Effect of maize plant morphology on the formation of apical kernels at different sowing dates and under different plant densities

In maize (Zea mays L.), final grain yield is largely determined by kernel number at harvest. The goal of this research was to assess the source of kernel loss and identify the main factors in the process of kernel formation among maize hybrid genotypes comprising semi-compact (JH5) and compact (LD9066) genetic backgrounds (with contrasting tolerance to stand density) under field conditions. Field experiments were conducted in 2014 and 2015. Two hybrids were grown at three sowing dates (early April: SD1, early May: SD2, and late May: SD3) and under three plant densities: D1 (5.25 plants m−2), D2 (6.75 plants m−2), and D3 (8.25 plants m−2). Two sources of kernel loss were assessed and associated with 1) silks failing to emerge from the husk 7 days after silking (loss1) and 2) kernel abortion during the grain-filling period (loss2). The results demonstrated that floret number was significantly influenced by maize hybrid: the semi-compact maize hybrid JH5 had more florets than did the compact maize hybrid LD9066. However, the final kernel number per plant (FKNP) of JH5 was slightly lower than that of LD9066. JH5 had a larger value for both loss1 and loss2 than did LD9066 (22.6% vs 11.9% and 21.0% vs 12.1%, respectively). Moreover, sowing date and plant density also significantly influenced the source of kernel loss. SD1 exhibited a greater value of loss1 than did SD2 and SD3 (24.9%, 15.8% and 11%, respectively) but a lower value of loss2 than did SD2 and SD3 (10.0%, 20.6% and 19.6%, respectively). Plant density had no significant effect on loss1; however, the highest plant density (D3) resulted in the highest loss2 among three plant densities (9.4%, 15.9% and 25.9%). The dynamics of visible silking showed that JH5 requires more time (+1.5 days) to reach 50% maximum visible silks than does LD9066 and has a lower proportion of maximum visible silks at the seventh day after silking. In addition, the apical kernels of JH5 had a relatively longer Dmax (days needed to reach the maximum grain-filling rate) and greater Gmax (maximum grain-filling rate) than did those of LD9066 (+0.5 day and +3.5 mg kernel−1 d−1, respectively). However, the Wmax (kernel weight at the maximum grain-filling rate) and P (active grain-filling period) of JH5 were much lower than those of LD9066 (-6.4 mg kernel−1 and −8.2 days, respectively). Compared with the semi-compact hybrid, the compact hybrid showed an enhanced ability to resist abiotic stress tolerance, with good fertilization synchronization and a low percentage of kernel abortion. These above-mentioned traits should be considered in breeding programs to further increase the maize yield in China.