Light-saturated CO2 Assimilation Rate Research Articles

An increase in water deficit has no impact on the photosynthetic capacity of field-grown Mediterranean plants.

In the Mediterranean, annual mean precipitation has continuously decreased over the last three years (by ca 36% in Barcelona), and the decrease has been dramatic during the summer (by ca 78 and 64% during July and August, respectively). The impact of increased drought on the photosynthetic capacity of Mediterranean vegetation is currently unknown. In this study, two native Mediterranean plants [rosemary (Rosmarinus officinalis L.) and lavender (Lavandula stoechas L.)] were grown outdoors and subjected to two water regimes (50 mm month-1 during the summer, or no supplementary water at all). Rosemary and lavender plants watered with 50 mm month-1 during the summer had higher relative leaf water content and water potential than non-watered plants. Changes in water status were accompanied by large decreases in parameters of gas exchange [i.e. the light-saturated rate of CO2 assimilation, the maximum velocity of ribulose-1,5-bisphosphate (RuBP) carboxylation by Rubisco and the capacity for RuBP] and of modulated chlorophyll fluorescence (i.e. the relative quantum efficiency of PSII photochemistry and the efficiency of energy capture by open PSII reaction centres) during the summer, but no differences were found in any photosynthetic parameters for leaves subjected to the two water regimes. The drought-induced decreases in the relative quantum efficiency of PSII photochemistry in rosemary and lavender plants were attributable to 'downregulation' of electron transport. Photodamage to PSII in the field appeared to be a later effect of drought in these plants. Photorespiration was not a major mechanism protecting the photosynthetic apparatus of these plants from photodamage in the field. After the autumn rainfall, photosynthetic capacity fully recovered. We conclude that rosemary and lavender are well adapted to drought, and that an increase in water deficit is unlikely to have a significant impact on the photosynthetic capacity of leaves.

Growth, photosynthesis and ozone uptake of young beech ( Fagus sylvatica L.) in response to different ozone exposures

Beech seedlings (Fagus sylvatica L.) were exposed to episodes of O3 in environmentally controlled growth chambers during one growing season. Three treatments were applied: charcoal-filtered air, charcoal-filtered air with the addition of 40 ppb O3 for seven episodes of 5 days' duration (9000–1700 hours), and charcoal-filtered air with the addition of 100 ppb O3 for seven episodes of 5 days' duration (9000–1700 hours). The accumulated exposure over a threshold of 40 ppb in the last treatment reached 13,911 ppb h. Throughout the growing season we measured growth as well as photosynthetic properties and related effects to external and calculated internal doses of O3, using stomatal conductance (gs) data. Growth, measured as diameter increment and biomass, was not significantly affected by the O3 treatments. In the 100-ppb treatment, light-saturated CO2 assimilation rates and chlorophyll content were significantly reduced, and the chlorophyll fluorescence parameter Fv/Fm was significantly reduced at times of high uptake rates and coincided with strong reductions of assimilation rates. O3 uptake was lowered in the 100-ppb treatment due to reduced gs. There was serious visible damage by the end of the exposure period in the 100-ppb treatment, while the treatment with 40 ppb O3 did not seem to cause any significant changes.

Effects of drought on photosynthesis in Mediterranean plants grown under enhanced UV-B radiation

