6.2.5  Light x nutrients

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Figure 6.15 Plant-nutrient productivity (biomass formed per unit plant nutrient per unit time) can be inferred from analysis of plants grown in aeroponic culture, and varies with daily irradiance. In (a), plant-N productivity of birch seedlings (Betula pendula) grown under continuous illumination in cabinets was saturated around 30 MJ m-2 d-1. In (b), plant-P productivity of Eucalyptus grandis seedlings in naturally illuminated phytotron cabinets remained unsaturated up to 24 MJ m-2 d-1 ((a) Based on lngestad and McDonald 1989, (b) based on Kirschbaum 1991)


Light and nutrients are not only prerequisites for growth, but show a positive interaction in their effect on growth indices. Plant biomass formed per unit plant nutrient (plant-nutrient productivity) increases with irradiance. Birch seedlings grown in aeroponic units under 24 h illumination and constant environment at Uppsala (Figure 6.15a) and Eucalyptus grandis seedlings in Ingestad units under natural light (Figure 6.15b) provide examples of light effects on N and P productivity. In both cases, nutrient productivity has been calculated in terms of whole-plant biomass formed per day per unit plant N or plant P.

Recall from Equation 6.6 that NAR = (1/A) (dw/dt) = productivity per unit area. In that case, carbon assimilation (biomass gain) was referenced to leaf area per plant. By analogy, nutrient productivity can be referenced to N or P content per plant, so that nitrogen productivity (designated NARN) would be

Similarly, phosphorus productivity (NARP) would be

Both indices are integrated over successive harvests as with NARA, and the same caveats apply, namely both whole-plant biomass and nutrient element content must be increasing exponentially so that a linear relationship exists between whole-plant biomass (W) and plant content of N or P. Leaf-N productivity and leaf-P productivity (i.e. whole-plant biomass increase per unit leaf N or leaf P per unit time) can be derived in the same way.

Plant-N productivity from birch seedlings increases with photon irradiance and approaches an asymptote around 30 mol quanta m–2 d–1 (Figure 6.15a). Plant-P productivity from E. grandis seedlings (Figure 6.15b) can be described by a linear function to c. 24 mol quanta m–2 d–1 and returns numeric values an order of magnitude higher, reflecting the contrasting requirements of these two nutrient elements (Chapter 16). Corresponding estimates of NARN and NARP on a leaf basis can be used as parameters in process-based models of plant growth where canopy assimilation (and hence biomass gain) is simulated from data on canopy light climate and nutrient concentration in leaves (see Sands 1996 and literature cited).