CO2 Diffusion Resistance Research Articles

Photosynthetic limitation of similar-height Cryptomeria japonica trees growing at different rates

We compared light-saturated photosynthetic rates and their stomatal limitations among Cryptomeria japonica trees with a similar height but different current growth rates. Although slow-growing trees had a lower stomatal conductance and a higher carbon isotope ratio in shoots, the stomatal limitations in photosynthesis were not different. Large mesophyll CO2 diffusion resistance contributed to the low photosynthetic rate of the slow-growing trees.

Why do Polytrichaceae have lamellae?

Light-response curves of chlorophyll-fluorescence parameters from 12 species of Polytrichaceae show photosynthetic electron flow giving a good fit to negative-exponential saturation curves up to high irradiances; 95%-saturation irradiances for the species of unshaded habitats approached or exceeded noon summer sunlight. The species showed little or no indication of the non-saturating electron flow at high irradiances, or the very high values of non-photochemical quenching, seen in many other mosses of dry, sun-exposed habitats. Calculation of CO2 diffusion rates from first principles on a cell-area basis indicates that photosynthesis of mosses with unistratose leaves is likely to be limited by CO2 diffusion resistance at high irradiances. In Polytrichaceae the area for CO2 uptake is often >6 times the projected leaf area. By lowering the whole-leaf resistance to CO2 uptake, the lamellae remove the CO2-diffusion constraint that limits mosses with unistratose leaves and allow Polytrichaceae to utilize efficiently the high irradiance of direct sunlight.

Spatial and age-dependent modifications of photosynthetic capacity in four Mediterranean oak species.

Drought is one of the most important limitations of photosynthesis in Mediterranean climates. However, Mediterranean sclerophyllous species with long-lived leaves also support extensive and dynamic canopies, with potentially large spatial and age-dependent gradients. We studied within-canopy and temporal patterns in foliage structure, chemistry and photosynthesis in the evergreen species Quercus coccifera L., Q. ilex L. subsp. ballota (Desf.) Samp. in Bol. and Q. suber L. and in the semi-deciduous marcescent species Q. faginea Lam. to determine the role of within-canopy shading and leaf age on foliage functioning. There was a 2.5-fold within-canopy gradient in leaf dry mass per unit area (MA) that was accompanied by a 3-fold range in area-based leaf nitrogen (N) content, the capacity for photosynthetic electron transport (Jmax) and maximum Rubisco carboxylase activity (Vcmax), while the fractional investments of leaf nitrogen in electron transport (FB) and in Rubisco (FR) were relatively constant within the canopy. Leaf aging led to increased MA, larger or constant mass-based N content, larger phosphorous (P) and structural carbon contents, but decreased movable cation contents. Age-dependent increases in MA and N per dry mass meant that Jmax and Vcmax per area were weakly related to leaf age, with a trend of decreasing values in older leaves. However, Jmax and Vcmax per unit dry mass decreased 4-fold across the range of leaf age, primarily owing to decreases in apparent N investments in photosynthetic machinery. This decrease in apparent N investments in photosynthetic machinery was possibly the result of a larger fraction of N bound to cell walls, or of an enhanced CO2 diffusion resistance from the outer surface of cell walls to the chloroplasts in older leaves with thicker and more lignified cell walls. The age-dependent variation in apparent fractional investments of N in photosynthetic machinery reduced the generality of leaf nitrogen v. photosynthesis relationships. Photosynthetic characteristics qualitatively fitted the same patterns with leaf age in all species, but at a common leaf age, area-based leaf photosynthetic potentials depended on species-specific values of MA. These data collectively demonstrate important canopy and age-dependent controls on leaf structure, chemistry and photosynthetic potentials that should be included in larger-scale photosynthesis simulations in Mediterranean climates.

Sink-source transition in tobacco leaves visualized using chlorophyll fluorescence imaging.

• The sink-source transition of developing Nicotiana tabacum (tobacco) leaves was studied here using chlorophyll fluorescence imaging. • In accordance with leaf development, the quantum efficiency of PSII, showed a steep gradient across the leaf with increasing values towards the tip. • The linear electron transport rate (ETR) saturated at higher CO2 concentrations in the younger, than in the mature, part of the leaf, probably due to a lower Rubisco activity or a higher CO2 diffusion resistance. • The induction of ETR at CO2 concentrations near the compensation point after long-term dark adaptation of the young leaf, showed distinct responses; ETR rose rapidly in the basal but more slowly in the apical regions. There was a correlation between fast induction and carbohydrate import, as measured by 14 C-translocation. In the basal regions, larger pools of metabolic intermediates are expected due to imported carbohydrates. These might be used in the Calvin cycle directly after dark-light transition providing the electron acceptors for the faster induction of ETR. Additionally, a higher mitochondrial respiration can provide CO2 for the Calvin cycle in these regions.

