, with the cob proportion decreasing as the grain develops. Early studies on the physical characteristics of the earshoot and cob go back more than a century (Harshberger, 1893). More recent research has examined cob shape (Srinivas et al., 1991;Bhattacharya et al., 1996;Orr et al., 1997;Pearson, 2000) and the genetic control of cob color (Nguetta and Cross, 1997;Sidorenko et al., 1999). A few studies have also focused on sugars (BeMiller and Hoffmann, 1972), nitrogen (N;Crawford et al., 1982), minerals (Mozafar, 1990), and antioxidants (Cevallos and Cisneros, 2003) contained within maize earshoots.A typical view of maize cobs is that they serve as a temporary storage organ and as a conveyor of nutrients to the developing kernels (Crawford et al., 1982). A larger metabolic role of the cob in kernel growth, however, can be inferred from the observation that an attached cob ...

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The relative role of the maternal source and the filial sink in controlling the composition of maize (Zea mays L.) kernels is unclear and may be influenced by the genotype and the N supply. The objective of this study was to determine the influence of assimilate supply from the vegetative source and utilization of assimilates by the grain sink on the final composition of maize kernels. Intermated B73×Mo17 recombinant inbred lines (IBM RILs) which displayed contrasting concentrations of endosperm starch were grown in the field with deficient or sufficient N, and the source supply altered by ear truncation (45% reduction) at 15 d after pollination (DAP). The assimilate supply into the kernels was determined at 19 DAP using the agar trap technique, and the final kernel composition was measured. The influence of N supply and kernel ear position on final kernel composition was also determined for a commercial hybrid. Concentrations of kernel protein and starch could be altered by genotype or the N supply, but remained fairly constant along the length of the ear. Ear truncation also produced a range of variation in endosperm starch and protein concentrations. The C/N ratio of the assimilate supply at 19 DAP was directly related to the final kernel composition, with an inverse relationship between the concentrations of starch and protein in the mature endosperm. The accumulation of kernel starch and protein in maize is uniform along the ear, yet adaptable within genotypic limits, suggesting that kernel composition is source limited in maize.

ABSTRACTe need to intensify agricultural production due to a growing human population requires yield gaps to be closed. In 2009 and 2010, ve management factors were assessed for their individual and cumulative contributions to reducing the corn (Zea mays L.) yield gap and yield components in a corn-soybean [Glycine max (L.) Merr.] rotation. Five management factors (plant population, transgenic insect resistance, fungicide containing strobilurin, P-S-Zn fertility, and N fertility) were evaluated. An incomplete factorial design with these factors resulted in 12 treatments, including two controls: high technology (HT) and standard technology (ST), comprising all ve factors applied at the supplemental or the standard level, respectively. e HT control yielded 2.9 Mg ha -1 (2.12-3.50 Mg ha -1 across sites and years) more grain (28%) than the ST control, demonstrating the yield gap between traditional farm practice and attainable yield using available technologies. All management factors except plant population were necessary for reducing the yield gap. Fungicide and Bacillius thuringiensis gene (Bt) traits provided the greatest yield increases compared to the ST system. Averaged over sites and years, if each factor was withheld from the HT system, yield decreased by decreasing kernel number. Increased plant population reduced the yield gap when all other inputs were applied at the supplemental level. Kernel number was more signi cant for increasing yield than kernel weight. e yield contribution of each factor was greater when applied as part of a full complement of supplemental inputs than when added individually to the standard input system. Abbreviations: Bt, Bacillius thuringiensis gene; -Bt, HT control system with non-Bt variety; +Bt, ST control with Bt-containing variety added; CU, Champaign-Urbana study site; DS, Dixon Springs Research Center site; -Fung, HT control treatment without fungicide; +Fung, ST control treatment with added fungicide; HT, high technology level; +N, additional side-dress N to ST control; -N, HT control without side-dress application; +Pop, high population treatment added to ST control; -Pop, decreased population treatment within HT control; +P-S-Zn, P, S, and Zn combination fertilizer added to ST control; -P-S-Zn, HT control treatment without P-S-Zn combination fertilizer; ST, standard technology level; VT, tasseling.Numerous recent papers have established that agricultural production must increase substantially to meet the increasing per capita demand for food, feed, fuel, and fi ber of a burgeoning human population (Keyzer et al., 2005, Food and Agriculture Organization, 2009. From a global perspective, it is generally agreed that agricultural intensifi cation (increasing agricultural production per unit area) is preferable to extensifi cation (expanding agriculture onto new areas) as a means of increasing crop production Burney et al., 2010;McLaughlin, 2011;Tilman et al., 2011;Foley et al., 2011). In contrast to the question of whether yield increases are needed, the more practical i...