CAM-cycling in the cycad Dioon edule Lindl. in its natural tropical deciduous forest habitat in central Veracruz, Mexico

Abstract

The cycad Dioon edule Lindl. inhabits a seasonally-dry tropical forest along with associated CAM plants such as bromeliads and cacti. To test the hypothesis that D. edule might also be a CAM plant, diel total-acid fluctuation was measured through the dry to wet seasons of 4 consecutive years on adult D. edule plants in their natural forest habitat in Veracruz, Mexico. Correlations between acid fluctuation index and climatic data, and also soil water potential were determined over this period. Laboratory trials were followed up to estimate diel patterns of CO2 exchange and estimation of δ13C value. A comparison of stomatal density cm−2 with other C3, CAM and CAM-facultative plants was made. The diel total titratable-acid fluctuation values, although variable, were found to be consistent and significant for the dry season. Carbon dioxide exchange was found to be typical of C3 plants when hydrated but when the plant was stressed by withholding water, although the leaf remained healthy, there was no significant dark-period CO2 output. Stomatal density was comparable to other CAM and CAM-facultative plants. It was concluded that D. edule is a C3 plant that shows CAM-cycling metabolism when water stressed. © 2002 The Linnean Society of London, Botanical Journal of the Linnean Society, 138, 155–161.