CO2 diffusion resistance of bundle sheath cells in Amaranthus edulis, as revealed from the study of a PEP carboxylase deficient mutant

A mutant of the NAD-ME type C4 plant Amaranthus edulis, that lacks PEP carboxylase in the mesophyll cells was studied. Analysis of CO2 response curves of the mutant in the absence of a functional C4 cycle, provided a direct means of determining the liquid phase diffusive resistance of atmospheric CO2 to the sites of RuBP carboxylation inside the bundle sheath (BS) chloroplasts (rbs) within intact plants. The magnitude of rbs has been difficult to determine and previous estimates using different methods have ranged from 15 -1400 m2 s mol-1. The values of rbs in A. edulis were 70-180 m2 s mol-1 and they increased as the leaf matured. This is about 70-fold higher than the liquid phase resistance or diffusion of CO2 to Rubisco in the mesophyll cells of C3 plants. The values of rbs in A. edulis are sufficient for C4 photosynthesis to elevate the CO2 concentration inside the BS cells and thus reduce photorespiration. High reassimilation of photorespired CO2 was demonstrated in both mutant and wild type plants at limiting CO2 concentrations, which can be explained by high rbs. Based on O2 sensitivity analysis of wild type plants, the calculated CO2 concentration in BS cells under maximum rates of CO2 fixation in young leaves (lower rbs) and in expanded leaves (higher rbs) was about the same (1000-1200 m bar), which is sufficient to minimise photorespiration. This indicates that overcycling of the pathway and the rate of leakage of CO2 from the BS cells was higher in the younger leaves.

COENOGONIUM: A GREEN ALGAL LICHEN WITHOUT PHOTOSYNTHETIC DEPRESSION AT HIGH WATER CONTENTS

For experiments run at 350 ppm CO 2, the subtropical lichen Coenogoniumdoes not exhibit net photosynthetic depression at water contents above those at which initial maximal values are reached. This is in contrast to most lichens for which such a depression occurs. We infer that the results reflect the unusual morphology of Coenogonium, in which the alga is the dominant symbiont, occurring as filaments at the lichen surface. This arrangement should minimize CO 2diffusion resistance to the algal surface. With light saturation occurring by 70 μmol photons m −2s −1, Coenogoniiumhas one of the lowest light saturation values reported among lichens.

Coenogonium: A Green Algal Lichen Without Photosynthetic Depression at High Water Contents

For experiments run at 350 ppm CO2, the subtropical lichen Coenogonium does not exhibit net photosynthetic depression at water contents above those at which initial maximal values are reached. This is in contrast to most lichens for which such a depression occurs. We infer that the results reflect the unusual morphology of Coenogomum, in which the alga is the dominant symbiont, occurring as filaments at the lichen surface. This arrangement should minimize CO2 diffusion resistance to the algal surface. With light saturation occurring by 70 μmol photons m−2 s−1, Coenogoniium has one of the lowest light saturation values reported among lichens.

Photosynthetic Depression at High Thallus Water Contents in Lichens: Concurrent Use of Gas Exchange and Fluorescence Techniques with a Cyanobacterial and a Green Algal Peltigera Species

Abstract: Lichens, being poikilohydric, have varying thallus water contents (WC) and show a complex interaction between net photosynthesis (NP) and WC. NP can be depressed at low WC (desiccation effects) and, in some species, also at high WC. In the latter case the depression is normally ascribed to increased CO2 diffusion resistances through water blockage. Recently, an earlier explanation, that the depression at high WC is due to recycling of CO2 from increased dark respiration processes (DR), has been given renewed prominence.The two explanations were distinguished by the concurrent use of gas exchange and chlorophyll fluorescence techniques to investigate NP: WC relationships in the lichens Peltigera leucophlebia (green algal) and P. neckeri (cyanobacterial). Both species had a distinct optimal WC for NP with depressed values at low and high WC. The maximal quantum yield for both CO2 fixation (initial slope of light response curves of NP) and photosystem II (fluorescence signals of dark‐adapted thalli) was depressed only at low WC and remained high at optimal and greater WC. In contrast, the relative electron transport rate (ETR, derived from fluorescence signals of thalli in the light) tracked NP and was depressed at low and high WC. The depression of both NP and ETR at high WC (not that at low WC) could be prevented by using elevated external CO2 concentrations. A single, linear relationship was found between all values of gross photosynthesis (NP + DR) and ETR regardless of external CO2 concentration or WC.Our results show that, for these lichens, the depression in NP at high WC is a real fall in photosynthetic rate of the photobionts and is not due to recycling of CO2. The removal of the depression in NP and ETR at high WC by using elevated external CO2 levels allows us to conclude that an additional CO2 diffusion resistance is present.

