Simulating daily field crop canopy photosynthesis: an integrated software package

Photosynthetic manipulation is seen as a promising avenue for advancing field crop productivity. However, progress is constrained by the lack of connection between leaf-level photosynthetic manipulation and crop performance. Here we report on the development of a model of diurnal canopy photosynthesis for well watered conditions by using biochemical models of C3 and C4 photosynthesis upscaled to the canopy level using the simple and robust sun–shade leaves representation of the canopy. The canopy model was integrated over the time course of the day for diurnal canopy photosynthesis simulation. Rationality analysis of the model showed that it simulated the expected responses in diurnal canopy photosynthesis and daily biomass accumulation to key environmental factors (i.e. radiation, temperature and CO2), canopy attributes (e.g. leaf area index and leaf angle) and canopy nitrogen status (i.e. specific leaf nitrogen and its profile through the canopy). This Diurnal Canopy Photosynthesis Simulator (DCaPS) was developed into a web-based application to enhance usability of the model. Applications of the DCaPS package for assessing likely canopy-level consequences of changes in photosynthetic properties and its implications for connecting photosynthesis with crop growth and development modelling are discussed.

Quantum Yield of Photosystem II and Efficiency of CO2 Fixation in Flaveria (Asteraceae) Species under Varying Light and CO2

The quantum yields of electron transport from photosystem II (PSII) (Φe, determined from chlorophyll a fluorescence), and CO2 assimilation (ΦCO2, photosynthetic rate/light intensity) were measured simultaneously in vivo with representative species of Flaveria which show a progression in development between C3 and C4 photosynthesis and in reduction of photorespiration. These were F. pringlei (C3), F. sonorensis (C3-C4, but lacking a C4 cycle), F. floridana (C3-C4, with partially functional C4 cycle), F. brownii (C4-like) and F. bidentis (C4). The level of PSII activity with varying CI under 210 mbar O2 was very similar in all species. However, the progressive development of C4 characteristics among the species produced an increased efficiency in utilisation of PSII derived energy for CO2 assimilation under 210 mbar O2, due to reduced photorespiratory losses at low CO2 levels. In all species, when photorespiration was limited by low O2 (20 mbar), there was a linear or near linear relationship between the quantum yield of PSII v. the quantum yield of CO2 fixation with varying intercellular levels of CO2 (Ci) indicating that CO2 fixation in this case is linked to PSII activity. When switching from 20 to 210 mbar O2 at atmosphere levels of CO2, there was a similar decrease in the efficiency in utilising PSII activity for CO2 assimilation at different light intensities, but the degree of sensitivity to O2 progressively decreased among the species concomitant with the development of C4 photosynthesis. These results may help explain why there is an advantage to evolution of C4 photosynthesis in environments where Ci becomes limiting.