Compensation Point Of Photosynthesis Research Articles

Water relations and gas exchange of fan bryophytes and their adaptations to microhabitats in an Asian subtropical montane cloud forest.

Fan life forms are bryophytes with shoots rising from vertical substratum that branch repeatedly in the horizontal plane to form flattened photosynthetic surfaces, which are well suited for intercepting water from moving air. However, detailed water relations, gas exchange characteristics of fan bryophytes and their adaptations to particular microhabitats remain poorly understood. In this study, we measured and analyzed microclimatic data, as well as water release curves, pressure-volume relationships and photosynthetic water and light response curves for three common fan bryophytes in an Asian subtropical montane cloud forest (SMCF). Results demonstrate high relative humidity but low light levels and temperatures in the understory, and a strong effect of fog on water availability for bryophytes in the SMCF. The facts that fan bryophytes in dry air lose most of their free water within 1h, and a strong dependence of net photosynthesis rates on water content, imply that the transition from a hydrated, photosynthetically active state to a dry, inactive state is rapid. In addition, fan bryophytes developed relatively high cell wall elasticity and the osmoregulatory capacity to tolerate desiccation. These fan bryophytes had low light saturation and compensation point of photosynthesis, indicating shade tolerance. It is likely that fan bryophytes can flourish on tree trunks in the SMCF because of substantial annual precipitation, average relative humidity, and frequent and persistent fog, which can provide continual water sources for them to intercept. Nevertheless, the low water retention capacity and strong dependence of net photosynthesis on water content of fan bryophytes indicate a high risk of unbalanced carbon budget if the frequency and severity of drought increase in the future as predicted.

Complementary ecophysiological strategies combine to facilitate survival in the hostile conditions of a deep chlorophyll maximum

In the deep, cooler layers of clear, nutrient-poor, stratified water bodies, phytoplankton often accumulate to form a thin band or "deep chlorophyll maximum" (DCM) of ecological importance. Under such conditions, these photosynthetic microorganisms may be close to their physiological compensation points and to the boundaries of their ecological tolerance. To grow and survive any resulting energy limitation, DCM species are thought to exhibit highly specialised or flexible acclimation strategies. In this study, we investigated several of the adaptable ecophysiological strategies potentially employed by one such species, Chlamydomonas acidophila: a motile, unicellular, phytoplanktonic flagellate that often dominates the DCM in stratified, acidic lakes. Physiological and behavioural responses were measured in laboratory experiments and were subsequently related to field observations. Results showed moderate light compensation points for photosynthesis and growth at 22°C, relatively low maintenance costs, a behavioural preference for low to moderate light, and a decreased compensation point for photosynthesis at 8°C. Even though this flagellated alga exhibited a physiologically mediated diel vertical migration in the field, migrating upwards slightly during the day, the ambient light reaching the DCM was below compensation points, and so calculations of daily net photosynthetic gain showed that survival by purely autotrophic means was not possible. Results suggested that strategies such as low-light acclimation, small-scale directed movements towards light, a capacity for mixotrophic growth, acclimation to low temperature, in situ exposure to low O(2), high CO(2) and high P concentrations, and an avoidance of predation, could combine to help overcome this energetic dilemma and explain the occurrence of the DCM. Therefore, corroborating the deceptive ecophysiological complexity of this and similar organisms, only a suite of complementary strategies can facilitate the survival of C. acidophila in this DCM.

