Photosynthetic activity of cotyledons and foliage leaves of young angiosperm seedlings

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Abstract
Absorption of 14CO2 and distribution of 14C-photosynthate from cotyledons and foliage leaves were studied for 35 days after seed germination in Acer rubrum, Robinia pseudoacacia, and Ulmus americana seedlings. Differences occurred among species in total carbohydrate requirements, strengths of various carbohydrate sinks, and importance of cotyledons or foliage leaves as sources of carbohydrates. Robinia cotyledons fixed appreciable 14C 2 to 4 days after seedling emergence. Acer and Ulmus cotyledons fixed only small amounts of 14C 2 days after seedling emergence, but a significant increase in fixation occurred 4–6 days after seedling emergence. Photosynthesis in very young cotyledons was inhibited more by lack of stomata than by a deficiency of chlorophyll. As cotyledons reached maximum size, and as other plant parts were rapidly expanding, the cotyledons retained less 14C-photosynthate, and a higher proportion of label was translocated basipetally to the stem and roots and acropetally to the stem and leaves. Compared with cotyledons, young hypocotyls fixed small quantities of 14C and insignificant 14C was fixed by stems 10 days after seedling emergence. Fully expanded foliage leaves accounted for an increasingly larger percentage of total 14C fixation, and eventually more label was exported from leaves than from cotyledons. In 10- to 15-day-old plants much more 14C was fixed by cotyledons of young Robinia plants than by cotyledons of Acer or Ulmus. This difference was largely the result of more cotyledon tissue in Robinia rather than in more efficient photosynthesis per unit of cotyledon tissue.

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