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RESEARCH PRODUCT

The Effect of Different Light Intensities on the Frequency and Size of Stomata, the Size of Cells, the Number, Size and Chlorophyll Content of Chloroplasts in the Mesophyll and the Guard Cells during the Ontogeny of Primary Leaves of Sinapis alba

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Summary Plants of Sinapis alba were cultivated under high light (60 W m -2 PhAR) and low light (6 W m -2 PhAR) conditions. High light intensity during growth increased the stomatal frequency but there were only small changes in the length of the stomatal pore. High-light leaves had more than twice as many stomata per unit area as low-light leaves. The decrease of stomatal diffusive resistance in high-light leaves is primarily caused by the increase in stomatal density. There were striking changes in stomatal frequency during the ontogeny of primary leaves. High light caused a stronger development of the assimilating mesophyll. The volume of the palisade cells increased to a much higher extent under strong light conditions. The development of leaves as well as the light intensity applied during growth had also a strong effect on the size and number of chloroplasts. The mean volume of chloroplast showed a large and characteristic increase under both light conditions during the ontogenesis of primary leaves. In expanded high-light leaves the palisade cells had 3.5 times and the spongy cells 2.7 times more chloroplasts than the corresponding cells of low-light leaves. The number of chloroplasts per guard cell, however, was similar in high-light and low-light leaves. The number of chloroplasts per unit cell volume was similar in full-grown highlight and low-light leaves. Low-light leaves had more chlorophyll per unit fresh weight than high-light leaves. But the chlorophyll content per unit leaf area was lower in low-light leaves. The ratio of chlorophyll a/chlorophyll b was lower under low light conditions. Chloroplasts from low-light leaves had about twice as much chlorophyll as chloroplasts from high-light leaves. The consequences of these results concerning the photosynthetic unit are discussed. Low-light adaptation of the photosynthetic apparatus on the level of the light reactions results in a decrease of electron transport components and a combination of several chlorophyll-protein complexes to a bigger photosynthetic unit.