Sign up for email alerts of new Fluid Journal issues!
Fluid Journal : Summer 2016
18 The Fluid Journal Summer 2016 with the stressed environments and they found a non-linearity between seed number per m2 and crop growth rate, suggesting a decoupling between vegetative growth and reproduction that may constrain yield potential. These results, using multiple numbers of accessions, will be necessary to determine the factors that limit yield potential. Nitrogen Nitrogen is key determinant of plant productivity and fulfilling the future food needs of the world will require an adequate supply of N. The projected increase in N supply for the increased production and the amount of N to guarantee food security has been estimated to be 250 Mt year-1 (Tilman et al., 2011). If we examine Eq  and the two efficiency terms ei and ec , then the essential role of N in the production of plant proteins and the expansion and functioning of photosynthetic area is necessary to allow plants to achieve their potential in growth and yield (Grindlay, 1997; Sinclair and Horie, 1989). The foundation for gauging the N response can be seen through RUE, which is a representation of the photosynthetic capacity of the leaves and canopies that are directly related to concentration of N per unit area of leaf (Monteith, 1977). Hatfield and Parkin (2015) observed that maintenance of green leaf area and delayed senescence was positively related to grain yield in maize. As these N concentrations fall below a critical threshold, RUE decreases (Massignam et al. 2009) and as N supply decreases, leaf area expansion decreases and senescence increases (Massignam et al. 2011). Three responses to a shortage of N for the vegetative period of growth are possible as defined by Lemaire et al. (2008): (i) reduce leaf area (light capture) and maintain leaf N content (photosynthetic capacity), creating a reduction in the radiation interception while maintaining RUE; (ii) maintain leaf area expansion and reduce leaf N content, which maintains the radiation interception but reduces RUE, or (iii) a combination of both responses. Management of N will become a major limitation for plant productivity because of the need to increase N use efficiency (NUE). Two avenues need to be more fully explored by agronomists to achieve greater NUE: (i) utilizing enhanced efficiency fertilizers to increase the availability of N later in the growing season (Hatfield and Parkin 2014); and (ii) understanding the potential of increased soil biological fertility in which the activity of the microbial system recycles nutrients in the soil profile and makes them available to the plant (Abbott and Murphy, 2007; Lazcano et al. 2013, Rogers and Burns, 1994). In both of these cases, there are indications that these N management practices increase the duration of green leaf area and the photosynthetic efficiency of the canopies. This response is similar to the observations by Gous et al. (2013) on the maintenance of green leaf area in sorghum offering an advantage to grain yield. Hatfield and Parkin (2014) demonstrated that both a chlorophyll summation index and a senescence index based on remote sensing were related to grain yield because of the greater duration of green leaf area in the grain-filling period induced by the enhanced efficiency fertilizer. The yield parameter affected by this change was the weight per 100 kernels, suggesting that increased photosynthetic area and duration would increase the capacity of the maize plant to fill the grain. Improved management of N along with all nutrients will be necessary to capture the yield potential of all crops and increase vegetative and grain quality. Teixeira et al. (2014) observed an interaction between water and N stresses on maize, which affected the ability of the plant to achieve optimum use of the natural resources. This evaluation of NUE and WUE under water or N stresses provides a framework for evaluating genotypic responses. Nitrogen requirements of worldwide crops to achieve nutritional security are estimated to be in excess of 150 million MT by 2050. Other challenges IfweutilizetheGxExMapproach as our foundation for meeting future food needs, we have the opportunity to examine the potential of each component. One of the largest components will be the environment, which includes the soil and atmosphere. However, the weather variations within and among growing seasons and among locations is expected to increase, leading to more variation of crop growing conditions. Ray et al. (2015) concluded that climate variation led to global yield variation in maize, rice, wheat, and soybeans. The magnitude of this variation was nearly 40%. Climate variation in temperature and precipitation were considered the primary climate factors affecting crop yield. Since drought is a major limitation to crop yield, supplying irrigation water is a viable option for some locations; however, attention to improving water availability for the crop by increasing the potential storage of soil water in the soil profile and decreasing the evaporation component of evapotranspiration also offer promise (Hatfield et al. 2001). Degradation. One of the first challenges in meeting global food demands will be maintenance of a high- quality soil resource and its ability to provide adequate water and nutrients, and sufficient rooting depth for the plant to obtain these resources (Sakschewski et al. 2014). Lal (1993) proposed that soil degradation (chemical, physical, and biological) is extensive throughout the world and especially so in the tropics and subtropics. Because water availability is critical to the crop, the changes in soil structure and saturated hydraulic conductivity related to cropping systems, i.e., tillage and residue removal, led to a degradation of soil structure in the profile causing maize yield reductions as large as 50% (Wang et al., 1985). This decrease in yields could be partially explained by shallow root growth and limitations of water availability to the growing plant. Impacts of poor soil structure on plant growth and yield can be significant and thus continued degradation of the soil resource will have a major impact on the ability of the plant to produce grain, fiber, or forage. Degradation of the soil resource will also exaggerate the effects of a variable climate and limit the ability of the genetic resource to achieve its potential. Water use by crops is dependent on the available water resource in the soil and under rain-fed conditions. Wessels et al. (2007) found that degraded soils have reduced precipitation use efficiency for rangeland soils. De Vita et al. (2007) observed increased durum