The relationship between photosynthesis and hybrid vigour in Arabidopsis

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Heterosis, or hybrid vigour, is the phenomenon where a F1 hybrid exceeds the parents in biomass and seed production. Hybrids are used in production in rice, maize and other crops. In Arabidopsis, biomass heterosis occurs in hybrids at an early developmental stage and throughout development. Previous findings suggest that heterosis is associated with altered gene expression, especially for the genes involved in the photosynthesis pathway, but the relationship between photosynthesis and the generation of biomass heterosis is not clear. The aims of this project were to analyse Arabidopsis hybrids for their photosynthetic parameters and growth patterns and compare them to the corresponding parents to elucidate the relationship between heterosis and photosynthesis. To investigate whether photosynthetic properties in hybrids were different from the parents, the chlorophyll fluorescence and the CO₂ gas-exchange on a per leaf area basis were measured as indicators of the capacity of the light and dark reactions, respectively; the content of chlorophyll and the number of chloroplasts per mesophyll cell were analysed as indicators of the density of photosynthetic machinery in leaves; and leaf parameters that might affect gas-exchange in the leaf, including leaf thickness and the number of mesophyll cell layers, were examined. In all hybrids these photosynthetic parameters were at levels either between the two parents or similar to the better parent, showing that photosynthetic processes were highly conserved in hybrids and parents. Increasing photosynthesis via increasing the growth irradiance conditions did not enhance the heterosis level of hybrids compared to the parents. These results indicate that the biomass heterosis was not due to changes in photosynthetic processes. The growth patterns of the leaves of the heterotic C24/L𝑒𝑟 hybrids, showed that biomass heterosis was due largely to the newly developed leaves in the hybrid being larger than those of the parents at any point in time during development. The heterosis in leaf growth was due to greater cell size and increased cell number. Although the unit leaf area photosynthetic rate in the hybrids was not greater than the parents, the hybrids had a greater total leaf area to intercept more light to sustain the increased demands of energy and building blocks for the fast-growing new leaves. The growth heterosis in early leaf development might be a prerequisite for biomass heterosis throughout development. A critical role for photosynthesis in cotyledons in the generation of biomass heterosis was demonstrated by a mutant, snowy cotyledon2, which had impaired chloroplast biogenesis, specifically in the cotyledons. Whereas Ws/L𝑒𝑟 hybrids had a considerable biomass heterosis compared to the parents, mutant Ws/L𝑒𝑟 hybrids that were unable to carry out photosynthesis in cotyledons showed no heterosis relative to the homozygous mutant parents. This result indicates that the generation of biomass heterosis in early seedling development depends on photosynthesis in the cotyledons and that this is important for the biomass heterosis in subsequent stages.
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