Dynamic Nature of Heterosis and Determination of Sink Size in Maize
Heterosis, the phenotypic superiority of first generation progeny over that of its inbred parents, has been extensively investigated. However, differences in the phenology and dynamic growth patterns between parents and offspring provide challenges in understanding causal factors behind superior trait values. For maize, manipulation of heterosis to increase grain yield has been of primary importance, and the number of spikelets that develop on the female inflorescence is the primary determinant of grain yield. The initial experiment examined heterosis in genetic backgrounds that led to minimal differences in phenology and plant architecture. Growth curves were used to characterize the dynamic expression of heterosis between the hybrid and the inbred parents for a series of vegetative and reproductive traits across stages of development. The second experiment was conducted to determine the effects that stress due to planting density might have on the number of properly developed spikelets, as the first experiment, along with results from the literature, provided evidence to suggest that a proportion of the later forming spikelets found on the distal portion of the female inflorescence were not capable of producing kernels. Results from the initial experiment suggested that expression of heterosis for individual characteristics – such as fresh weight, whose percent mid-parent heterosis was 82% at V4 and declined steadily to 17% at V11 – begins at a high level and decreased during development. On a whole plant level – as determined by a combined analysis of the values and growth rates of the individual characteristics – heterosis increases throughout development until it reaches a steady-state level. Results from the second experiment indicated that increasing plant density did not affect the total number of spikelets per ear but decreased the number of kernels per ear, and it was found that the number of properly developed spikelets per ear was equal to the number of kernels per ear in eight of the nine genotypes tested. Optimal growth stages for more in-depth investigation of transcriptomic changes that may identify causal genetic factors of heterosis for yield were not found, and stress increases the proportion of improperly developed spikelets causing a loss in kernel number.