Canopy Photosynthetic Capacity and Light Response Parameters of Rubber (Hevea Brasiliensis) with Reference to Exploitation

H. K. L. K. Gunasekera^1*, W. A. J. M. De Costa², A. Nugawela³

¹Department of Agricultural and Plantation Engineering, Faculty of Engineering Technology, The Open University of Sri Lanka.

²Department of Crop Science, Faculty of Agriculture, University of Peradeniya, Sri Lanka.

³Rubber Research Institute of Sri Lanka.

DOI : http://dx.doi.org/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 A_max values.In each clone, A_max of the tapped trees was greater than the A_max 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.

Keywords:

Rubber, Canopy Photosynthesis, Clone, Light Response Parameters, Canopy Layer

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