The effects of drought on the photosynthetic characteristics of three Mediterranean plants (olive, Olea europea L.; rosemary, Rosmarinus officinalis L.; lavender, Lavandula stoechas L.) exposed to elevated UV-B irradiation in a glasshouse were investigated over a period of weeks. Drought conditions were imposed on 2-year-old plants by withholding water. During the onset of water stress, analyses of the response of net carbon assimilation of leaves to their intercellular CO2 concentration were used to examine the potential limitations imposed by stomata, carboxylation velocity and capacity for regeneration of ribulose 1,5-bisphosphate on photosynthesis. Measurements of chlorophyll fluorescence were used to determine changes in the efficiency of light utilization for electron transport, the occurrence of photoinhibition of photosystem II photochemistry and the possibility of stomatal patchiness across leaves. The first stages of water stress produced decreases in the light-saturated rate of CO2 assimilation which were accompanied by decreases in the maximum carboxylation velocity and the capacity for regeneration of ribulose 1,5-bisphosphate in the absence of any significant photodamage to photosystem II. Leaves of rosemary and lavender were more sensitive than those of olive during the first stages of the drought treatment and also exhibited increases in stomatal limitation. With increasing water stress, significant decreases in the maximum quantum efficiency of photosystem II photochemistry occurred in lavender and rosemary, and stomatal limitation was increased in olive. No indication of any heterogeneity of photosynthesis was found in any leaves. Drought treatment significantly decreased leaf area in all species, an important factor in drought-induced decreases in photosynthetic productivity. Exposure of plants to elevated UV-B radiation (0.47 W m(-2)) prior to and during the drought treatment had no significant effects on the growth or photosynthetic activities of the plants. Consequently, it is predicted that increasing UV-B due to future stratospheric ozone depletion is unlikely to have any significant impact on the photosynthetic productivity of olive, lavender and rosemary in the field.

Effects of drought on photosynthesis in Mediterranean plants grown under enhanced UV-B radiation.

The effects of drought on the photosynthetic characteristics of three Mediterranean plants (olive, Olea europea L.; rosemary, Rosmarinus officinalis L.; lavender, Lavandula stoechas L.) exposed to elevated UV-B irradiation in a glasshouse were investigated over a period of weeks. Drought conditions were imposed on 2-year-old plants by withholding water. During the onset of water stress, analyses of the response of net carbon assimilation of leaves to their intercellular CO2 concentration were used to examine the potential limitations imposed by stomata, carboxylation velocity and capacity for regeneration of ribulose 1,5-bisphosphate on photosynthesis. Measurements of chlorophyll fluorescence were used to determine changes in the efficiency of light utilization for electron transport, the occurrence of photoinhibition of photosystem II photochemistry and the possibility of stomatal patchiness across leaves. The first stages of water stress produced decreases in the light-saturated rate of CO2 assimilation which were accompanied by decreases in the maximum carboxylation velocity and the capacity for regeneration of ribulose 1,5-bisphosphate in the absence of any significant photodamage to photosystem II. Leaves of rosemary and lavender were more sensitive than those of olive during the first stages of the drought treatment and also exhibited increases in stomatal limitation. With increasing water stress, significant decreases in the maximum quantum efficiency of photosystem II photochemistry occurred in lavender and rosemary, and stomatal limitation was increased in olive. No indication of any heterogeneity of photosynthesis was found in any leaves. Drought treatment significantly decreased leaf area in all species, an important factor in drought-induced decreases in photosynthetic productivity. Exposure of plants to elevated UV-B radiation (0.47 W m(-2)) prior to and during the drought treatment had no significant effects on the growth or photosynthetic activities of the plants. Consequently, it is predicted that increasing UV-B due to future stratospheric ozone depletion is unlikely to have any significant impact on the photosynthetic productivity of olive, lavender and rosemary in the field.

Effects of ozone exposure in open-top chambers on poplar (Populus nigra) and beech (Fagus sylvatica): a comparison

Rooted cuttings of poplar (Populus nigra) and seedlings of beech (Fagus sylvatica) were exposed to ozone in open-top chambers for one growing season. Three treatments were applied: charcoal-filtered (CF), non-filtered (NF) and non-filtered air plus 30 ppb (nl l−1) ozone (NF+). Extra ozone was only added on clear days, from 09:00 until 17:00–20:00. The AOT40s (accumulated exposure over a threshold of 40 ppb), calculated from April to September were 4055 ppb.h for the NF and 8880 ppb.h for the NF+ treatments. For poplar ozone exposure caused highly significant reductions in growth rate, light-saturated net CO2 assimilation rate, stomatal conductance, Fv/Fm and chlorophyll content. The largest effects were observed in August at which time ozone concentrations were elevated. A reduction was noticed in new leaf production, while accelerated ageing and visible damage to leaves caused high leaf losses. For beech the responses were similar but less pronounced: ozone exposure resulted in non-significant growth reductions, slight changes in light-saturated photosynthesis and accelerated leaf abscission. The chlorophyll content of beech leaves was not affected by the ozone treatments. The results confirmed previous observations that fast-growing tree species, such as most poplar species and hybrids, are more sensitive and responsive to tropospheric ozone than slower-growing species, such as beech. The growth reductions observed and reported here for beech were within the range of those reported in relationship to the AOT40 (accumulated exposure over a threshold of 40 ppb) critical level for ozone.

