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

Gas exchange responses of mangosteen (Garcinia mangostana L.) leaves t o photosynthetic photon flux density (PPFD), internal CO2 concentratio n (C(i)), leaf-air vapor pressure deficit (VPD), leaf temperature (T(l )) and time of day were investigated in plants grown in three shade tr eatments. Maximal photosynthetic rate (P(n(max)) per unit leaf area at light saturation did not differ significantly among plants grown in t he different shade treatments despite significant morphological differ ences. Light compensation point (9-15 mumol m-2s-1) and quantum yield (0.022-0.023) did not differ significantly among treatments, whereas l ight 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 assimi lation 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% shad e treatment (4.57 mumol m-2s-1). Stomatal conductance (g(s)) decreased with increasing CO2 concentrations. Vapor pressure deficits higher th an 2.5 kPa significantly decreased P(n) and g(s), whereas P(n) remaine d steady over a 24-33-degrees-C temperature range in leaves in 80% sha de and over a 27-36-degrees-C range in leaves in 20 and 50% shade. Hig hest carbon gain during a 12-hour photoperiod was observed for leaves grown in 50% shade. The results imply that mangosteen is a shade-toler ant, lower canopy tree adapted to humid tropical lowlands. Providing l ow 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 a daptability to the seasonally wet and dry tropics of northern Australi a.