The objective of the present investigation was to examine the extent of var
iations in single leaf net photosynthetic rate (P-N) and its relative depen
dence on stomatal conductance (g(s)) and the mesophyll capacity to fix carb
on in 12 clones of the natural rubber plant. There were significant variati
ons in P-N measured at low and saturating photon flux density (PFD); the ex
tent of variation was larger at low than at saturating PFD. The compensatio
n irradiance (CI) and apparent quantum yield of CO2 assimilation (phi(c)) c
alculated from the P-N/PFD response curves showed significant variations am
ong the clones. P-N at low irradiance was positively correlated with phi(c)
. Thus a clone with large P-N at low irradiance, high phi(c), and low CI ma
y tolerate shade better and thus produce a high tree stand per hectare. A s
trong positive correlation existed between P-N saturated with radiant energ
y (P-sat) and carboxylation efficiency (CE) estimated from the response cur
ves of PN on intercellular CO2 concentration (C-i), but g(s) showed a poor
correlation with P-max. High CO2 compensation concentration (Gamma) led to
low CE in Hevea clones. A clone with large P-sat, high CE, low g(s), and lo
w Gamma is the one in which photosynthesis is more dependent on the mesophy
ll factors than stomata. Such a clone may produce relatively high biomass a
nd maintain high water use efficiency.