The physiological basis of genetic improvement in maize ... - UQ eSpace [PDF]

The physiological basis of genetic improvement in maize (Zea mays L.) yield in the US Corn Belt Andres F. Reyes Ponce Master of Science

A thesis submitted for the degree of Doctor of Philosophy at The University of Queensland in 2016 Queensland Alliance for Agriculture and Food Innovation

Abstract Maize, along with rice and wheat, provides the foundation of human food supply. Ecological intensification of maize cropping systems and accelerated genetic improvement of maize yield will be necessary to satisfy an increasing demand from rapid population growth (Cassman 1999). Understanding of the physiological basis underpinning genetic improvement of maize can inform breeding efforts and improve tailoring of maize hybrids to intensified cropping systems. Maize yields in the US Corn Belt have increased at a rate of 1% a year for over 70+ years. A series of field studies using the so-called ERA hybrid set (Duvick et al. 2004a), which represent commercially successful hybrid releases over that period, demonstrated that yield improvement resulted from the interaction between genotype and plant population, and documented that yield advance was associated with increased leaf angle score and stay-green score, and decreases in anthesis–silking interval (ASI), tassel size scores, and barrenness. Changes in leaf angle scores and stay green scores have been implicated as determinants of increased radiation use efficiency and water capture (Hammer et al. 2009a; Messina et al. 2009). Increased kernel number per unit area via reduced barrenness, shorter ASI, and reduced tassel size suggest that biomass allocation to the ear may have changed as the result of selection for yield in the ERA hybrids (Duvick et al. 2004a; Hammer et al. 2009a). Increased total biomass and increased partitioning to the kernels around silking and during early ear growth have been implicated as the major physiological determinants of the yield increase in short-season maize (Tollenaar et al. 2006a). We have a cursory understanding of the physiological drivers of yield improvement in temperate maize. This study provides empirical evidence to evaluate previously proposed hypotheses (i.e., water capture; Hammer et al. (2009a)) and deductions from field observations. This study utilizes a crop growth and development framework structured on concepts of resource capture, utilization efficiency and allocation to guide field experimentation. The experimental component of this study includes field experiments in managed stress environments located in USA and Chile during the growing seasons 2012 and 2011/2012 respectively, seeking to quantify genotypic differences in light and water capture, crop and ear growth, and resource allocation. Two seasons of experiments in 2012 utilizing the lysimeter and root chamber facilities located at UQ have been used to quantify genotypic differences in transpiration dynamics and efficiency, biomass allocation, and sensitivities to drought stress. Experimental work used a contrasting subset of single crosses from the ERA hybrids with seed available in Australia. All the experiments were planted at the highest plant density used in each location and platform.

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Measurement of soil water during two years of field experiments showed that total water capture under water-limited conditions did not differ between hybrids from another subset of the ERA hybrids released at different decades. However, there was evidence of changes in timing of water uptake over the season. No significant trends with year of release were observed in transpiration or transpiration efficiency in the two experiments conducted at the Gatton lysimeter platform, though significant differences existed between hybrids according to pairwise comparison (p-value