Biotic Stress and Yield Loss

partitioning of nitrogen to leaves. Similarly, what will be the influence of nitrogensupply on the partitioning of new biomass among plant organs?In a recent study in rice, Hasegawa and Horie 73 suggested that the effect of nitrogensupply on leaf area development was more important to crop growth and yieldthan its effect on N L,iand subsequent CO 2assimilation. Although maize and velvetleafleaf nitrogen content clearly differed among nitrogen application treatments(Figure 13.2), 57 maximum observed LAI and height of maize and velvetleaf arestrongly influenced by nitrogen application rate (Figure 13.3). This observation suggeststhat when nitrogen is in short supply, plants make a tradeoff when allocatingnew biomass between tissues necessary for acquiring nitrogen vs. light. In otherwords, if nitrogen uptake is reduced due to limited nitrogen supply, plants partitionless new growth to leaves and more to roots. This provides the plant with greater rootbiomass and volume so it can acquire more nitrogen, and it allows the plant to maintaina relatively constant nitrogen concentration in existing leaves. However, it limitsthe production of leaf area and therefore potential growth rate. Because maize is a C 4species and expected to have greater nitrogen and radiation use efficiency than velvetleaf,root growth will need to increase more in velvetleaf than maize to maintainA max (umol CO 2 m -2 s -1 )5040302010Nrate = 0Nrate = 20Nrate = 40Nrate = 80Nrate = 16000.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5Leaf N (g N -2 )FIGURE 13.2 Maize (open symbols) and velvetleaf (closed symbols) net CO 2assimilationrate (A max, mol CO 2m 2 s 1 1.7536 10 5 g CO 2m 2 s 1 ) under full sun conditions inresponse to nitrogen content (N L) of the most recent fully expanded leaf in plots with nitrogenapplication (Nrate) ranging from zero to 160 kg N ha 1 . Redrawn from Lindquist andMortensen. 57