Long-term responses of the green-algal lichen Parmelia caperata to natural CO2 enrichment.

Acclimation to elevated CO2 was investigated in Parmelia caperata originating from the vicinity of a natural CO2 spring, where the average daytime CO2 concentration was 729 ± 39 μmol mol-1 dry air. Thalli showed no evidence of a down-regulation in photosynthetic capacity following long-term exposure to CO2 enrichment in the field; carboxylation efficiency, total Ribulose bisphosphate carboxylase/oxygenase (Rubisco) content, apparent quantum yield of CO2 assimilation, and the light-saturated rate of CO2 assimilation (measured under ambient and saturating CO2 concentrations) were similar in thalli from the naturally CO2 enriched site and an adjacent control site where the average long-term CO2 concentration was about 355 μmol mol-1. Thalli from both CO2 environments exhibited low CO2 compensation points and early saturation of CO2 uptake kinetics in response to increasing external CO2 concentrations, suggesting the presence of an active carbon-concentrating mechanism. Consistent with the lack of significant effects on photosynthetic metabolism, no changes were found in the nitrogen content of thalli following prolonged exposure to elevated CO2. Detailed intrathalline analysis revealed a decreased investment of nitrogen in Rubisco in the pyrenoid of algae located in the elongation zone of thalli originating from elevated CO2, an effect associated with a reduction in the percentage of the cell volume occupied by lipid bodies and starch grains. Although these differences did not affect the photosynthetic capacity of thalli, there was evidence of enhanced limitations to CO2 assimilation in lichens originating from the CO2-enriched site. The light-saturated rate of CO2 assimilation measured at the average growth CO2 concentration was found to be significantly lower in thalli originating from a CO2-enriched atmosphere compared with that of thalli originating and measured at ambient CO2. At lower photosynthetic photon flux densities, the light compensation point of net CO2 assimilation was significantly higher in thalli originating from elevated CO2, and this effect was associated with higher usnic acid content.

Leaf CO2 assimilation and leaf dynamics in catch crops during autumn and winter at two levels of nitrogen supply

This study relates the leaf CO2 assimilation and leaf dynamics of nitrogen catch crops to environmental conditions. Winter rye (Secale cereale) and fodder radish (Raphanus sativus) were grown as catch crops in an outdoor pot experiment at two rates of nitrogen supply (N2 higher than N1) in Wageningen, Netherlands, from August 1993 until April 1994. Biomass increased with increasing N supply. There was no net growth after mid-November. The number of tillers in rye and of appeared leaves in radish were higher in N2 than in N1. Leaf appearance rate increased with temperature in fodder radish and rye. Tillering in rye ceased in mid-October. Leaf lifespan was related to the temperature sum between leaf emergence and leaf death. Leaf lifespan was 478 +or- 68 degrees C day in fodder radish. In rye, the leaf lifespan gradually decreased from 592 +or- 66 to 389 +or- 25 degrees C day and from 545 (1 observation) to 401 +or- 64 degrees C day in N1 and N2, respectively. In young leaves, Amax (light-saturated CO2 assimilation rate) was approximately 1.2 mg CO2 m-2 leaf s-1 in September and 0.5 mg CO2 m-2 leaf s-1 later on, independent of species and N supply, N supply affected the organic N concentrations only in the older leaves. Amax was not dependent on temperature at measurement (range: 12-19 degrees C in September, 5-15 degrees C from November until March), but, in contrast, strongly related to temperature and irradiance during the preceding growth period. Leaf nitrate concentrations increased with N supply. Water-soluble carbohydrate concentrations were higher in N1 than in N2 and higher in rye than in fodder radish. They fluctuated during the season. It is concluded that overall growth rates were limited by process rates other than that of leaf CO2 assimilation per unit leaf area in both N1 and N2.

