Carbon Assimilation Characteristics of the Aquatic CAM Plant, Isoetes howellii

Publisher Website
Google Scholar
Abstract
The relationship between malic acid production and C assimilation was examined in the submerged aquatic crassulacean acid metabolism (CAM) plant, I. howellii Engelmann. Under natural conditions free-CO2 level in the water was highest at 0600 h and 14CO2 assimilation rates in I. howellii were also highest at this time. After 0900 h there was a similar pattern in rate of free-CO2 depletion from the water, reduction of C assimilation rates and rate of deacidification in leaves. Rates of daytime deacidification increased under CO2-free conditions and as irradiance intensity increased. Nighttime CO2 uptake was estimated to contribute 1/3-1/2 of the total daily gross C assimilation. CO2 uptake accounted for only 1/3-1/2 of the overnight malic acid accumulation. Internal respiratory CO2 may be a substrate for a large portion of overnight acid accumulation as leaves incubated overnight without CO2 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 decrease in leaf succulence. .delta.13C values of I. howellii were not clearly distinguishable from those for associated non-CAM submerged macrophytes.