Call number: LD2668 .T4 1950 P8 / Master of Science

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The relationship between carbon flux pathways in the plant must be explained. Fixation of carbon by the plant is the first step in dry matter yield production. Photorespiration (PR) appears to depress dry matter yield potential through its release of carbon, potentially fixed by photosynthesis, in the light. If PR rate could be reduced genetically in the plant, net carbon gain might be increased. A population of alfalfa (Medicago sativa L.) was screened for low PR potential per unit of leaf area. 'Hayden' alfalfa, grown under competitive stand field conditions, was investigated for five harvests for carbon flux relationships. Field-grown shoots of the plants were analyzed in the laboratory for per unit leaf area total carbon uptake (TCU), PR, apparent photosynthesis (AP), and dark respiration (DR) rates. An exceptionally low PR plant, designated ∅5, was isolated. Nine randomly selected Hayden plants were crossed on to the ∅5 and seed collected from the latter. Forty F₁ progeny, grown under space planted field conditions, were analyzed by the same means for carbon flux. The 40 genotypes approached a normal distribution for PR, TCU, AP, and DR rates. Significant differences were found among genotypes for the pathways sufficient variability was present for selection of low PR rate plants. Clone cuttings of a high (number 40) and a low (number 12) PR F₁ selection, plus an ∅5 parent clone, were grown in the greenhouse for carbon flux measurements and analyzed as described previously. No significant differences were found for PR, TCU, AP, or DR between genotypes. In the F₁ population, 12 and 40 were concomitant extremes for PR and TCU rates. Correlation between PR and TCU rates in the population was positive but moderate with an r = 0.53. Genotypic expression of PR was observed in the field plants, but only trends toward these differences were expressed in the greenhouse. Environmental factors in the greenhouse masked the genotypic expression of PR, TCU, AP, and DR. This indicates that greenhouse selection of PR levels may not be possible. The selection of low PR plants under Arizona field conditions may be a feasible operation, if significance between genotypes and ranking persistence is maintained over seasons.

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