WALLACE, RAYMOND H. (U. Connecticut, Storrs.) , AND HELEN M. HABEHMANN. Genetic history and general comparisons of two albino mutations of Helianthus annuus. Amer, Jour. Bot. 46(3): 157-162. IlIus. 1959.--The genetic history of the progeny of a single ultrasonically-treatedseedling of H elianthus annuus L. has been summarized for the 6 generations for which quantitative data are available. A yellow mutation was found in the F2 generation and later in the F" a second, white mutation occurred. Both mutants have been grown to maturity by grafting tbem onto normal green host plants and they have set viable seeds. Both pigment-deficient conditions are inherited as single recessive factors. These albino strains form chlorophyll during their seedling stages if they are grown at low light intensities. Chlorophyll is destroyed, however, under bright illumination and, once bleached, the capacity for chlorophyll formation appears to be lost. The yellow mutant contains xanthophyll but no traces of carotene have been found. In the white mutant, neither carotene nor xanthophyll have been detected. Flower color in the yellow mutant is normal while the flowers of the white mutant have no apparent pigmentation. The growth pattern of grafted yellow mutants is normal, save for a stiffer and woodier condition and a greater resistance to wilting. These characteristics have also been observed in grafted white mutants. In addition, there is a pronounced reduction in leaf size in the white mutant.
Leaves of the xantha mutant of Helianthus annuus have a higher rate of transpiration and a lower diffusive resistance in the light than in the dark. Stomates of this nonphotosynthetic mutant open in the light and close in the dark.Comparative studies of tobacco, xantha mutant, and wildtype sunflower stomatal opening over a range of light intensities in isolated portions of the spectrum reveal two patterns of response: (a) a low intensity opening in the green and far red characterized by partial opening, absence of a threshold, and saturation of the response at low light intensities; (b) a high intensity response in the blue characterized by a threshold (intensities greater than 100 microwatts per square centimeter needed for opening) and a linear opening response at higher incident light intensities. In xantha mutant stomates only the low intensity system appears to be operational, while both low and high intensity systems are present in the wild-type sunflower and tobacco.Red light has an inhibiting effect on stomatal opening in both mutant and wild-type sunflowers. They require prior exposure to far red for opening to occur in red light. This redfar red antagonism suggests the involvement of phytochrome.The stomatal apparatus controls not only transpirational water loss but also the exchange of gases between the interior and exterior of leaves. Stomatal opening or closing is a consequence of turgor changes in the guard cells that are influenced by a number of environmental factors including CO2 and 02 tension, water supply, temperature, and light. Of these controlling factors, the influence of light is perhaps the least understood.
Summary
1. Helianthus chloroplasts to which flavin, MnC12, catalase and ascorbic acid have been added catalyze the photoxidation in red light of ascorbic acid in a three‐step reaction with the absorption of not one but two equivalents of oxygen.
2. In white or blue light the chloroplasts can be omitted and the entire sequence of photochemical reactions occurs unchanged with flavin as the light absorbing dyestuff.
3. New to this reaction are the continuation beyond the oxidation to dehydroascorbic acid and the peculiar kinetics which treat successive intermediate products not as competing but as successive substrates of various photochemical steps.
4. The sequence of reactions depends specifically on the combination of flavin, manganous ion and iron porphyrin. To photoxidize dehydroascorbic acid to a 2 carbon and 4 carbon oxidative product none of the three components can be omitted from the reaction mixture.
5. Kinetics and specificity of the reaction are discussed on the basis of a complex which imitates the action of natural flavin‐iron porphyrin oxidase.
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