We appraised the literature and described an approach to estimate the parameters of the Farquhar, von Caemmerer and Berry model using measured CO2 assimilation rate (A) and photosystem II (PSII) electron transport efficiency (F2). The approach uses curve fitting to data of A and F2 at various levels of incident irradiance (Iinc), intercellular CO2 (Ci) and O2. Estimated parameters include day respiration (Rd), conversion efficiency of Iinc into linear electron transport of PSII under limiting light [k2(LL)], electron transport capacity (Jmax), curvature factor (q) for the non-rectangular hyperbolic response of electron flux to Iinc, ribulose 1·5-bisphosphate carboxylase/oxygenase (Rubisco) CO2/O2 specificity (Sc/o), Rubisco carboxylation capacity (Vcmax), rate of triose phosphate utilization (Tp) and mesophyll conductance (gm). The method is used to analyse combined gas exchange and chlorophyll fluorescence measurements on leaves of various ages and positions in wheat plants grown at two nitrogen levels. Estimated Sc/o (25°C) was 3.13 mbar mbar -1 ; Rd was lower than respiration in the dark; Jmax was lower and q was higher at 2% than at 21% O2; k2(LL), Vcmax, Jmax and Tp correlated to leaf nitrogen content; and gm decreased with increasing Ci and with decreasing Iinc. Based on the parameter estimates, we surmised that there was some alternative electron transport.

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Bundle-sheath conductance (gbs) affects CO2 leakiness, and, therefore, the efficiency of the CO2-concentrating mechanism (CCM) in C4 photosynthesis. Whether and how gbs varies with leaf age and nitrogen status is virtually unknown. We used a C4-photosynthesis model to estimate gbs, based on combined measurements of gas exchange and chlorophyll fluorescence on fully expanded leaves of three different ages of maize (Zea mays L.) plants grown under two contrasting nitrogen levels. Nitrogen was replenished weekly to maintain leaf nitrogen content (LNC) at a similar level across the three leaf ages. The estimated gbs values on leaf-area basis ranged from 1.4 to 10.3 mmol m -2 s -1 and were affected more by LNC than by leaf age, although gbs tended to decrease as leaves became older. When converted to resistance (rbs = 1/gbs), rbs decreased monotonically with LNC. The correlation was presumably associated with nitrogen effects on leaf anatomy such as on wall thickness of bundle-sheath cells. Despite higher gbs, meaning less efficient CCM, the calculated loss due to photorespiration was still low for high-nitrogen leaves. Under the condition of ambient CO2 and saturating irradiance, photorespiratory loss accounted for 3-5% of fixed carbon for the highnitrogen, versus 1-2% for the low-nitrogen, leaves.

Intimate relationships exist between form and function of plants, determining many processes governing their growth and development. However, in most crop simulation models that have been created to simulate plant growth and, for example, predict biomass production, plant structure has been neglected. In this study, a detailed simulation model of growth and development of spring wheat (Triticum aestivum) is presented, which integrates degree of tillering and canopy architecture with organ-level light interception, photosynthesis, and dry-matter partitioning. An existing spatially explicit 3D architectural model of wheat development was extended with routines for organ-level microclimate, photosynthesis, assimilate distribution within the plant structure according to organ demands, and organ growth and development. Outgrowth of tiller buds was made dependent on the ratio between assimilate supply and demand of the plants. Organ-level photosynthesis, biomass production, and bud outgrowth were simulated satisfactorily. However, to improve crop simulation results more efforts are needed mechanistically to model other major plant physiological processes such as nitrogen uptake and distribution, tiller death, and leaf senescence. Nevertheless, the work presented here is a significant step forwards towards a mechanistic functional-structural plant model, which integrates plant architecture with key plant processes.

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