The objective of the present investigation was to examine the extent of variations in single leaf net photosynthetic rate (PN) and its relative dependence on stomatal conductance (gs) and the mesophyll capacity to fix carbon in 12 clones of the natural rubber plant. There were significant variations in PN measured at low and saturating photon flux density (PFD); the extent of variation was larger at low than at saturating PFD. The compensation irradiance (CI) and apparent quantum yield of CO2 assimilation (φc) calculated from the PN/PFD response curves showed significant variations among the clones. PN at low irradiance was positively correlated with φc. Thus a clone with large PN at low irradiance, high φc, and low CI may tolerate shade better and thus produce a high tree stand per hectare. A strong positive correlation existed between PN saturated with radiant energy (Psat) and carboxylation efficiency (CE) estimated from the response curves of PN on intercellular CO2 concentration (Ci), but gs showed a poor correlation with Psat High CO2 compensation concentration (Γ) led to low CE in Hevea clones. A clone with large Psat, high CE, low gs, and low Γ is the one in which photosynthesis is more dependent on the mesophyll factors than stomata. Such a clone may produce relatively high biomass and maintain high water use efficiency. and K. N. Nataraja, J. Jacob.
The pH-dependent quenching of chlorophyll (Chl) fluorescence (the high energy quenching) was characterized by stationary Chl-a-fluorescence in the thylakoid membranes and photosystem (PS) 2 preparations. The variable part of fluorescence was quenched, when the pH in the thylakoid lumen decreased below 5.5, i.e., at high ApH. This quenching was caused by an inhibition of electron donation from the manganese cluster to the reaction centre of PS 2. The pH response of quenching suggested that a 1 H'''-transition with an apparent pK of 4.7 was involved. Parallel to quenching at low pH, a Ca^‘'‘-release was measured (1 Ca^'*' per 200 Chl). When the ApH relaxed and the pH on the lumen side increased again, fluorescence recovered provided Ca^'*' was present (Kd = 100 pM). Both the quenching at low pH and the reactivation at pH > 5.5 are light-dependent processes. In the presence of high concentration of extemal Ca^"^, fluorescence recovered even at low pH. Inhibition of the donor-side of PS 2 directly affected the acceptor-side of PS 2, as seen by a shift of the redox potential of Qa from -120 mV (pH 7.0) to +40 mV (pH 4.2). We propose that at high ApH (7) the water splitting side is inactivated by release of Ca from a high afflnity binding site, and (2) Qa is converted to a high-potential form. Excitatíon energy is then dissipated at the PS 2 reaction centres by a recombination reaction between donor and acceptor side. As a result, Qa (and the intersystem electron transport chain) remains oxidized, even in the excessive light.