Photosynthetic carbon assimilation by Crassula helmsii.

Photosynthesis of Crassula helmsii, an amphibious aquatic macrophyte weed species, has been measured with respect to pH and irradiance. C. helmsii shows a marked diel fluctuation in titratable acidity, which can be accounted for by changing levels of malic acid. C. helmsii is unable to use HCO inf3sup- for photosynthesis and exhibits generally low photosynthetic rates when CO2 is not limiting. The photon flux density at which the onset of light saturation of photosynthesis is reached (E K ) is low for aquatic macrophytes. Some advantages conferred on C. helmsii by the possession of crassulacean acid metabolism are an extension of the period of assimilation of dissolved inorganic carbon, resulting in a reduction in the limitation imposed on photosynthesis in aquatic environments by a very high CO2 diffusion resistance.

Photobiont-related differences in carbon acquisition among green-algal lichens

The photosynthetic properties of a range of lichens (eight species) containing green algal primary photobionts of either the genus Coccomyxa, Dictyochloropsis or Trebouxia were examined with the aim of obtaining a better understanding for the different CO2 acquisition strategies of lichenized green algae. Fast transients of light/dark-dependent CO2 uptake and release were measured in order to screen for the presence or absence of a photosynthetic CO2-concentrating mechanism (CCM) within the photobiont. It was found that lichens with Trebouxia photobionts (four species) were able to accumulate a small pool of inorganic carbon (DIC; 70–140 nmol per mg chlorophyll (Chl)), in the light, which theoretically may result in, at least, a two to threefold increase in the stromal CO2 concentration, as compared to that in equilibrium with ambient air. The other lichens (four species), which were tripartite associations between a fungus, a cyanobacterium (Nostoc) and a green alga (Coccomyxa or Dictyochloropsis) accumulated a much smaller pool of DIC (10–30 nmol·(mg Chl)−1). This pool is most probably associated with the previously documented CCM of Nostoc, inferred from the finding that free-living cells of Coccomyxa did not show any signs of DIC accumulation. In addition, the kinetics of fast CO2 exchange for free-living Nostoc were similar to those of intact tripartite lichens, especially in their responses to the CCM and the carbonic anhydrase (CA) inhibitor ethoxyzolamide. Trebouxia lichens had a higher photosynthetic capacity at low and limiting external CO2 concentrations, with an initial slope of the CO2-response curve of 2.6–3.9 μmol·(mg Chl)−1·h−1·Pa−1, compared to the tripartite lichens which had an initial slope of 0.5–1.1 μmol-(mg Chl)−1·h−1·-Pa−1, suggesting that the presence of a CCM in the photobiont affects the photosynthetic performance of the whole lichen. Regardless of these indications for the presence or absence of a CCM, ethoxyzolamide inhibited the steady-state rate of photosynthesis at low CO2 in all lichens, indicating a role of CA in the photosynthetic process within all of the photobionts. Measurements of CA activity in photobiont-enriched homogenates of the lichens showed that Coccomyxa had by far the highest activity, while the other photobionts displayed only traces or no activity at all. As the CCM is apparently absent in Coccomyxa, it is speculated that this alga compensates for this absence with high internal CA activity, which may function to reduce the CO2-diffusion resistance through the cell.

Nitrogen Source Regulation of Growth and Photosynthesis in Beta vulgaris L.

