Glycolate metabolism of three submersed aquatic angiosperms during ageing

Under controlled experimental conditions, the influences of light and temperature on the growth and metabolism of Egeria densa Planch, Hydrilla verticillata Royle, and Myriophyllum spicatum L. were comparatively examined. Light was controlled at six levels ranging between 5 and 75% of full sunlight at solar noon. Water temperature was controlled at five levels ranging between 16? and 32?C. Growth considerations included morphology, biomass, and nutrition. Photosynthesis, respiration, and CO2 compensation points were determined to evaluate physiological differences in plant growth as affected by the experimental ranges of temperature and light. External morphology in these species was significantly affected by the different experimental light and temperature conditions. Both low light and high temperature promoted extensive shoot elongation and associated canopy formation. Biomass production and carbon metabolism in all species were influenced more by temperature than by light. Each of the species demonstrated metabolic acclimation to light over a broad range. Conversely, the macrophyte species considered here were not strictly capable of acclimating to temperature. Growth rate and the seasonal progression of senescence were interrelated in these species. Higher temperatures stimulated growth and promoted a compression of the growth cycle. The relationship between photosynthesis and respiration (P:R) was appreciably reduced by senescence, but the CO2 compensation point did not reflect this condition. In the species examined, CO2 compensation points decreased with increasing temperature, suggesting adaptations to low free CO2 levels in the environ- ment. Latitudinal differences in integral seasonal temperature, in relation to species-specific ranges of thermal tolerance, appear to be important in influencing the geographical distribution of the species considered here. Light may be the primary determinant of their depth distribution, but its importance in this regard could be somewhat diminished by their significant abilities to extend to the water surface under low light conditions.

Comparative Influences of Light and Temperature on the Growth and Metabolism of Selected Submersed Freshwater Macrophytes

The functional significance of different photosynthetic CO2 fixation pathways is a question which can be answered in many ways, each being appropriate to certain scales of enquiry. In physiological ecology our purpose should be to integrate these different scales of enquiry as comprehensively as possible, and to show how photosynthesis contributes, directly or indirectly, to performance and survival of plants in diverse habitats. Studies of photosynthetic CO2 fixation were afflicted with a post-Calvin cycle chauvinism in the 1950’s, which may have been responsible for the slow and tentative revelation of the C4 pathway of photosynthetic carbon assimilation in the USA and USSR (Burr et al. 1957; Karpilov 1960; Kortschak et al. 1965). Elucidation of this pathway undoubtedly stimulated new interest in the carbon metabolism of photosynthesis in the next decade (Hatch and Slack 1966, 1970; CC Black 1973) and led to an upsurge in comparative studies of higher plant photosynthesis (Black 1971; Bjorkman 1973). Largely as a result of this stimulus, the peculiar dark CO2 fixation processes of succulent plants, known as crassulacean acid metabolism (CAM), were also recognized as a distinctive photosynthetic process (Kluge and Ting 1978; Osmond 1978). It also led to the present revival of interest in the photosynthesis of aquatic plants.

Functional Significance of Different Pathways of CO 2 Fixation in Photosynthesis

Photosynthetic carbon assimilation in aquatic macrophytes

Underwater photosynthesis in flooded terrestrial plants: a matter of leaf plasticity.

The CO 2 compensation point of the submersed aquatic macrophyte Hydrilla verticillata varied from high (above 50 microliters per liter) to low (10 to 25 microliters per liter) values, depending on the growth conditions Plants from the lake in winter or after incubation in an 11 C/9-hour photoperiod had high values, whereas summer plants or those incubated in a 27 C/14-hour photoperiod had low values The plants with low CO 2 compensation points exhibited dark 14 CO 2 fixation rates that were up to 30% of the light fixation rates This fixation reduced respiratory CO 2 loss, but did not result in a net uptake of CO 2 at night The low compensation point plants also showed diurnal fluctuations in titratable acid, such as occur in Crassulacean acid metabolism plants However, dark fixation and diurnal acid fluctuations were negligible in Hydrilla plants with high CO 2 compensation points Exposure of the low compensation point plants to 20 micromolar 14 CO 2 resulted in 60% of the 14 C being incorporated into malate and aspartate, with only 16% in sugar phosphates At a high CO 2 level, the C 4 acid label was decreased A pulse-chase study indicated that the 14 C in malate, but not aspartate, decreased after a long (270-second) chase period; thus, the C 4 acid turnover was much slower than in C 4 plants Phosphoenolpyruvate carboxylase activity was high (330 micromoles per milligram chlorophyll per hour), as compared to ribulose bisphosphate carboxylase (20 to 25), in the plants with low compensation points These plants also had a pyruvate, Pi dikinase activity in the leaves of 41 micromoles per milligram chlorophyll per hour, which suggests they are not C 3 plants NAD- and NADP + -malate dehydrogenase activities were 6136 and 245 micromoles per milligram chlorophyll per hour, respectively Of the three decarboxylating enzymes assayed, the activities of NAD- and NADP + -malic enzyme were 1042 and 237 micromoles per milligram chlorophyll per hour, while phosphoenolpyruvate carboxykinase was only 02 Low compensation point Hydrilla plants fix some CO 2 into C 4 acids, which can be decarboxylated for later refixation, presumably into the Calvin cycle Refixation would be advantageous in summer lake environments where the CO 2 levels are high at night but low during the day Hydrilla does not fit any of the present photosynthetic categories, and may have to be placed into a new group, together with other submersed aquatic macrophytes that have environmentally variable CO 2 compensation points

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Papers

C4 Acid Metabolism and Dark CO2 Fixation in a Submersed Aquatic Macrophyte (Hydrilla verticillata)