Canopy Photosynthetic Capacity and Light Response Parameters of Rubber Hevea brasiliensis with Reference to Exploitation

H Gunasekera,A Nugawela,W De Costa

doi:10.12944/carj.1.1.01

Abstract

The main objective of this study wasto investigate the relationship between canopy photosynthetic capacityand light response parametersof tapped and untapped trees of twoHeveabrasiliensis genotypes, i.e. RRISL 211 and RRIC 121. Moreover, attempts have been made to develop correlations between canopy photosynthesis and light response parameters Heveawith reference to exploitation. The canopy photosynthetic rates measured under optimal environmental conditions clearly showed clonal differences in CO2 assimilation rates. The photosynthetic capacities of leaves from all strata of RRISL 211 were greater than the corresponding strata values in RRIC 121. A greater canopy photosynthetic rate was observed in clone RRISL 211 despite its leaf area index being 2% lower than in RRIC 121. This could be because of the greater photosynthetic capacity of RRISL 211, as indicated by the greater Amax values.In each clone, Amax of the tapped trees was greater than the Amax of untapped trees, and this difference was greater in RRISL 211 than RRIC 121. Another reason for the greater canopy photosynthesis of clone RRISL 211 was the presence of a higher percentage of leaf area in the top canopy layer as compared to clone RRIC 121. Even though, the light saturation point, LSP (i.e. the light intensity at which photosynthetic rate reaches maximum), did not differ significantly between different canopy layers within a clone for both clones, RRIC 121 had greater LSP for corresponding layers than RRISL 211. Moreover, it was evident that, due to the more open canopy architecture of clone RRIC 121, LSP of its middle canopy layer was very close to LSP of the upper canopy layer.In both clones QE of all canopy layers did not show a consistent variation between tapped and untapped treatments The Rd rates of corresponding canopy layers were always slightly greater in RRISL 211 than in RRIC 121. In both clones there was a gradual reduction in Rd rates when moving from upper through middle to bottom layers of the canopy. However, detailed analysis of Rd rates in the different canopy layers between tapped and untapped treatments showed clonal differences. Nevertheless, in both clones Rd of all canopy layers did not show a consistent variation pattern between tapped and untapped treatments. The overall results of both clones clearly showed that tapped trees have a greater photosynthetic capacity as compared to untapped trees because tapping exerts a stimulatory effect on photosynthesis. This trend was more evident in clone RRISL 211.

Highlights

Canopy photosynthesis is governed by the photosynthetic process itself, and by the canopy architecture
It was evident that, due to the more open canopy architecture of clone RRIC 121, Light saturation point (LSP) of its middle canopy layer was very close to LSP of the upper canopy layer.In both clones Quantum efficiency (QE) of all canopy layers did not show a consistent variation between tapped and untapped treatments
In the upper canopy layer of both clones, Amax was higher in the untapped trees as compared to the tapped trees (Tables [3, 4, 5] and 6)

Summary

INTRODUCTION

Canopy photosynthesis is governed by the photosynthetic process itself, and by the canopy architecture. The major differences in photosynthetic activity in sun and shade adapted leaves within a canopy are correlated with differences in the concentration of the electron transport chain, photosystem activity and photosynthetic enzyme activity (Beadle et al, 1985). It appears that canopy photosynthesis is naturally optimized by partitioning of photosynthetic capacity among the leaves with respect to natural light exposure. Attempts have been made to develop correlations between canopy photosynthesis and light response parameters of Hevea with reference to exploitation

MATERIALS AND METHODS

RESULTS

DISCUSSION