Seasonal leaf phenotypes in the canopy of a tropical dry forest: photosynthetic characteristics and associated traits.

We evaluated the hypothesis that photosynthetic traits differ between leaves produced at the beginning (May) and the end (November-December) of the rainy season in the canopy of a seasonally dry forest in Panama. Leaves produced at the end of the wet season were predicted to have higher photosynthetic capacities and higher water-use efficiencies than leaves produced during the early rainy season. Such seasonal phenotypic differentiation may be adaptive, since leaves produced immediately preceding the dry season are likely to experience greater light availability during their lifetime due to reduced cloud cover during the dry season. We used a construction crane for access to the upper canopy and sampled 1- to 2-month-old leaves marked in monthly censuses for six common tree species with various ecological habits and leaf phenologies. Photosynthetic capacity was quantified as light- and CO2-saturated oxygen evolution rates with a leaf-disk oxygen electrode in the laboratory (O2max) and as light-saturated CO2 assimilation rates of intact leaves under ambient CO2 (Amax). In four species, pre-dry season leaves had significantly higher leaf mass per unit area. In these four species, O2max and Amax per unit area and maximum stomatal conductances were significantly greater in pre-dry season leaves than in early wet season leaves. In two species, Amax for a given stomatal conductance was greater in pre-dry season leaves than in early wet season leaves, suggesting a higher photosynthetic water-use efficiency in the former. Photosynthetic capacity per unit mass was not significantly different between seasons of leaf production in any species. In both early wet season and pre-dry season leaves, mean photosynthetic capacity per unit mass was positively correlated with nitrogen content per unit mass both within and among species. Seasonal phenotypic differentiation observed in canopy tree species is achieved through changes in leaf mass per unit area and increased maximum stomatal conductance rather than by changes in nitrogen allocation patterns.

Production Temperatures Influence Growth and Physiology of Leatherleaf Fern

Leatherleaf fern [Rumohra adiantiformis (Forst.) Ching] fronds produced under a high-temperature regime (HTR, 30 day/25C night) grew faster and produced sori earlier than those in a low-temperature regime (LTR, 20 day/15C night). Abaxial diffusive conductance was lower for HTR-grown fronds. Light-saturated net CO2 assimilation rates (Pn) and dark respiration were lower for HTR fronds, but light-saturated Pn efficiencies (chlorophyll basis); light compensation points; and soluble sugars, starch, and nonstructural carbohydrate levels were similar for the two regimes. Transpiration and water-use efficiency (mass basis) at light saturation were similar for fronds from both temperature treatments. Comparison of physiological characteristics of fronds from the two temperature regimes revealed no differences that might account for reduced postharvest longevity of fronds produced at the higher temperatures.

Physiological Benefits of Stem Curling for Resurrection Plants in the Field

The stems of the resurrection plant Selaginella lepidophylla, a piokilohydrous desert pteridophyte, curl dramatically as the plants dry and uncurl when rewetted. We tested the hypothesis that stem curling is a morphological feature that may serve to limit bright—light and/or thermal damage in resurrection plants with a field experiment at a Chihuahuan desert site in west Texas. Experimental plants were irrigated for 3 d and then permitted to dry. The plants wee either allowed to curl normally or mechanically restrained to prevent curling and either shaded (10% of ambient irradiance) or left fully exposed during desiccation. Nine days after complete desiccation, the plants were harvested and returned to the laboratory. When rehydrated, plants that had been unshaded and restrained from curling during field desiccation had significantly lower photosystem II electron transport rates, higher intrinsic chlorophyll fluorescence (Fo) yields, lower variable to maximum chlorophyll fluorescence (Fv/Fm) yield ratios, lower chlorophyll contents, and both lower whole—plant photosynthetic quantum efficiencies and light—saturated CO2 assimilation rates relative to plants from the other desiccation treatments. The results of both low—irradiance and high—irradiance hydrations in the laboratory suggest that the curling during desiccation and the uncurling during rehydration of S. lepidophylla in its natural habitat serves to protect the plant from damage due to high irradiance, high temperature, or perhaps a synergistic combination of the two. This protection permits the rapid recovery of photosynthetic competence when next the plants are wetted, thereby helping to maximize net carbon balance over time.

