Water hyacinth (Eichhornia crassipes [Mart.i Solms) plants were grown in environmental chambers at ambient and enriched CO2 levels (330 and 600 microliters CO2 per liter). Daughter plants (ramets) produced in the enriched CO2 gained 39% greater dry weight than those at ambient C02, but the original mother plants did not. The CO2 enrichment increased the number of leaves per ramet and leaf area index, but did not significantly increase leaf size or the number of ramets formed. Flower production was increased 147%. The elevated CO2 increased the net photosynthetic rate of the mother plants by 40%, but this was not maintained as the plants acclimated to the higher CO2 level. After 14 days at the elevated C02, leaf resistance increased and transpiration decreased, especially from the adaxial leaf surface. After 4 weeks in elevated as compared to ambient CO2, ribulose bisphosphate carboxylase activity was 40% less, soluble protein content 49% less, and chlorophyll content 26% less; whereas starch content was 40% greater. Although at a given CO2 level the enriched CO2 plants had only half the net photosynthetic rate of their counterparts grown at ambient C02, they showed similar internal CO2 concentrations. This suggested that the decreased supply of CO2 to the mesophyll, as a result of the increased stomatal resistance, was counterbalanced by a decreased utilization of CO2. Photorespiration and dark respiration were lower, such that the CO2 compensation point was not altered. The photosynthetic light and CO2 saturation points were not greatly changed, nor was the 02 inhibition of photosynthesis (measured at 330 microliters CO2 per liter). It appears that with CO2 enrichment the temporary increase in net photosynthesis produced larger ramets. After acclimation, the greater total ramet leaf area more than compensated for the lower net photosynthetic rate on a unit leaf area basis, and resulted in a sustained improvement in dry weight gain.For many plants, especially those with C3 photosynthesis, an increase in CO2 produces an increase in net photosynthetic rate (9). This is largely attributable to the elevated CO2 competing with 02 to promote the activity of RuBP carboxylase but inhibit that of RuBP oxygenase (30). Although there are notable examples where these short-term increases in photosynthetic efficiency are directly extrapolated into long-term growth improvements (6,22,28,33), this is not always the case (20,21,24 plants exhibit some form of acclimation to high CO2, such that the initial increase in net photosynthesis is moderated or even lost, and the expected gains in productivity are not realized (10,16,25). The nature of this acclimation has not been fully elucidated, and it differs among species and developmental stages (1 1, 13, 21). It does not seem to affect directly the competitive interaction between CO2 and 02 for fixation by RuBPCase,2 but may be mediated by more indirect effects, such as increased stomatal resistance or starch production (7,13,17).This study was designed to evaluate the pho...

Inducible C4—like photosynthetic metabolism in Hydrilla verticillata leaf tissue elicits variability in photosynthetic phenotype, expressed as CO2 compensation point (Τ). We conducted a field and laboratory study to investigate the ecological and adaptive significance of this physiological phenomenon. Spatial horizontal environmental heterogeneity was observed within clonal populations of H. verticillata in Florida, USA. Measured at midday, the edge habitat at the expanding periphery of the clone exhibited a dissolved inorganic carbon (DIC) concentration of 0.7 mmol/L, pH 7.1, a dissolved oxygen (DO) level of 0.13 mmol/L, and biomass of 0.2 kg/m2. The mat habitat, located 200 cm towards the interior of the surface mat, exhibited DIC 0.1 mmol/L, pH 10.2, DO 0.48 mmol/L, and biomass 0.8 kg/m2. DIC depletion and DO supersaturation characterized the mat habitat for most of the day and much of the growing season. Furthermore, net photosynthesis, daily carbon gain, and relative growth rate (RGR) of H. verticillata were reduced 80% by mat conditions compared to edge conditions. Τs of H. verticillata were positively correlated with CO2 and bicarbonate concentration, and negatively correlated with pH, DO, and biomass. Low and high Τ photosynthetic phenotypes were associated with the mat and edge habitats, respectively. Photosynthetic phenotype of H. verticillata appears to acclimate to environmental heterogeneity within a clone in the field. Net photosynthesis and daily carbon gain of low Τphenotype H. verticillata was 128% and 40% greater than the high Τ phenotype when measured in the mat habitat, but was 21% lower than the high Τphotosynthetic phenotype when measured in the edge habitat under low quantum flux. Laboratory experiments showed a negative curvilinear relationship between the Τ of H. verticillata and plant density. The data demonstrate that plasticity in photosynthetic phenotype of H. verticillata is a density—dependent, physiological response that optimizes carbon gain within a stressful heterogeneous environment. Evolution of facultative C4—like photosynthetic metabolism in H. verticillata may have been an adaptation to the contraints imposed upon carbon gain by DIC and quantum flux limitation in the mat habitat.

