© 2003 Heron Publishing—Victoria, Canada
Growth, photosynthetic performance and shade adaptation of rubber (Hevea brasiliensis) grown in natural shade
A. M. W. K. Senevirathna (1, 2, 3), C. M. Stirling (1) and V. H. L. Rodrigo (3)
1. School of Agricultural and Forest Sciences, University of Wales, Bangor, LL57 2UW, U.K. / 2. Author to whom correspondence should be addressed (wasan@sltnet.lk) / 3. Rubber Research Institute of Sri Lanka, Dartonfield, Agalawatta, Sri Lanka / Received July 24, 2002; accepted December 30, 2002; published online June 2, 2003
Summary
We compared growth, photosynthetic performance and shade adaptation of rubber (Hevea brasiliensis Muell. Arg.) plants growing in natural shade (33, 55 and 77% reduction in incoming radiation) to control plants growing in
full sunlight. Stem diameter and plant height, measured over a 15-month period, were greatest in plants grown in full sunlight,
and both parameters decreased with increasing shade. At 7 and 14 months after planting (MAP), total plant dry mass was highest
in control plants and lowest in plants in 77% shade. Expansion of the fourth leaf whorl, monitored at 5–6 MAP, was slowest
in plants in 77% shade and fastest in unshaded plants, which had more leaves and higher leaf areas and inter-whorl shoot lengths.
In response to increasing shade, specific leaf area increased, whereas leaf weight ratio and relative growth rate decreased.
Chlorophyll a/b ratio decreased with increasing shade, indicating shade-induced partitioning of chlorophyll into light-harvesting
complexes. Compared to the response in unshaded plants, CO2 assimilation saturated at lower photosynthetic photon flux densities in plants in 77% shade, with a lower upper-asymptote
to the light response curve. Chlorophyll fluorescence revealed no evidence of sustained photoinhibitory damage in unshaded
plants. Dynamic photoinhibition decreased with increasing shade, with the greatest depression in the ratio of variable to
maximal fluorescence around midday. We conclude that shade adaptation and shade-induced reductions in dynamic photoinhibition
account for the enhanced early growth of rubber in light shade.
Keywords:
chlorophyll fluorescence, dynamic photoinhibition, leaf area ratio, leaf weight ratio, relative growth rate, shade-plants,
specific leaf area, sun-plants.