Sugar beets (Beta vulgaris L. cv F58-554H1) were grown hydroponically in a 16-h light, 8-h dark period at a photosynthetic photon flux density of 0.5 mmol m-2 s-1 for 4 weeks in half-Hoagland culture solution containing only nitrate-nitrogen. Half of the plants were then transferred to half-Hoagland solution with ammonium-nitrogen (7.35mM), while the other half continued on 7.5 mM nitrate. Growth analysis was carried out by sampling the plants at 3-d intervals over a period of 21 d. Compared to plants supplied with nitrate, ammonium initially slowed the growth of shoots more than roots. Ammonium reduced both the area expansion of individual leaves and the relative water content of these leaves, but increased the amount of dry matter/area. The increase in specific leaf weight in ammonium-grown leaves was associated with a doubling of chloroplast volume, as much as a 62% rise in chlorophyll content, and a 4.3-fold higher accumulation of soluble protein. Ammonium nutrition substantially decreased the rate of expansion of photosynthetic (leaf) surface but did not decrease the rate of photosynthesis per area; in fact, net photosynthetic CO2 exchange rates were slightly higher than in nitrate plants, due to the build-up in stromal enzymes of the Calvin cycle, several of which increased in total extractable activity on a leaf area basis, e.g. ribulose-1,5- biphosphate carboxylase oxygenase, sedoheptulose-1,7-biphosphatase. Nitrogen source had no effect on stomatal conductance. Rates of photosynthesis per chlorophyll were decreased slightly in ammonium-grown leaves, possibly due to an increased CO2-diffusion resistance associated with the enlarged chloroplasts.

Effects of water deficit on photosynthesis in wheat plants. II. The physiological basis for the difference in photosynthetic sensitivity to water stress among plant parts.

In the previous paper, it was pointed out that the extent of depression in photosynthesis by soil water deficit treatment was different from part to part of the wheat plant. The objective of the present study was to find out the physiological basis for this phenomenon from the viewpoint of water potential and CO2 diffusion resistances, i.e. leaf resistance (r1) and mesophyll resistance (rm) using the same materials as those for the previous paper. The results are summarized as follows : 1. Water potential of each plant part was decreased by water deficit treatment. The extent of decrease in water potential was the same in every plant part. It was concluded form this finding that the difference in photosynthetic sensitivity among plant parts is not be attributable to the difference in the extent of decrease in water potential of plant parts (Figs. 1, 2). 2. Both r1 and rm were increased by the water deficit treatment. The extent of increase in r1 and rm was largest just after the start of the water deficit treatment. However, when the water deficit condition was prolonged, r1 and rm tended to decrease and come close to those of unstressed plants (Figs. 6-9). 3. Both r1 and rm increased to a larger extent in lower leaves than in upper ones by the water deficit treatment (Figs. 3-4). The extent of increase in r1 and rm resulted from water deficit was always larger in lower leaves than in upper ones (Figs. 5-9). Since there was no difference in the extent of decreasse in water potential by the water deficit treatment between lower and upper leaves, both stomata and the photosynthetic mechanism of lower leaves seemed to have greater sensitivity to decrease of water potential than those of upper leaves.

Influence of differences in leaf anatomy on net photosynthetic rates of some cultivars of Cassava

Gas exchange measurements and leaf anatomy of 10 cassava cultivars were conducted to study the interrelationship between the relatively high photosynthetic rates and the factors limiting internal CO2 diffusion. The internal mesophyll surface area per unit leaf surface area (A(mes)/A) and the intracellular components of CO2 diffusion and fixation resistance (R(cell)CO2) were determined. Among the group of cultivars tested net CO2 exchange rates were 26±2.5 μmol CO2 m(-2) s(-1) in normal air and intense light and A(mes)/A ranged from 14 to 38. Estimated R(cell)CO2 ranged from 4300 to 13,000 s m(-1). The combined and compensating effects of A(mes)/A and R(cell)CO2 accounted for both the high net photosynthetic rates (Pn) and the lack of large differences in Pn among cultivars.

Determination of Photosynthetic Capacity of Lichens in the Field - a Method for Measurement of Light Response Curves at Saturating CO2 Concentration

Summary Reproducible determination of the photosynthetic capacity of lichens is desirable in order to identify patterns of natural change in lichen metabolic activity and in order to effectively use lichens as bioindicators to monitor influences of air pollution. However, lichen photosynthetic CO2 exchange is highly dependent on the actual thallus water content, and experimental control of the state of hydration is difficult. At superoptimal water content and under natural ambient CO2 conditions, net photosynthetic rates are low due to large CO2 diffusion resistance in the thallus. However, high levels of external CO2 can establish saturating CO2 partial pressure at the sites of carboxylation. Thus at high CO2, one obtains a constant and reproducible photosynthetic response with lichens over a large range in high thallus water content. A newly developed, portable minicuvette system is described which allows quick and reliable determination of photosynthetic capacity of lichens in the field. Small, fully water saturated thallus samples (0.15 to 0.3 g dry mass) are included in a climatized chamber with attached light source. Carbon dioxide exchange is measured by means of an infrared gas analyzer at saturating external CO2 concentration (2,500 ppm) and under defined light and temperature conditions. Light response curves at constant temperature are demonstrated for several epiphytic lichen species. Several types of information, such as the maximal photosynthetic rate, quantum utilization efficiency, and light compensation point can be obtained from these measurements. We suggest that such parameters can be used to assess changes in lichen photosynthetic activity, for instance with respect to impact of pollutants. When combined with measurements of actual net photosynthesis rates obtained in time courses, the described methods will also help us to better understand on a physiological basis the behavior of lichens in their natural habitat.