During a shipboard expedition to Andros Island (Bahamas), photosynthetic measurements for the macroalgae Cladophoropsis membranacea (Chlorophyta), Dilophus guineensis, Turbinaria turbinata, and Lobophora variegata (Phaeophyta), and Laurencia papillosa (Rhodophyta), taken directly from their marine habitat, showed that only Cladophoropsis was saturated at seawater inorganic carbon levels (2.5 mM). The photosynthetic k0.5 values for inorganic carbon ranged from 1.1 to 3.2 mM. Decreasing the pH at 2.5 mM inorganic carbon, and thus enhancing the CO2 by 30-fold, only slightly increased photosynthesis, suggesting that bicarbonate was the major assimilated form of inorganic carbon. At 2.5 mM inorganic carbon, only Lobophora exhibited a Warburg effect on photosynthesis (49%), but at 0.5 mM, Turbinaria and Laurencia were also inhibited by O2. Ribulosebisphosphate carboxylase–oxygenase appeared to be the predominant carboxylation enzyme, but in Dilophus and Laurencia extracts, its activity was rivaled by phosphoenolpyruvate carboxylase and carboxykinase. Malate pools were detected in Turbinaria and Laurencia, and in the latter they were greater at night than during the day. However, this diel fluctuation was too small to implicate crassulacean acid metabolism. The data indicate that the bicarbonate concentration in seawater is insufficient to overcome O2 inhibition effects on photosynthesis, unless the macroalga has some form of CO2 concentrating system, based on bicarbonate uptake or C4 acid metabolism. In addition, the inorganic carbon in seawater may be a nutrient limiting the photosynthesis and productivity of certain macroalgae.

Ceratophyllum demersum L. remained physiologically active beneath ice of a southeastern Michigan lake. The effect of seasonally low photosynthetic photon flux density (PPFD) and cold but nonfreezing temperature on whole-plant physiology was studied. Net photosynthesis was measured at six temperatures and 12 PPFDs. Net photosynthesis, soluble protein concentration, ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) protein concentration, and Rubisco activity of winter plants were 32, 31, 33, and 70% lower, respectively, than those of plants collected in the summer. Optimum temperatures for net photosynthesis of winter and summer plants were 5 and 30'C, respectively. Dark respiration of winter plants was up to 313% greater than that of summer plants. Reduced Rubisco adivity and increased dark respiration interacted to reduce net photosynthesis. lnteraction of reduced net photosynthesis and increased dark respiration increased CO, and light compensation points and the light saturation point of winter plants. Crowth of C. demersum was limited by the ambient phosphorus concentration of lake water during summer. Apical stem segments of winter-collected plants had 54 and 35% more phosphorus and nitrogen, respectively, than summer-collected plants. Physiologically active perennation beneath ice enabled C. demersum to accumulate phosphorus during the winter when it was most abundant. Partia1 uncoupling of phosphorus acquisition from utilization may reduce phosphorus limitation upon growth during the summer when phosphorus concentration is seasonally the lowest.

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