Effects of limitation stress and of disruptive stress on induced antigrazing defense in the bladder wrack Fucus vesiculosus

We assessed the effects of light limitation and temperature shift on palatability and induced antiherbivore defense in the brown alga Fucus vesiculosus L. Incubation for 2 wk at light intensities above the compensation point of photosynthesis and in the absence of grazers increased the palatability of F. vesiculosus and its subsequent consumption by the omnivorous isopod Idotea baltica Pallas. This effect correlated with an increased C:N ratio and mannitol content in the algal tissue, presumably due to increased photosynthetic carbon fixation. Mannitol, the primary product of photosynthesis in F. vesiculosus, proved to be a feeding cue for I. baltica, and depletion of the mannitol pool may therefore account for the reduced palatability during light limitation. At light intensities above the compensation point of photosynthesis, F. vesiculosus responded with decreasing palatability when it was exposed to I. baltica grazing. Irrespective of the preceding light regime, such defense induction was prevented during incubation under light limitation. Thus, under low light, defense induction is not only inhibited, but also less necessary due to the relative absence of feeding cues. Upward or downward shifts in water temperature by approximately 10°C also inhibited inducible defense in F. vesiculosus. However, such shifts did not affect algal growth and were therefore the consequence of an impairment of specific defense-related components rather than of resource limitation, unless compensatory growth was given priority over defense.

The uptake of amino acids by the cyanobacterium Planktothrix rubescens is stimulated by light at low irradiances

The rates of uptake of five amino acids--alanine, glutamate, glycine, leucine and serine--by axenic cultures of the cyanobacterium Planktothrix rubescens were measured over a range of irradiances using the (14)C-labelled amino acids at the nanomolar concentrations observed in Lake Zürich. The rates in the light exceeded the dark rates by as much as two- to ninefold. The light-affinity constants for stimulation were similar, indicating a similar process for each of the five amino acids. The E(k) (light saturation irradiance) for light stimulation was only 1 micromol m(-2) s(-1), less than the compensation point for photosynthesis and autotrophic growth, and much lower than the E(k) for either process. The E(k) for amino acid uptake was also less than the irradiance at which filaments obtain neutral buoyancy, which determines the depth at which they stratify and the irradiance they receive. This indicates that stimulation of amino acid uptake by light of low irradiances provides a mechanism for supplementing growth of filaments stratifying deep in the metalimnion, which, while able to grow at low irradiances, are often left with insufficient light to sustain them. Acetate uptake was also stimulated by light, but the kinetics differed.

Regulation of root anaerobiosis and carbon translocation by light and root aeration in Isoetes alpinus

ABSTRACTThe activity of alcohol dehydrogenase (ADH) was measured in corms and roots of the submerged freshwater macrophyte Isoetes alpinus Kirk. growing in situ, and related to its capacity for internal oxygen transport and to carbohydrate translocation. ADH activity was present in roots but not corms at uniform activity (0.15–0.35 × 10−6 mol g−1 fresh weight s−1) over the entire plant depth range (3–7 m depth), and was intermediate to that developed in excised roots after 1‐week exposure to either dissolved oxygen at air‐saturation or to anoxia. Responses of photosynthesis and root oxygen release to light intensity confirmed that shoot‐to‐root oxygen transport saturated at similar light intensities to photosynthetic oxygen evolution, but was positive in the dark and at irradiances below the compensation point for photosynthesis, due to contributions to transport by oxygen diffusion from the external medium. Transport of 14C‐labelled photo‐assimilates to roots nevertheless ceased when intact plants were exposed to a combination of leaf darkness and root external anoxia, even when high 14C concentrations were present in shoots, but remained high when the roots were provided with external oxygen. The lack of any control over permeability of the root surface to gases in this species suggested that ADH activity and reduced translocation is most likely caused by development of hypoxic tissues in the apical tissue. These results suggest that reductions in ambient light intensity may have indirect effects on I. alpinus viability by increasing the degree of root hypoxia and impairing carbon partitioning.

