The CO2 exchange of several species of fresh water and marine algae was measured in the laboratory to determine whether photorespiration occurs in these organisms. The algae were positioned as thin layers on filter paper and the CO2 exchange determined in an open gas exchange system. In either 21 or 1% 02 there was little difference between 14CO2 and 12CO2 uptake. Apparent photosynthesis was the same in 2, 21, or 50% 02. The compensation points of all algae were less than 10 id 1-1.CO2 or 14CO2 evolution into C02-free air in the light was always less than the corresponding evolution in darkness. These observadons are inconsistent with the proposal that photorespiration exists in these algae.Algae are C3 plants but there is still controversy regarding the existence or nature of photorespiration (7,8,33). Many algae have been shown to produce glycolate (28,33) and to possess the enzymes of the glycolate pathway (28, 33). The inhibition of photosynthesis by 02 or the Warburg effect was first shown in algae, and has since been extensively studied (2, 34). Some workers, however, have found no effect of 02 on CO2 fixation in algae (1, 16, 18) and many algae show low compensation points (4,25,35) and release less CO2 in the light compared to the dark (7,17).In higher plants, CO2 evolution and CO2 exchange are the most extensively studied aspects of photorespiration (36). In algae, although CO2 evolution during photosynthesis has been implied (33), few studies have been performed on CO2 exchange because of the difficulties imposed by the aqueous medium. In view of the paucity of information and the diversity of observations and opinions on CO2 exchange in algae, we decided to investigate the CO2 exchange of several species of algae using an open gas analysis system such as that used for measurement of CO2 exchange in higher plants (15,26). MATERIALS AND METHODSThe following algae were obtained from the Indiana University Culture Collection, and were grown on Gorham's culture medium No. 11 (21) Scotia. These marine species were grown on a shaker without aeration with a 16-hr photoperiod.All species were grown at 25 C and a quantum flux density of 80 to 100 ,ueinsteins m-2 sec-'. All cultures were axenic with the exception of Mougeotia. Cells were harvested during linear growth phase, and concentrated, where necessary, by settling and decanting the medium. Algae were suspended as an "artificial leaf" (12) to facilitate gas exchange and rapid changes in gas composition. The cells were filtered on Whatman No. 3 filter paper, producing an even double layer of cells (4 x 4 cm). This square was cut out and enclosed in the leaf chamber for the gas analysis measurements. The total volume of medium on the filter paper was approximately 0.3 ml. Similar results were obtained when the algae were suspended on 10-gm Nitex nylon mesh (B and S. H.The open gas analysis system was as previously described (15, 26). Measurements were started about 30 min after the algae were placed in the leaf chamber because the rate of apparent ...

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1977. Photosynthesis and photorespiration in submerged aquatic vascular plants. . In experiments carried out in a water-vapour-saturated atmosphere, the photosynthesis of submerged leaves ofPotr~rnogeton and Myriophyll~~rn was light saturated at a quantum flux of 200 pEm-, s-I. Saturationofphotosynthesis with C 0 2 , however, could not beachieved, even at 3500 p1 ! -I COZ. In contrast, floating or aerial leaves of these plants showed saturation with light only at 01. above a quantum flux of 1200 p E m-, ssl. In floating leaves ofPotarnogetoti photosynthesis was saturated at 1000 p1 P1 CO, and in aerial leaves of Myriophyllrrtn saturation was being approached at 2100 p1 P1 COz.The high resistance of submerged leaves to CO, transfer makes accurate measurements of photosynthesis and photorespiration difficult. Apparent photosynthesis, though, was stimulated in 2% 0, and inhibited in 50% O2 compared with the rates in air. Oxygen sensitive CO? evolution into C0,-free air in the light was measured and compensation points of 31 to 75 p1 ! -I CO, were determined. These observations indicate that photorespiration exists in submerged aquatic vascular plants, but the rate of the process cannot be accurately measured. LLOYD, N . D. H., D. T. CANVIN, et J. M. BRISTOW. 1977. Photosynthesis and photorespiration in submerged aquatic vascular plants. Can. J. Bot. 55: 3001-3005. Au cours d'experiences realisees dans une atmosphere saturCe en vapeur d'eau. la photosynthese de feuilles submergkes d'un Potatnogetotz et d'un Myriophyll~~tn a Cte saturee par la lumiere avec un flux quantique de 200 p E m-*s-'. Cependant la saturation de la photosynthese par le CO, n'a pas pu &tre realisee, mZme avec 3500 pI ! -I de CO,. Au contraire, les feuilles flottantes ou ae~iennes de ces plantes ont Cte saturees par la lumiere uniquement avec un flux quantique Cgnl ou superieur h 1200 p E m-,s-'. Chez les feuilles flottantes du Potamogetoti, la photosynthese a e t i saturke avec 1000 pI de CO, et chez ies feuilles akriennes du My~~iophyll~rnz, la saturation a kt6 presque atteinte avec 2100 ~1 !-I de CO, . -~a f o r t e resistance des feuilles ~u b m e ;~e e s au transfert du CO, rend difficilement rCalisables les mesures plicises de la photosynthese et de la photorespiration. La photosynthese apparente est . .cependant stirnulee pa;2% d'0, et inhibee pa;50% d 7 d Z , comparativement aux taux h I'air. Des points de compensation de 31 75 p1 ! -I de CO, sont obtenus lorsqu'on mesure I'emission du CO, sensible i I'oxygene dans de I'air sans CO, et en presence de lumiere. Ces observations montrent que la photorespiration existe chez les plantes vasculaises aquatiques submergees. mais que le taux du processus ne peut pas &re mesure avec precision.[Traduit par le journal]

SUMMARY Six races of Sesleria caerulea from widely separated parts of the geographical range of the species exhibited different rates of photosynthesis and transpiration when grown under standard conditions. These can be traced to various attributes of the stomatal apparatus. The stomata differ in frequency, size and the depth to which they are sunk below the epidermal surface in the different races, but quantitative consideration of these parameters leads to the conclusion that the apertures to which the stomata can open, an attribute which correlates with the depth to which they are sunk, determines the maximum rates of photosynthesis and transpiration. Evidence is presented to suggest that, although this complex of stomatal modifications may be interpreted as representing xerophytic adaptations, they may have evolved not so much for conservation of water per se, but to conserve the energy balance of the leaves, thereby reducing evaporative heat loss and so maintaining leaf temperature. A leaf temperature of 1–2°C above ambient may increase the rate of photosynthesis by 10–20% in the range from 10–20°C.