Photosynthetic Responses of Giant Granadilla (Passiflora quadrangularis L.) to Salinity

Abstract

The giant granadilla is distributed throughout the tropics and may deserve wider use as a commercial crop (Martin et al., 1987). Knowledge of the factors that affect photosynthesis of tropical species is a prerequisite for successful cultivation (Possingham, 1986). Utsunomiya and Shigenaga (1988) observed a reduction in giant granadilla shoot growth and stomatal conductance when plants were irrigated with NaCl levels up to ≈4.5 dS·m. Salinity effects on plant growth and yield depend in part on factors that influence the rate of C gain (Yeo, 1983). Understanding the influence of salinity on photosynthesis of this species may thus be of benefit in managing fruit production in areas with salinity problems. The objective of this study was to determine the effects of substrate salinity on the photosynthetic process of giant granadilla. Giant granadilla cuttings from a single clone were taken on 6 July 1990 and rooted in perlite under intermittent mist. The plants were potted on 1 Aug. in a medium of equal parts of perlite, sand, peat, and bark chips (by volume) in 2.6-liter containers and grown on raised benches under glasshouse conditions (50% ambient light by whitewash). A topdressing of 6 g complete granular fertilizer was applied every 8 weeks. All plants were pruned to two nodes on 6 Dec. and spaced 30 cm in a diamond pattern. Regrowth of a single shoot was trained to 90cm bamboo stakes. When the leaves near the top of the stakes were fully expanded, uniform plants were chosen and the growth above the stakes was pruned. The gas exchange of the three terminal leaves of each plant did not differ based on preliminary data obtained before the salinity treatment. These leaves were chosen at random on each plant for all gas exchange measurements throughout the study.