GAS-EXCHANGE CHARACTERISTICS OF MANGOSTEEN (GARCINIA-MANGOSTANA L) LEAVES

Gas exchange responses of mangosteen (Garcinia mangostana L.) leaves to photosynthetic photon flux density (PPFD), internal CO2 concentration (C(i)), leaf-air vapor pressure deficit (VPD), leaf temperature (T(l)) and time of day were investigated in plants grown in three shade treatments. Maximal photosynthetic rate (P(n(max)) per unit leaf area at light saturation did not differ significantly among plants grown in the different shade treatments despite significant morphological differences. Light compensation point (9-15 mumol m-2s-1) and quantum yield (0.022-0.023) did not differ significantly among treatments, whereas light saturation point was significantly higher for leaves grown in 20% shade than for leaves grown in 50 or 80% shade (951,645 and 555 mumol m-2s-1, respectively). Shade treatments significantly affected assimilation responses to varying CO2 concentrations. At CO2 concentrations between 600 and 1000 mumol mol-1, leaves from the 20% shade treatment recorded higher P(n) (6.44 mumol m-2s-1) than leaves from the 80% shade treatment (4.57 mumol m-2s-1). Stomatal conductance (g(s)) decreased with increasing CO2 concentrations. Vapor pressure deficits higher than 2.5 kPa significantly decreased P(n) and g(s), whereas P(n) remained steady over a 24-33-degrees-C temperature range in leaves in 80% shade and over a 27-36-degrees-C range in leaves in 20 and 50% shade. Highest carbon gain during a 12-hour photoperiod was observed for leaves grown in 50% shade. The results imply that mangosteen is a shade-tolerant, lower canopy tree adapted to humid tropical lowlands. Providing low VPD in the nursery may result in maximal growth. However leaves of seedlings grown in 50 or 20% shade tolerated high VPD and temperatures better than leaves of seedlings grown in 80% shade, indicating some adaptability to the seasonally wet and dry tropics of northern Australia.