Unlike all other major grain crops, maize (Zea mays L.) is a monoecious species, with a separate male flower (tassel) located at the top of the stem and female flowers (ears) located about midway down on the same plant. This spatial arrangement of the flowers facilitates both selfing (pollination of the female flower with pollen from the same plant) and crossing (pollination of the female flower with pollen from a different plant). Several characteristics make it an attractive genetic system retaining the major strength of plant genetics (Strable and Scanlon 2009). This use of maize is particularly important for increasing our understanding of the agronomic and the genetic bases of yield performance, related to the genomic changes that have occurred with domestication and breeding (Solomon et al., 2014; Shi and Lai, 2015).
Maize is one of the best model plants to combine physiological and agronomic studies. It is a C4 grass with a specific cellular compartmentation that allows a better metabolic efficiency in terms of carbon and nitrogen assimilation (Cliquet et al. 1990; Oaks, 1994). Compared with C3 crops, maize uses Co2, solar radiation, water and nitrogen more efficiently during photosynthesis. The water-use efficiency of maize is approximately double that of C3 crops grown at the same sites. In addition, maize was chosen as a model system, as for a given genotype, the number of leaves that emerge under specific growth conditions is genetically determined (Fournier and Andrieu 1999). Therefore each leaf can be easily numbered, and a single plant provides a large set of leaves in which there is a gradation of the remobilization process when senescence is progressively taking place.
Maize life cycle has different growth stages with distinctive physiological and agronomic features
Maize life cycle is divided into two main growth stages; vegetative and reproductive. Each of these stages has its own physiological characteristics. Vegetative growth stages are determined using the Leaf Collar method. A plant is assigned a growth stage depending on the number of visible leaf collars present. Vegetative stages defined by this method are named ‘V’ stages. Reproductive stages are defined by using an ‘R-stage’ and subdivided into two stages; anthesis or male flowering (R1) and physiological maturity (R6). R1 is the growth stage at which pollen shed begins whereas R6 describes the physiological maturity which is identified by the appearance of black layer at the conjunction of maize kernels with the cob (Hill, 2007; IITA and CIMMYT, 2007).