Needs for Increased N Uptake and NUE

Factors influencing crop production and yield such as increased photosynthesis (see above), or determining mineral nutrient availability (as influenced by rainfall patterns) will impact on demands for nutrients and efficiency of use; this applies equally to all nutrients but is exemplified by N, a major limit on yield and a costly input for production. As additional N inputs are to be preferentially avoided for both economic and environmental reasons, increases in NUE are preferred. The overall trait of NUE is the product of two complex subtraits, namely N uptake efficiency (NUpE) and N utilization efficiency (NUtE) (Fig. 8.2). NUpE (N taken up by the crop as a function of the available N) is a trait associated with root characteristics of both architecture and function, including activity of transporters and assimilatory pathway enzymes. NUtE (yield of harvested material as a function of the total N taken up by a crop) is dependent on canopy functions such as photosynthesis and nutrient recycling within the plant. These are independent traits although in some cases crop height/dwarfism, which will strongly influence harvest index and utilization efficiency, may be related to root growth (Wojciechowski et al., 2009). In general, each of these subtraits needs to be independently selected for in relation to the required crop ideotype and the anticipated climate conditions of the production regions.

One impact of elevated CO2 may be increased C allocation to below-ground material, which ought to enhance both N and water acquisition ability. Selection for varieties which are responsive with respect to such root proliferation capacity will be beneficial for improving N and water-use efficiency (WUE), particularly as other consequences of climate change may be to reduce water availability and consequently limit nutrient acquisition. In addition to this, decreased transpiration and mass flow brought about by high CO2 will also negatively impact on N acquisition. Crop improvement in root functioning has been suggested as a possible next Green Revolution (Lynch,

Fig. 8.2. Possible targets to increase N-use efficiency (NUE) in crops (see text for explanation). NUpE, N uptake efficiency; NUtE, N utilization efficiency.

2007) and will be pivotal for maximizing water and mineral acquisition in challenging environments.

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