Effect of leaf nitrogen and water regime on the photosynthetic capacity of kenaf (Hibiscus cannabinus L.) under field conditions

To understand the influence of nitrogen (N) supply on the productivity of kenaf (Hibiscus cannabinus L.), the dependence of photosynthetic capacity, measured as light-saturated leaf CO2 assimilation rate (CA) and crop radiation use efficiency (RUE), on specific leaf nitrogen (SLN) was examined under well-watered conditions in the field. Photosynthetic capacity increased hyperbolically with SLN and was most responsive at SLN values less than c. 1.5 g m-2. The relationship reached a plateau at a maximum CA of 37 8mol m-2 s-1 and a maximum RUE of 1.5 g MJ-1 Increasing N application from 0 to 24 g N m-2 increased both RUE and SLN. Where ample N was applied, RUE and SLN were higher in those plots which previously experienced water deficit than in those that were always fully irrigated. The stimulation in growth following alleviation of water deficit was directly associated with the increase in SLN during water deficit. SLN varied according to differences in N uptake, the proportion of N allocated to leaves and the leaf area index. In particular, N application had much less effect on SLN and consequently RUE, than on N uptake, due to lower N partitioning to leaves and higher LAI where a high rate of N was applied. The importance of quantitative information on the relationship between photosynthetic capacity and SLN, in order to predict crop performance under varying environmental conditions, is discussed.

Partitioning of leaf nitrogen with respect to within canopy light exposure and nitrogen availability in peach (Prunus persica)

Relationships between leaf nitrogen content and within canopy light exposure were studied in mature nectarine peach trees (Prunus persica cv. Fantasia) that had received 0, 112, 196, 280 or 364 kg of fertilizer nitrogen per hectare per year for the previous 3 years. The relationships between light saturated leaf CO2 assimilation rates and leaf nitrogen concentration were also determined on trees in the highest and lowest nitrogen fertilization treatments. The slope of the linear relationship between leaf N content per unit leaf area and light exposure was similar for all nitrogen treatments but the y-intercept of the relationship increased with increasing N status. The slope of the relationship between leaf N content per unit leaf area and light saturated CO2 assimilation rates was greater for the high N trees than the low N trees, but maximum measured leaf CO2 assimilation rates were similar for both the high and low N treatments. A diagrammatic model of the partitioning of leaf photosynthetic capacity with respect to leaf light exposure for high and low nitrogen trees suggests that the major influence of increased N availability is an increase in the photosynthetic capacity of partially shaded leaves but not of the maximum capacity of highly exposed leaves.

Growth and Photosynthetic Response to Light and Nutrients of Flindersia brayleyana F. Muell., A Rainforest Tree With Broad Tolerance to Sun and Shade

