An Improvement of Semiempirical Method of Evaluating the Total Photosynthesis of a Plant Community

Abstract

In this paper a semiempirical method in evaluating the total photosynthesis of a plant community was improved as a preliminary to further experimental studies, by taking into consideration the influence of the concentration of carbon dioxide and of the leaf age within the community on photosynthesis of leaves.The following assumptions were used to derive the equation for evaluating the total photosynthesis:1. p/p∞=I/K+I′ (1)where p and p∞ are the photosynthesis and the maximum value of it, I the radiation intensity at leaves surface, and K a constant (the value of I at which p/p∞=0.5).2. p∞=DcC, (2)where Dc is the total integral exchange coefficient between surrounding airs and the chroloplast, C the carbon-dioxide concentration in the air.3. Dc=DcH+βF, C=CH+γF, (3)where DcH and CH are the values of Dc and C at the top surface of plant canopy, β and γ the vertical gradients in Dc and C within the canopy, respectively.4. I=IH/(1+αF), (4)where IH is the radiation intensity at the top surface, α a canstant depending upon the canopy structure, and F the downward cummulative leaf area index.Substituting Eqs. (2), (3) and (4) into Eq. (1) and integrating it with respect to the leaf area depth (subject to the conditions Pt=0, Ft=0) givePt=IH/B[lnA+BFt/A{a-bA/B+cA2/B2}+(b-cA/B)Ft+c/2Ft2], where Pt is the total photosynthesis of the plant community with leaf area index of Ft, A=K+IH, B=Kαa, a=DcH⋅CH, b=(DcHγ+CHβ), and c=βγ.Fig. 3 represents, for a canopy with Ft=5, the variation of the maximum of photosynthesis (p∞) and the photosynthesis (p) camputed from the reciprocal law and exponential law for the light attenuation. With increasing the vertical gradient in CO2 concentration and integral CO2 exchange coefficient within the conopy, p and p∞ appreciably decreased with increase of the leaf area depth. It can be concluded from this result that an assumption concerning the constancy of the maximum value (p∞) of photosynthesis over the canopy, as has been adopted by many researchers, may not be valid in practical plant canopies.Fig. 4 suggests that the variation of integral CO2 exchange coefficient (Dc) with the leaf area depth is a conspicously most important factor controlling the total photosynthesis (Pt) of the plant community. Thus, much attention must be paid into the vertical change of photosynthetic activity of plant leaves in relation to the foliage structure. Fig. 4 revealed that though the CO2 concentration at the top of the canopy gives much influence on the total photosynthesis (Pt), the variation in the concentration within the conopy is not very influential on it. The optimum leaf area index was computed from Eq. (17). Results obtained are shown in Fig. 5 as a function of radiation intensity and the vertical gradient of integral exchange coefficient. The following relation was obtained to define the dependence of the total photosynthesis on both the conentration in carbon dioxide and the radiation intensity:dPt/dCH=IH/0.75+2.75IH.A similar relation was also obtained from the data of gross photosynthesis of cotton leaves.