Maize grain yield can be dissected into component whole crop level physiological processes that occur during various development phases in the life cycle of the plant (Tollenaar and Lee, 2006). Grain yield is the product of accumulating dry matter (source) and allocating a portion of the total dry matter to the grain (sink).
Maize grain yield improvements over the decades have been attributed to 40 % agronomic practices (Duvick, 2005) and 60% being attributed to improved genetic advances; however, realistically 100% of the increase in grain yield is actually due to the interaction between genetics and agronomic practices (Tollenaar and Lee, 2002). Duvick (2005) observed that there were some traits that breeders intended to change and, on the other hand, there were other traits that improved simultaneously when breeders were narrowly focused on enhancing grain yield. One trait that is of consistent focus is the enhancement in source and sink strength, as well as improving the efficiency of nutrient partitioning from source to sink (Tollenaar and Lee, 2011).
One way that abiotic stress acts on the maize plant is to shift source and sink processes out of balance with one another. Excess source capacity, relative to sink capacity, results in other tissues (e.g., leaves, stalks) acting as sinks. Purpling of leaves, sheath tissues, and stalks during the grain-filling period (GFP) are classic symptoms of excess source capacity. Excess sink capacity, relative to source capacity, results in premature senescence of leaves and stalks during the GFP.
Genetic improvements in maize——Physiological overview
Genetic modifications associated with this yield improvement were documented extensively in experiments comparing hybrids from different eras in the same environment (Russell, 1984; Tollenaar et al., 1992; Duvick et al., 2004a; Duvick, 2005; Li et al., 2011; Smith et al., 2014). The fold increase in yield could not have occurred without an increase in source activity per unit area, which has both a rate (i.e., canopy photosynthesis, g CO2 m–2 GA d–1, where GA is ground area) and a time component (principally the duration of kernel filling; Egli, 2011). These modifications include smaller tassels, erect leaves, increased stay-green characteristics of the leaves during kernel filling, less lodging, and improved disease, insect resistance and shorter anthesis – silkng intervals and decrease in seed protein content. Higher radiation use efficiency also contributed to the increase in productivity (Tollenaar and Aguilera, 1992). The increase in DMA can be attributed, in part, to quantifiable changes in light interception due to increased leaf area index (LAI) and changes in light utilization due to more erect upper leaves. Another part of the improvement in DMA is attributable to maintenance of green leaf area and leaf photosynthesis during the GFP.
The relatively constant harvest index during hybrid improvement (Duvick, 2005) also suggests that the productivity of the source increased in step with yield (i.e., higher yields were not due to a simple change in partitioning). Leaf senescence was slower in newer hybrids compared with old hybrids (Ding et al., 2005; Echarte et al., 2008). Longer kernel filling periods also contributed to higher yields (Cavalieri and Smith, 1985). Genetic improvement of maize yield can be related to increased tolerance to various types of stress (e.g., low soil water and/or N availability, increased population density, competition from weeds) and also to increased post-silking dry matter accumulation (Tollenaar and Wu, 1999). The increase in source activity could be responsible for the decrease in barren plants exhibited by modern hybrids (Pendleton et al., 1968; Hernandez et al., 2014) which contributed to the description of modern hybrids as having greater stress tolerance (Tollenaar and Wu, 1999; Duvick et al., 2004b). A key aspect of the productivity of the source per unit area is the interception of solar radiation, which must be near 100% to maximize canopy photosynthesis (Duncan, 1975).
Genetic improvement in visual stay-green ratings during in maize has been well documented (e.g., Duvick et al., 2004). The “visual stay-green” phenotype is defined as a delay in onset of leaf senescence and is visually characterized by maintenance of green leaf area during the GFP. Thomas and Howarth (2000) have defined five types of stay-green, types A to E. The stay-green inherent in modern maize hybrids can be classified as type C—chlorophylls being retained indefinitely, but photosynthetic capacity of the leaf is declining indicating that senescence is occurring (Thomas and Howarth, 2000). Leaf photosynthesis is at maximum at silking and declines during the GFP (Ying et al., 2000; Fig. 5.). This second aspect of the type C definition, decline in photosynthetic capacity, is what we refer to as “functional stay-green.” The genetic improvement in functional stay-green is illustrated by the smaller decline in leaf photosynthesis during the GFP in newer relative to older genotypes (Ying et al., 2000; Tollenaar et al., 2000).