The physiological basis of genetic improvement in maize (Zea mays L.) yield in the US Corn Belt

Abstract

Maize, along with rice and wheat, provides the foundation of human food supply. Ecologicalintensification of maize cropping systems and accelerated genetic improvement of maize yield willbe necessary to satisfy an increasing demand from rapid population growth (Cassman 1999).Understanding of the physiological basis underpinning genetic improvement of maize can informbreeding 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 seriesof field studies using the so-called ERA hybrid set (Duvick et al. 2004a), which representcommercially successful hybrid releases over that period, demonstrated that yield improvementresulted from the interaction between genotype and plant population, and documented that yieldadvance was associated with increased leaf angle score and stay-green score, and decreases inanthesisnsilking interval (ASI), tassel size scores, and barrenness. Changes in leaf angle scores andstay green scores have been implicated as determinants of increased radiation use efficiency and watercapture (Hammer et al. 2009a; Messina et al. 2009). Increased kernel number per unit area via reducedbarrenness, shorter ASI, and reduced tassel size suggest that biomass allocation to the ear may havechanged 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 duringearly ear growth have been implicated as the major physiological determinants of the yield increasein short-season maize (Tollenaar et al. 2006a).We have a cursory understanding of the physiological drivers of yield improvement in temperatemaize. This study provides empirical evidence to evaluate previously proposed hypotheses (i.e., watercapture; Hammer et al. (2009a)) and deductions from field observations. This study utilizes a cropgrowth and development framework structured on concepts of resource capture, utilization efficiencyand allocation to guide field experimentation. The experimental component of this study includesfield experiments in managed stress environments located in USA and Chile during the growingseasons 2012 and 2011/2012 respectively, seeking to quantify genotypic differences in light and watercapture, crop and ear growth, and resource allocation. Two seasons of experiments in 2012 utilizingthe lysimeter and root chamber facilities located at UQ have been used to quantify genotypicdifferences in transpiration dynamics and efficiency, biomass allocation, and sensitivities to droughtstress. Experimental work used a contrasting subset of single crosses from the ERA hybrids with seedavailable in Australia. All the experiments were planted at the highest plant density used in eachlocation and platform.Measurement of soil water during two years of field experiments showed that total water captureunder water-limited conditions did not differ between hybrids from another subset of the ERA hybridsreleased at different decades. However, there was evidence of changes in timing of water uptake overthe season. No significant trends with year of release were observed in transpiration or transpirationefficiency in the two experiments conducted at the Gatton lysimeter platform, though significantdifferences existed between hybrids according to pairwise comparison (p-valuel0.05). Fieldexperiments in Chile and California revealed a trend in radiation use efficiency for a subset of theERA hybrids that could account for some of the yield advance in well-watered environments.Genotypic differences in RUE were observed, and RUE increased at a rate of 0.0057 g MJ-1 y-1,which translates into 5 g m-2 y-1 in yield. This value is roughly 50% of the well documented trendin yield for irrigated corn. Two hypotheses are derived from this research; a) the improved RUE wasdue to improved tolerance to stress determined by leaf erectness reducing radiation load and canopytemperature, and improving water status, and b) the improved RUE is a manifestation of higher Nstatus due to improved pre-flowering N uptake, staygreen during grain fill, and maintenance ofassimilation.Finally, based on the experimental data resulting from field and the lysimeter platforms, genetic gainin yield and increase in number of kernels per unit land area were found to be underpinned by changesin partitioning around flowering. These changes were not equivalent in magnitude to the totaldifferences in reported grain mass, but enough to alter the changes in carbon flow and partitioning infavor of the ear and kernels to support increased kernel set. These changes, in turn, enabled theincrease in planting densities that support observed increases in grain mass. Trends were observedwith year of release in specific leaf nitrogen (increase) and tassel size (decrease). Morphologicalchanges in leaves during vegetative growth may underpin increased capacity to store N. Aroundflowering time both carbon and hormonal (e.g. cytokinin and abscisic acid) regulation, associatedwith a higher carbon assimilation due to increased RUE and reduced tassel size can increasereproductive sink size. Increased nitrogen concentration in leaves could support higher carbonassimilation during grain fill resulting in an increased grain yield.Results from this project provide information to improve the understanding of the physiologicaldeterminants of yield improvement in temperate maize, and document genotypic differences thatcould inform phenotyping and maize breeding efforts worldwide.