Environmental effects on photosynthesis of C3 plants : scaling up from electron transport to the canopy (study case: Glycine max L. Merr)

Abstract

Atmospheric CO₂ concentration is expected to rise from its current level of 354 ppm to 530 ppm by the year 2050 and 700 ppm by the year 2100 (Watson et al., 1990). This change in CO₂ concentration (and other infra-red absorbing gases) in the atmosphere are expected to produce a greenhouse warming of the global surface of 3 to 4 °C by 2100 (Bretherton et al., 1990). Predicting how plants will respond to these changes is very important in understanding the impacts of atmospheric change on both natural ecosystems and crop growth. Photosynthesis is the process by which plants both sense and respond to change in atmospheric CO₂ concentration, and hence, understanding how this process is affected by rising atmospheric CO₂ and accociated climate change is very important. Farquhar & von Caemmerer (1980) have shown that at steady state, the responses of C₃ leaf photosynthesis CO₂ uptake to light, temperature and ambient CO₂ concentration can be described by the biochemical properties of just two steps in the process, the carboxylation reaction and the regeneration of the acceptor for carboxylation. This mechanistic model, has been widely validated as an accurate predictor of photosynthetic carbon uptake by leaves with variation in environmental conditions. However, the description of the light and temperature dependencies of most parameters remains empirical and needs parameterisation. This thesis examines the effects of change in temperature, [CO₂], light intensity and nitrogen nutrition on photosynthesis, and parameters used in the modelling of photosynthesis in a C₃ plants, i.e. soybean (Glycine max L. Merr.). The parameters examined are those used in the mechanistic C₃ leaf-photosynthesis models of Farquhar et al. (1980) and Farquhar & von Caemmerer (1982), i.e. Vcmax (maximum rate of Rubisco activity in the leaf), lmax (potential electron transport rate), Rct (dark respiration), a2 (quantum yield of electron transport) and e (the curvature factor of the light response curve). Experiments involved gas exchange and fluorescence techniques on plants grown at different temperature, [CO₂], light intensity and nitrogen nutrient concentration in a controlled growth chamber. Short-term responses of photosynthesis and values for derived parameters from plant material grown at different controlled environmental conditions, are presented in Chapters 2, 3, and 4 and discussed in terms of methods used and comparison with other published data. The effect of wall conductance on the derived parameters was examined in quite an extent in Chapter 5. Long-term acclimation responses to CO₂ concentration and temperature is examined in Chapter 2 and 3 and the parameter derived from these analysis are used in Chapter 5 for a simulation of temperature response of the rate of CO₂ assimilation for various temperature growth acclimation. Chapter 6 shows the scaling up of models used and parameters obtained at the leaf level to the canopy level using the sun-shade model of de Pury & Farquhar ( 1997) and big-leaf model of Lloyd & Farquhar (1996). It presents algorithm for calculating diurnal canopy assimilation rate given parameters of the single leaf light and CO₂ concentration responses, the canopy extinction coefficient for light and nitrogen, canopy leaf area index, nitrogen concentration at the top of the canopy, daily solar irradiance and daily maximum and minimum temperatures. The simulation was run for Muara Climatology Station in Bogor, Indonesia (6.67 °S, 106.75 °E), where the fraction of diffuse irradiance is remarkably high (on average 0.64 for 1978). This chapter discusses the importance of diffuse fraction of irradiance in canopy assimilation rate and efficiency. Chapter 7 shows general conclusion of the manuscript and the applicability of the approach used for further work. Some important results that can be extracted from the thesis are as follows : (1) Long-term acclimation of Vcmax and lmax occurred for plants grown at different temperature. The parameter derived is sensitive to whether wall conductance is assumed finite or infinite; (2) The decline in electron transport rate after reaching its optimum temperature and up to 43 °C is reversible if exposure is of short duration; (3) A new and simpler empirical formulation for the rate of electron transport as a function of temperature is developed. This new equation describes the short-term temperature response quite well, and can be used to analyze acclimation to growth temperature; (4) The light gradient developed within the leaf during growth in the chamber affects the value of the curvature factor of the leaf's light response of photosynthesis when measured using conventional gas exchange system and hence affects the derived parameters a2 and Jmax· It is concluded that much of the curvature at the leaf level may result from a 'mismatch' between the profiles of light intensity and electron transport capacity within the leaf; (5) The fraction of diffuse irradiance is important in determining canopy assimilation efficiency. Efficiency (denoted as Light Use Efficiency) increases as the diffuse fraction of irradiance increases. (6) The difference between sun-shade and big-leaf model is negligible when canopy assimilation simulation was run under a high fraction of diffuse irradiance. The difference was significant under a very bright atmospheric condition. (7) The estimate of the optimum fraction of diffuse irradiance (fD) for maximum canopy assimilation rate is possible by using the sun-shade model. It is impossible to calculate with big-leaf model which would give optimum fD of zero. (8) The optimum fraction of diffuse irradiance, which gives highest canopy net assimilation rate at the chosen site, was around 30 %, with optimum LAI of 4. Overall, the flow of the work being carried out and reported in this manuscript is shown in the diagram below. It shows the equations used at the leaf level, parameterisation at the leaf level and the scaling up to the canopy level.