Net CO2 exchange rates and diffusion resistance of leaves from chicory selections during vegetative growth

Net CO2 exchange rates, stomatal and internal resistances for CO2-transport were followed on fully expanded Witloof chicory leaves (Cichorium intybus L. cv. Foliosum) for several months during vegetative growth. Maximum net CO2 exchange rate (Pmax) stayed high with a sudden drastic drop at the end of the growing season largely due to an increase in internal diffusion resistance.

The Dependence of the Relation between Light Quality and Photosynthesis on Optical and Anatomical Characteristics of Leaves

The relationship was investigated between stomatal resistance and CO2 absorption rate under different light-qualities. The action spectrum of photosynthesis of leaves was discussed from the view point of CO2 diffusion resistance. CO2 absorption rate and stomatal resistance were measured at the upper and the lower surfaces of cabbage, cucumber and lettuce leaves under an irradiation of three different lights (blue-green, yellow-red and red) from either the upper or the lower surface side of the leaf.Results were as follows:(1) Stomatal resistance of lettuce-leaf was smaller at the surface receiving the light than that at the opposite surface in every light. The resistances of cabbage and cucumber-leaves were smaller at the lower surfaces than those at the upper surfaces regardless of the direction of irradiation. The stomatal resistance at the light-receiving-surface was the smallest under blue-green light in all plants. At the nonirradiated surface, the light quality caused some effect on stomatal resistances in lettuce but not in cabbage and cucumber.(2) CO2 absorption rate was larger at the surface with a smaller stomatal resistance than that with a larger resistance for all plants. But, CO2 absorption at each surface was not always large under the light situation from which a smaller stomatal resistance was derived. At the lower surface the stomatal resistance did not so remarkably affect CO2 absorption rate. At the upper surface, the stomatal resistance was the limiting factor of CO2 absorption for cabbage and cucumber, but not for lettuce.(3) The ratio of CO2 absorption rate at the upper surface to that at the lower surface varied with the light-quality and the irradiation direction for all plants. In the case of irradiation directed to the upper surface, the ratio for lettuce was the largest and those for cucumber and then cabbage followed in every light. It is considered that the differences in photosynthetic action among plants were caused by the differences in the distribution, the activity of stomata, and in the anatomical characteristics of leaves.(4) The relative value of photosynthesis in blue region in the action spectrum of photosynthesis increased with increasing ratio of the upper surface to the lower one in CO2 absorption rate in the case of irradiation directed to the upper leaf surface. This phenomena was explained by the difference in the extinction coefficient for light within a leaf. The blue light has a large extinction coefficient within a leaf and is mostly absorbed near the surface layer. Therefore, higher the ratio, the higher photosynthetic activity occurred in blue region.

Carbon dioxide exchange in lichens: Estimation of internal thallus CO2 transport resistances

The gaseous exchange pathways of Sticla latifrons Rich. and Pseudocyphellaria amphisticta Kremp. were examined using both light and scanning electron microscopes. The size and frequency of the pores in the gas exchange structures (cyphellae and pseudocyphellae) and in the medulla were measured and from these CO2 diffusion resistances were calculated. Pseudocyphellae were found to be smaller and more widely spaced than cyphellae, consequently the resistance of the pseudocyphellae, was much greater than that of the cyphellae. Medulla resistances were low in both lichens and are probably unimportant, even at high water contents. No evidence of hyphal swelling was found. Gas exchange structure resistances were more than five fold greater than medulla resistances. It is suggested that this arrangement of resistances may simultaneously encourage refixation of respired CO2 and maintain a non desiccating environment for the lichen algae. The internal transport resistances calculated in this work approximate experimentally obtained values.

Moisture content and CO2 exchange of lichens : I. Influence of temperature on moisture-dependent net photosynthesis and dark respiration in Ramalina maciformis.

