Abstract
The relationship between malic acid production and carbon assimilation was examined in the submerged aquatic Crassulacean acid metabolism (CAM) plant, Isoetes howellii Engelmann. Under natural conditions free-CO(2) level in the water was highest at 0600 hours and (14)CO(2) assimilation rates in I. howellii were also highest at this time. After 0900 hours there was a similar pattern in (a) rate of free-CO(2) depletion from the water, (b) reduction of carbon assimilation rates, and (c) rate of deacidification in leaves. Rates of daytime deacidification increased under CO(2)-free conditions and as irradiance intensity increased. Nighttime CO(2) uptake was estimated to contribute one-third to one-half of the total daily gross carbon assimilation. CO(2) uptake, however, accounted for only one-third to one-half of the overnight malic acid accumulation. Internal respiratory CO(2) may be a substrate for a large portion of overnight acid accumulation as leaves incubated overnight without CO(2) accumulated substantial levels of malic acid. Loss of CAM occurred in emergent leaf tips even though submerged bases continued CAM. Associated with loss of CAM in aerial leaves was an increase in total chlorophyll, a/b ratio, and carotenoids, and a decrease in leaf succulence. delta(13)C values of I. howellii were not clearly distinguishable from those for associated non-CAM submerged macrophytes.