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Some interrelations of photosynthesis and photorespiration among species Downton, William John Sherwin


Photosynthesis, photorespiration and differences in their interactions among species were studied. In the first of two parts of the investigation, the CO₂ compensation concentration of members of the Gramineae and other plants was determined with an infrared CO₂ analyzer. In some cases the Initial products of ¹⁴C0₂ fixation and leaf anatomy were also examined. In plants with low compensation values (lacking photo-respiration) the initial products of photosynthesis were formed by the C₄-dlcarboxylic acid pathway. High compensation plants (with photorespiration) produced compounds typical of the Calvin cycle. The leaf veins of low compensation species were surrounded by a specialized parenchyma bundle sheath containing a high concentration of chloroplasts with large quantities of starch. Low compensation members of the Gramineae belonged to the arlstidoid, chloridoid-eragrostoid and panicoid lines of evolution. Of the genera confined to these phylogenetic lines, Panlcum was found to differ in photosynthetlc physiology and bundle sheath characteristics at the subgeneric level. This type of photosynthetlc diversity also occurred within Cyperus (Cyperaceae), Atriplex and Bassia (Chenopodiaceae). Despite these differences, the correlation between type of photosynthetlc pathway, compensation value and leaf anatomy was consistent. The literature indicates that low compensation grasses have photosynthetic rates that are about double those of plants with photorespiration correlated with a temperature optimum for photosynthesis of about 35°. Species with photorespiration have optima within the range 10-25°. Some simple assay procedures proposed on the basis of these correlations allow rapid determination of the physiological and biochemical status of plants with respect to photosynthesis. In the second part of the investigation, some published studies of photorespiration and glycolate oxidation were reexamined and correlated by infrared CO₂ analysis. Photosynthetic rate data at different O₂ tensions for wheat, oat and corn seedlings fed 3-(3,4-dichlorophenyl)-l, 1 dimethyl urea (DCMU) indicated that dark respiration continued in the light when photosynthesis was completely inhibited. Photorespiration was also inhibited. The O₂ sensitivity of glycolate-stimulated CO₂ production was found to be compatible with the proposal that glycolate is a substrate of photorespiration. Both 'in vivo' and 'in vitro' studies of the alga Nitella flexilis revealed a pathway of glycolate oxidation resembling that of higher plants. DCMU inhibition of photosynthesis by Nitella gave results similar to those for the monocotyledons tested. Under very low light intensity, CO₂ compensation in corn was measurable but was not sensitive to high O₂ concentration. It appears that the lack of photorespiration in this plant is not the end result of efficient internal recycling of CO₂ to photosynthesis.

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