Seed from four species of rainforest trees with widely contrasting sunlight requirements for growth and development were sown within disturbance gaps amidst mature forest on the Herberton Range in North Queensland. Observations on seedling persistence plus comparative growth of young trees of Acacia aulacocarpa, Toona australis, Flindersia brayleyana and Darlingia darlingiana (species ranked according to adaptation from full sun to deep shade) confirmed a broad tolerance of Flindersia to sunlight under all conditions, from wide to narrow gaps (minimum 0.6% full sun equivalent). Photosynthetic attributes which underlie such broad tolerance were subsequently inferred from single leaf gas exchange, plus foliar analyses of nitrogen, phosphorus and chlorophyll on tree seedlings held for 180 days under two nutrient × three irradiance levels adjusted to represent natural irradiance incident upon the forest floor (low), mid-canopy (medium) and emergent crowns (high irradiance treatment). Medium irradiance plus high nutrients proved optimal for leaf expansion, chlorophyll content and photosynthesis in air. Growth under low irradiance was characterised by thinner leaf palisade tissue, lower rates of dark respiration, increased leaf chlorophyll per unit nitrogen and lower light compensation point for photosynthesis. Such leaves retained a relatively high photosynthetic capacity despite these other shade-leaf attributes. High irradiance plus low nutrients proved supraoptimal for leaf expansion and expression of photosynthetic activity. Chronic photoinhibition appeared to prevail because apparent quantum yield was reduced, while photosynthetic processes on a nitrogen basis were substantially impaired. Nitrogen use efficiency, as inferred from leaf chlorophyll content, light saturated CO2 assimilation rate, electron transport rate and carboxylation rate on a nitrogen basis declined with increasing growth irradiance. Some ecological implications for the establishment and growth of these rainforest tree species in disturbance gaps are discussed.

Photosynthesis and stomatal characteristics of Striga hermonthica in relation to its parasitic habit

Photosynthesis and stomatal characteristics of the angiosperm parasite Striga her‐monthica (Del.) Benth. have been compared with those of Antirrhinum majus L. cv. Suttons Yellow Rust Resistant, a related, non‐parasitic species. The concentration of photosynthetic pigments in S. hermonthica leaves was less than 40% of those of A. majus leaves. Light saturated CO2 assimilation rate of S. hermonthica was less than 40% that of A. majus on a chlorophyll basis and under 20% than of A. majus on a leaf area basis.Stomata of Striga showed only partial closure in darkness, remained open in water stressed leaves and showed little response to exogenously applied abscisic acid. Stomatal conductance and transpiration were considerably higher in Striga compared with Antirrhinum. The high transpiration and low photosynthetic rates of S. hermonthica resulted in a low water use efficiency. The water relations of Striga leaves, while seemingly inappropriate for growth in drought prone environments, do appear to maximise nutrient and water acquisition from the host and as such may be an adaptation to the parasitic habit.

Photosynthetic responses to slowly decreasing leaf water potentials in Encelia frutescens.

The importance of reduced leaf conductance (stomatal and boundary layer) in limiting photosynthetic rates during water stress was studied in Encelia frutescens, a drought-deciduous leaved subshrub of the Mohave and Sonoran Deserts. Light-saturated CO2 assimilation rates of greenhouse grown plants decreased from 42.6±1.6μmol CO2 m-2 s-1 (x±s.e.) to 1.7±1.7 μmol CO2 m-2s-1 as leaf water potential decreased from-1.5 MPa to-4.0 MPa. The dependence of light saturated, CO2 assimilation rate on leaf intercellular CO2 concentrations between 60 and 335 μl l-1 was also determined as leaf water potential decline. This enabled us to compare the effects of leaf water potentials on limitations to carbon assimilation imposed by leaf conductance and by intrinsic photosynthetic capacity. Both leaf conductance and intrinsic photosynthetic capacity decreased with decreasing leaf water potential, but the decrease in leaf conductance was proportionately greater. The relative stomatal limitation, defined as the percent limitation in photosynthetic rate due to the presence of gas-phase diffusional barriers, increased from (x±s.e.) to 41±3% as water potentials became more negative. Since both leaf conductance and intrinsic photosynthetic capacity were severely reduced in an absolute sense, however, high photosynthetic rates could not have been restored at low leaf water potentials without simultaneous increases in both components.

Gas exchange responses of Chesapeake Bay tidal marsh species under field and laboratory conditions.

