The effects of CO2 fertilization and phosphorous availability on the photosynthesis of plants
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Rising CO2 levels can have a positive effect on the photosynthesis of plants through CO2 fertilization. At the same time changes are occurring in nutrient cycles through the use of fertilizers. The effects of CO2 fertilization together with phosphorus availability on photosynthesis have been tested Holcus lanatus Solanum dulcamara. Controlled growth chambers with CO2 levels of 200, 400 and 800 ppm have been used where plants were either treated with nutrient solutions of 1:1 or 45:1 N:P ratio. Through the means of photosynthesis measurements and A/Ci curves, different photosynthetic variables have been determined and analyzed. P treatment had an effect on light saturated photosynthesis (Asat) in the sense that high P availability led to higher Asat values for both species. Increased CO2 concentration did not lead to increased Asat for S. dulcamara. For H. lanatus higher CO2 level did cause higher Asat values with high P availability, but this was not observed under low P conditions. This resulted in an interaction effect between CO2 level and P availability, indicating that low P availability limits increase of Asat under elevated CO2 concentration. Photosynthetic capacity showed for both P treatments a decrease with increasing CO2 levels caused by downregulation of maximal Rubisco-limited rate of photosynthesis (Vcmax) and maximal electron transport-limited rate of photosynthesis (Jmax) with increasing CO2 level. Vcmax decreased to a larger extent than Jmax resulting in a decrease in Vcmax:Jmax. The changes in Vcmax, Jmax and Vcmax:Jmax were more profound in the transition from low to ambient CO2 level than from ambient to high CO2 level. Stomatal conductance (gs) declined with increasing CO2 level, but only showed an effect of P availability for H. lanatus, where high P treatment caused a higher gs. Positive correlations were found within the CO2 treatments when plotting Vcmax, Jmax and gs against Asat. As photosynthetic capacity declined with increasing CO2 but Asat did not decline it can be concluded that photosynthesis becomes more efficient by CO2 fertilization. Even without increase in Asat, plants can still benefit from elevated CO2 by for example increased water and nitrogen use efficiency. The larger transitions of variables from low to ambient CO2 suggest CO2 fertilization will have smaller effect in the future than it had until now. The results of the P treatments show nutrients cannot be excluded in looking at the effect on photosynthesis under elevated CO2.