The effect of elevated CO2 on ovemight malate accumulatíon in the CAM epiphyte Tillandsia ionantha and the CAM terrestrial species Crassula arborescens was compared. Both species showed an increase in nocturnal accumulatíon of malate with increasing CO2 concentrations. This study is the first to show an increase in nighttime malate accumulatíon with increasing levels of CO2 at near-ambient concentrations in a CAM plant. The results indicate that some CAM plants can respond to increasing levels of CO2 in the atmosphere, potentíally leading to an increase in productívity.
The effects of intra- and interspecific competition on growth, net photosynthetic rate (/^n), intemal CO2 concentration (cj), leaf conductance to CO2 (gi), and xylem leaf water potentials Q¥) were investigated among well-watered seedlings of the old-field species Abutilon theophrasíi. Ambrosia tri/ida, and Helianíhus annuus. In all comparisons, *F values of the target plants were unaffected by the presence of neighbors. On the other hand, revealed inhibitory effects of neighbors on A. theophrasíi and H. annuus. The above- and belowground biomass of all species was reduced in the presence of neighbors. Hence both inter- and intraspecific competition among seedlings may be important in influencing community stiucture in old-field communities.
Net CO2 exchange (Rn), stomatal conductance (gj, intemal CO2 concentration (Q), and water-use efficiency (WUE) of aging leaves were measured at the beginning of the dry season in 18 species in a tropical deciduous forest in Guanacaste, Costa Rica. In selected trees, gas exchange of old leaves was also compared with that of young leaves, and stomatal responses to irradiation and dark were measured. Based on comparisons with published data, and gj declined more rapidly with leaf age in deciduous than in evergreen species. Whereas both senescence and drought were implicated as causes of these dechneš in the deciduous species, the relative importance of factors responsible for the smaller decreases in physiological activity of old leaves of the evergreen species was less obvious. In the latter trees, and WUE of aging leaves, although not always as high as in young leaves, were high enough to refiite the suggestion that old leaves are solely a liability to tropical deciduous forest trees. On the other hand, stomatal closure in the dark was incomplete, potentially allowing water loss at night and during droughts.