Laboratory and field gas exchange measurements were made on C3 (Scirpus olneyi Gray) and C4 (Spartina patens (Ait.) Mahl., Distichlis spicata (L.) Green) species from an irregularly flooded tidal marsh on the Chesapeake Bay. Laboratory measurements were made on plants grown from root stocks that were transplanted to a greenhouse and grown under high light and high nutrient conditions. The two C4 species were similar in their laboratory gas exchange characteristics: both had higher net carbon exchange rates, higher mesophyll conductances, higher photosynthetic temperature optima and lower leaf conductances than the C3 species. The laboratory photosynthetic water use efficiency of the C4 species was approximately three times that of the C3 species.Field gas exchange responses of the above species were measured in situ a Chesapeake Bay tidal marsh. Despite differences in biological potential measured in the laboratory, all three species had similar in situ carbon exchange rates on a leaf area basis. On a dry weight basis, leaves of the two C4 species had about 1.4 times higher light saturated CO2 assimilation rates than the C3 species. Light saturation of CO2 exchange occurred at photosynthetic photon flux densities of 80 n Einstein cm-2s-1, compared with 160 n Einstein cm -2s-1 in the laboratory grown plants. Spartina patens and Scirpus olneyi had similar daily CO2 assimilation rates, but the daily transpiration rate of the C3 species was almost twice that of the C4 species. Spartina patens showed greater seasonal decrease in photosynthesis than Distichlis spicata and Scirpus olneyi. The two C4 grass species maintained higher mesophyll conductances and photosynthetic water use efficiencies than the C4 sedge.

Inhibition of the Capacity and Efficiency of Photosynthesis in Bean Leaflets Illuminated in a CO2-Free Atmosphere at Low Oxygen: A Possible Role for Photorespiration

Mature, intact, attached leaflets of Phaseolus vulgaris illuminated at 2000 μE m-2 s-1 in CO2-free N2 containing 1% O2 for a 3-h treatment period showed an 80% reduction in the light-saturated CO2 assimilation rate. Young bean leaflets showed photoinhibition but exhibited a partial recovery of CO2 assimilation rate within 2 h of treatment whereas mature leaflets did not. Photoinhibition by illumination under conditions which prevent photorespiration was markedly dependent upon the exposure time. Photoinhibition was not observed following illumination in the presence of CO2-free N2 containing above 7% O2 or in N2 containing 1% O2 and an intercellular CO2 partial pressure equivalent to the CO2 compensation point in 21% O2 (62 μbar, 6.2 Pa). Photoinhibition involved concomitant reduction in the light-saturated and the light-limited rate of CO2 assimilation; apparent quantum yield was reduced following treatment. These experiments show that internal generation of CO2 by the photorespiratory carbon oxidation cycle is sufficient to prevent the photoinhibition observed when photorespiration is prevented. Possible mechanisms and implications of these data are discussed.

THE PHOTOSYNTHETIC RHYTHM OF ACETABULARIA CRENULATA. II. MEASUREMENTS OF PHOTOASSIMILATION OF CARBON DIOXIDE AND THE ACTIVITIES OF ENZYMES OF THE REDUCTIVE PENTOSE CYCLE

1. The photosynthetic rhythm of Acetabularia crenulata affects both light (quantum yield) and dark reactions in a parallel manner.2. No significant difference was found between the activity of RuDP carboxylase in the extracts of samples taken at the middle of the light and dark periods nor was any difference detected in the affinity of this enzyme to CO2. The activity of RuDP carboxylase in the cell extracts was sufficient to account for the observed rates of photoassimilation of CO2 at saturating light intensities.3. The activities of eight other enzymes of the reductive pentose phosphate cycle were also shown not to differ to a significant extent in extracts of cells taken at the middle of the light and dark periods. Five of these enzymes (phosphoglycerate kinase, glyceraldehyde-3-P dehydrogenase, triose isornerase, R-5-P isomerase and Ru-5-P kinase) had activities considerably above those required for the observed light-saturated rate of CO2 assimilation while the activities of aldolase transketolase and transaldolase were of the same order of magnitude as the observed rates of photoassimilation.