Photosynthetic pathways in freshwater aquatic plants.

Recent studies show that generalizations about photosynthetic pathways, derived from terrestrial plant studies, do not apply to aquatic plants. Crassulacean acid metabolism (CAM) photosynthesis is of selective value not only in arid environments, where it enhances water-use efficiency, but also in aquatic plants of oligotrophic waters, where it enhances competitive ability in carbon acquisition. C(4) photosynthesis is present in many aquatic species, but in these species it is not coupled with the specialized anatomy of terrestrial C(4) plants. The ratio of the stable carbon isotopes, (13)C/(12)C, in the biomass of terrestrial plants is a marker of their photosynthetic pathway. In aquatic environments, additional resistances to carbon-isotope fractionation make this technique of limited use in detecting photosynthetic pathways.

Seed germination patterns of Salvia mellifera in fire-prone environments.

Salvia mellifera seeds from coastal sage, chaparal and desert scrub in southern California failed to germinate in the dark unless exposed to powdered charred wood. This pattern was observed for seeds given a one month stratification at 5 C and for ones not stratified and also for seeds incubated under continuous 23 C or a diurnal alternation of 13 C/23 C. Dark inhibition of germination was also overcome, but only in seeds from chaparral populations, if seeds were incubated on commerical potting soil under alternating 13 C/23 C. Seeds in the light germinated readily in all but one population from desert scrub vegetation. Germination of seeds from this population was markedly stimulated by dry heating of the seeds at either 70 C for 5 h or 115 C for 5 min. For all populations there were numerous significant interactions between incubation temperature, pre-chilling stratification, light, and heating/charred wood treatments. Timing of germination was remarkably consistent between populations; the vast majority of seeds germinated within the first week at 23 C (or 13 C/23 C) regardless of whether or not they had received a pre-chilling treatment.

Carbon Assimilation Characteristics of the Aquatic CAM Plant, Isoetes howellii.

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.

Hydrogen, oxygen, and carbon isotope ratios of cellulose from submerged aquatic crassulacean Acid metabolism and non-crassulacean Acid metabolism plants.

Isotope ratios of cellulose and cellulose nitrate from aquatic Crassulacean acid metabolism (CAM) and non-CAM plants were determined. Cellulose oxygen istope ratios for all plants that grew together were virtually identical, whereas large differences were observed for hydrogen isotope ratios of cellulose nitrate between CAM and non-CAM plants. Carbon isotope ratios of cellulose nitrate did not differentiate CAM from non-CAM plants.

Gas Exchange Characteristics of the Submerged Aquatic Crassulacean Acid Metabolism Plant, Isoetes howellii.

The submerged aquatic plant Isoetes howellii Engelmann possesses Crassulacean acid metabolism (CAM) comparable to that known from terrestrial CAM plants. Infrared gas analysis of submerged leaves showed Isoetes was capable of net CO(2) uptake in both light and dark. CO(2) uptake rates were a function of CO(2) levels in the medium. At 2,500 microliters CO(2) per liter (gas phase, equivalent to 1.79 milligrams per liter aqueous phase), Isoetes leaves showed continuous uptake in both the light and dark. At this CO(2) level, photosynthetic rates were light saturated at about 10% full sunlight and were about 3-fold greater than dark CO(2) uptake rates. In the dark, CO(2) uptake rates were also a function of length of time in the night period. Measurements of dark CO(2) uptake showed that, at both 2,500 and 500 microliters CO(2) per liter, rates declined during the night period. At the higher CO(2) level, dark CO(2) uptake rates at 0600 h were 75% less than at 1800 h. At 500 microliters CO(2) per liter, net CO(2) uptake in the dark at 1800 h was replaced by net CO(2) evolution in the dark at 0600 h. At both CO(2) levels, the overnight decline in net CO(2) uptake was marked by periodic bursts of accelerated CO(2) uptake. CO(2) uptake in the light was similar at 1% and 21% O(2), and this held for leaves intact as well as leaves split longitudinally. Estimating the contribution of light versus dark CO(2) uptake to the total carbon gain is complicated by the diurnal flux in CO(2) availability under field conditions.