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.

Bibliography: leaves [i]-xvi.

Most plants produce leaves with stomata on either both surfaces (amphistomatous) or on the lower surface only (hypostomatous). The importance of stomata to plant survival suggests that these two stomatal distribution patterns may be adaptive, and this problem is explored. It is concluded that amphistomaty is an adaptation to produce a high conductance to CO₂ diffusion into the leaf. As such it is advantageous to plants with high photosynthetic capacity leaves in high light environments, experiencing rapidly fluctuating or continuously available soil water. Plants meeting these criteria are found to be almost exclusively amphistomatous; those not meeting the criteria are mostly hypostomatous. Also investigated is the adaptive significance of differences in stomatal conductances and conductance responses to environmental factors between the two surfaces of amphistomatous leaves. Although differences in stomatal conductance are found between the two surfaces in sunflower, differences in conductance response to light intensity and water vapor pressure difference across the stomatal pore were neglible. Water stress relieved one day prior to experiments caused upper stomatal conductance to be reduced more than lower, but responses to light and water vapor pressure difference remained essentially parallel for the two surfaces. For these differences in conductance to be adaptive differences in photosynthetic characteristics between the two surfaces. In addition, estimation of the resistance to diffusion of CO₂ across the mesophyll yields values low enough to preclude steep gradients in CO₂ partial pressure in the mesophyll. In the absence of CO₂ gradients within the leaf, differences in photosynthetic characteristics between the two surfaces cannot exist. It is concluded that differences in stomatal conductance between the two surfaces of amphistomatous leaves are not adaptations to differences in CO₂ uptake characteristics.

Bibliography: leaves [i]-xvi / xi, 132, xvi, [137] leaves : ill ; 30 cm. / Title page, contents and abstract only. The complete thesis in print form is available from the University Library. / Thesis (Ph.D.)--University of Adelaide, 1984

The gas exchange responses of two cultivars of Hordeum vulgare L., to light, temperature, CO₂ and water stress were investigated in the laboratory. The optimum temperature for net CO₂ assimilation was found to be 25°C and 22.5°C for cv. Clipper and cv. Dayan respectively. Net CO₂ assimilation was reduced at 30°C in cv. Dayan. At low light intensity the highest quantum yield efficiency was 0.051 mol.mol⁻¹ at 30°C for cv. Clipper, and 0.066 mol.mol⁻¹ at 20°C for cv. Dayan. At the same temperature, cv. Clipper had a higher water use efficiency than cv. Dayan, but stomatal conductance for cv. Dayan was higher than cv. Clipper. Stomatal limitation to CO₂ was lowest at the optimum temperature for CO₂ assimilation in both cultivars. Stomata limited CO₂ assimilation in cv. Clipper to a larger degree than in cv. Dayan. Relative stomatal limitation for cv. Clipper at 25°C was 0.280 ± 0.010, and for cv. Dayan at 22.5°C was 0.028 ± 0.011. Short-term exposure to elevated CO₂ concentrations increased CO₂ assimilation in both cultivars, but more so for cv. Clipper. Transpiration rate at elevated CO₂ partial pressures were higher in cv. Dayan than in cv. Clipper. At very high CO₂ (860 μmol.m⁻²s⁻¹) partial pressure water use efficiency in cv. Clipper was higher than cv. Dayan, but at low CO₂ partial pressures water use efficiency in cv. Dayan was higher than cv. Clipper. Water stress reduced the relative leaf water content and net CO₂ assimilation in both cultivars. Cultivar Dayan was more tolerant to water stress, and CO₂ assimilation in this cultivar was less affected by water stress. In both cultivars water stress increased the concentration of chlorophyll a, chlorophyll b, and chlorophyll a+b. The chlorophyll a:b ratio remained relatively constant throughout the stress period. No correlation between relative leaf water content and total carotenoid concentration was observed.

Call number: LD2668 .T4 1985 M444 / Master of Science