Cyclic-AMP levels in the lichen Evernia prunastri are modulated by light quantity and quality

Changes in the accumulation of cAMP levels were measured by the isotope dilution assay using protein kinase A in the lichen Evernia prunastri at varying light conditions. cAMP levels decreased following exposure to low irradiance (20 μmol quanta m −2 s −1, and below the compensation point for photosynthesis) of red light (600–710-nm wave length) and increased by 50% after far-red light illumination (15 μmol quanta m −2 s −1, 710–800-nm wavelength). Far-red partially reverted the effect of red light when the former was supplied after the latter. cAMP increased to its maximum level under high irradiance supplied by a non-photomorphogenic yellow light source (400 μmol quanta m −2 s −1, reaching photosynthetic saturation). The addition of small quantities of red and far-red light, however, had profound restricting effects on cAMP accumulation. The addition of inhibitors of electron transport chains did not promote any significant change in cAMP levels in any of the treatments, indicating that cAMP accumulation could not depend on ATP synthesis. We propose that the response of cAMP accumulation at low irradiance comprises the activation of a morphogenic pathway through a red/far-red photoreceptor. In addition, at high irradiance the response would occur most likely through photosystems II and I acting as sensors of light quantity, that can be strongly modified by the red/far-red photomorphogenic system. Thus, cAMP would be involved in sensing the overall light environment.

Interactions between light and CO2 enhance the growth of Riccia fluitans

We grew the amphibious liverwort, Riccia fluitans, at saturating nitrogen and phosphorus concentrations in a cross-factorial design under three levels of light and three levels of CO2 making a matrix of nine treatments. Under these conditions, the relative growth rate (RGR) ranged from 0.011 d−1 at low light and CO2 availability to 0.138 d−1 at high light and CO2 with a significant positive interaction between light and CO2 on the RGR. After the growth experiments, a range of photosynthetic parameters were measured and in particular the maximum net photosynthesis (NPmax) showed a strong acclimation to light and CO2 availability. NPmax decreased significantly with increasing light intensities but increased with increasing CO2 concentration. Surprisingly, no positive correlation between the dark respiration (R) and the RGR was found. Rather, a strong positive correlation between NPmax and R was present and thus a positive correlation between R and RGR cannot be obtained since NPmax and RGR did not develop in parallel with increasing light and CO2 availability. The CO2 compensation point for photosynthesis was also strongly affected by the availability of light and CO2. The CO2 compensation point was very low a high light and low CO2 and increased at low light and high CO2 and there were significant interactions between light and CO2 on the CO2 compensation point throughout the entire experimental matrix. The observed responses to changes in light and CO2 availability and the interactions between the two will alleviate CO2 limitations in dense buoyant mats where the light is typically high. On the other hand, these interactions will also allow penetration to greater depths where light is scarce because the higher CO2 near the bottom can increase the light use efficiency.

Rapid inhibition of HCO3− use by high concentration of free CO2 in Elodea canadensis

The effect of exposure at a high concentration of free CO2 (denoted henceforth as CO2) in simple bicarbonate media on alkalization (increase of pH) of the media was investigated in the submerged macrophyte, Elodea canadensis Michx., in short-term experiments. Experimental plants were exposed at a high CO2 concentration of 0.45–1.35 mM in bicarbonate media of total alkalinity (TA) 0.3–9.8 mmol I−1 directly at the beginning of pH-drift experiments. In other cases, the plants were pretreated with 1 mM CO2 for a given period (1–60 min) and exposed afterwards. Plants having been exposed in the media with 1 mM CO2 for 30–60 min alkalized these media to a lesser extent thereafter, i.e. their photosynthetic HCO3− affinity was strongly reduced. The HCO3− compensation point of photosynthesis (carbon-based rates) was about 0.22 mM at pH 10.47 in the untreated controls, but about 0.69 mM at pH 9.60 in CO2-pretreated plants. The pretreated plants showwed a substantial reduction in HCO3− affinity lasting up to 12 h, although a very slow recovery of HCO3− affinity took place during this time. In Ceratophyllum demersum L., however, no marked reduction in HCO3− affinity was found after the CO2 pretreatment. In all measurements, the initial TA of media did not stay constant and dropped by 0.02–0.20 mmol 1−1. This finding shows that the employment of the pH-drift method is problematic.

CO 2 Compensation Point for Photosynthesis: Effect of Variable CO 2 and Soil Moisture Levels

CO 2 Compensation Point for Photosynthesis: Effect of Variable CO 2 and Soil Moisture Levels