Nitrogen relations for net CO 2 uptake by the cultivated hemiepiphytic cactus, Hylocereus undatus

Abstract

The net CO 2 uptake ability of a vine-like cactus native to shaded habitats, Hylocereus undatus, was hypothesized to adjust more rapidly to changes in the applied nitrogen concentration, which can have major impacts on fruiting, than the more massive cactus most widely cultivated for fruit, Opuntia ficus-indica. Specific objectives were to examine the effects of applied N on stem N concentrations and chlorophyll levels, which can affect net CO 2 uptake ability and hence growth. After 9 weeks, the total daily net CO 2 uptake for H. undatus was only 26% less for the relatively low N concentration of 0.32 mM than for 8 mM N (the N concentration in 0.5-strength Hoagland solution; all other nutrients were maintained at their concentration in 0.2-strength Hoagland solution), compared with 2 weeks for major changes for O. ficus-indica in response to changing applied N. Based on the maximal net CO 2 uptake rates at night for the Crassulacean acid metabolism H. undatus, the half-time for the shift in response to seven different N concentrations applied for 22 weeks was 12–13 weeks; the half-time for the attainment of the highest net CO 2 uptake rate of 10 μmol m −2 s −1 in response to subsequent application of 16 mM N for 11 weeks was 8–9 weeks. After 22 weeks, the stem N level in response to 0.16 mM N was 0.9% by dry mass and the chlorophyll content per unit stem area was 0.30 g m −2 compared with 2.5% and 0.63 g m −2 for 16 mM N. Subsequent application of 16 mM N for 11 weeks reversed the observable stem bleaching and raised the chlorophyll toward the highest levels. Both the chlorophyll content and the N level per unit stem area are highly correlated with the net CO 2 uptake ability of H. undatus and either could help assess the physiological status of this cactus.