Net photosynthesis (10 klx light intensity, 150 μE m-2 s-1 PAR) and dark respiration of the lichen Ramalina maciformis at different temperatures are measured in relation to thallus water content. Both first increase with increasing hydration. Dark respiration then remains constant with increased water content until thallus saturation. In contrast, a further increase in water content leads to a depression of net photosynthesis, as shown in previous studies, after a maximum of CO2 uptake has been attained. However, the extent of this depression depends strongly on temperature. In saturated thalli (160% water content in relation to lichen dry weight) the depression amounts to about 15% and 63% of the maximum unsaturated rate at 5°C and 25°C thallus temperature, respectively. The moisture compensation-point of net photosynthesis is also decisively determined by temperature (for 0°C at 20% water content; for 25°C at 15%), and the water content that allows maximum rates of CO2 uptake (for 0°C at 80%; for 25°C at less than 40% water content). An electrical analogue of CO2 exchange in a lichen thallus is presented, and it is suggested that the experimental results may be interpreted in terms of temperature-dependent CO2 diffusion resistances in imbibed lichen thalli.

Photosynthesis of Desiccating Leaves of Poplar

AbstractThe relation between photosynthesis and water content was investigated using detached leaves of Populus euramericana (Dode) Guinier cv. Robusta.The time course of photosynthesis was measured at different light intensities, at different CO2 contents of the air and at constant temperature during the desiccation of the leaves. The time course of decreasing water content was obtained from continuous measurement of water transpired from the leaves.A large reduction of light saturated (400 W × m−2) photosynthetic rates was observed with decreasing water contents between 78 and 64% (water potential between −14 and −24 atm (bar)). This reduction was much greater in air with 0.3 % CO2 than in air with 5 % CO2, indicating a significant influence of CO2 diffusion resistance on rate of photosynthesis. The reduction of the rate of light and CO2 saturated photosynthesis (at 400 W × m–2 and 5% CO2 in the air) is a measure of the inactivation of the photosynthetic enzyme system by desiccation. A proportional reduction of the light saturated and light limited rate of photosynthesis (for different H2O contents) was found, when measured in air containing a saturating amount of CO2 (5 %). The reduction of the light limited rate of photosynthesis (at 20 W × m−2) was the same at both CO2 levels.

The Dependence of the Relation Between Light Quality and Photosynthesis on Optical and Anatomical Characterisyics of Leaves (1)

The relationship was investigated between stomatal resistance and CO2 absorption rate under different light-qualities. The action spectrum of photosynthesis of leaves was discussed from the view point of CO2 diffusion resistance. CO2 absorption rate and stomatal resistance were measured at the upper and the lower surfaces of cabbage, cucumber and lettuce leaves under an irradiation of three different lights (blue-green, yellow-red and red) from either the upper or the lower surface side of the leaf.Results were as follows:(1) Stomatal resistance of lettuce-leaf was smaller at the surface receiving the light than that at the opposite surface in every light. The resistances of cabbage and cucumber-leaves were smaller at the lower surfaces than those at the upper surfaces regardless of the direction of irradiation. The stomatal resistance at the light-receiving-surface was the smallest under blue-green light in all plants. At the nonirradiated surface, the light quality caused some effect on stomatal resistances in lettuce but not in cabbage and cucumber.(2) CO2 absorption rate was larger at the surface with a smaller stomatal resistance than that with a larger resistance for all plants. But, CO2 absorption at each surface was not always large under the light situation from which a smaller stomatal resistance was derived. At the lower surface the stomatal resistance did not so remarkably affect CO2 absorption rate. At the upper surface, the stomatal resistance was the limiting factor of CO2 absorption for cabbage and cucumber, but not for lettuce.(3) The ratio of CO2 absorption rate at the upper surface to that at the lower surface varied with the light-quality and the irradiation direction for all plants. In the case of irradiation directed to the upper surface, the ratio for lettuce was the largest and those for cucumber and then cabbage followed in every light. It is considered that the differences in photosynthetic action among plants were caused by the differences in the distribution, the activity of stomata, and in the anatomical characteristics of leaves.(4) The relative value of photosynthesis in blue region in the action spectrum of photosynthesis increased with increasing ratio of the upper surface to the lower one in CO2 absorption rate in the case of irradiation directed to the upper leaf surface. This phenomena was explained by the difference in the extinction coefficient for light within a leaf. The blue light has a large extinction coefficient within a leaf and is mostly absorbed near the surface layer. Therefore, higher the ratio, the higher photosynthetic activity occurred in blue region.