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Articles

Photosynthetic Pod Wall of Pea (Pisum sativum L.)

Distribution of Carbon Dioxide-fixing Enzymes in Relation to Pod Structure ¹

Alastair M. Flinn^b and Trevor W. Steele^b

^a Department of Botany, University of Western Australia, Nedlands, Western Australia 6009, Department of Biological and Environmental Sciences, The New University of Ulster, Coleraine, Londonderry, Northern Ireland

The pod wall of pea (Pisum sativum L.) was shown to contain two distinct photosynthetic layers. The outer, comprising chlorenchyma of the mesocarp, captured CO₂ from the outside atmosphere; the inner, a chloroplast-containing epidermis lining the pod gas cavity, was involved in photoassimilation of the CO₂ released from respiring seeds.

Structural features of the pod included the thick cuticle and stomata of the outer epidermis, the inward projecting veinlets of the vascular network in the mesocarp, the sparsity of air spaces, the fiber and parenchyma layers of the endocarp, and the abundant chloroplasts, thin cuticle, and rounded outer contours of cells of the inner epidermis.

The inner epidermis showed high specific activities of ribulose 1,5-diphosphate (RuDP) carboxylase (EC 4.1.1.39) and phosphoenolpyruvate (PEP) carboxylase (EC 4.1.1.31), contained up to 20% of the pod's chlorophyll, and was capable of fixing 66% of the CO₂ released during the photoperiod to the pod gas space by the seeds of a fully grown fruit.

The in vitro carboxylation capacity of the pod exceeded the estimated gross photosynthesis of the fruit for all but the last few days of development. Chlorophyll content and carboxylation activity declined more markedly in the outer photosynthetic layers than in the inner epidermis.

The ratio of activities of RuDP carboxylase to PEP carboxylase in pod extracts varied from 2.4:1 to 12:1 as against 48:1 to 156:1 in extracts of leaves.

Structural and physiological properties of the pod were related to its capacity to conserve respired CO₂ and provide photosynthate to developing seeds.

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