Significance of Photosynthetic and Respiratory Exchanges in the Carbon Economy of the Developing Pea Fruit

Abstract

The nutritional economy of the developing fruit of Pisum sativum L. (cv. Greenfeast) was studied in terms of intake of translocate, incorporation of C and N into dry matter, transpiration, and CO₂ exchanges of the fruit with its external and internal atmospheres. The environmental conditions were 12-hr days (22 C, 850 μeinsteins m⁻² sec⁻¹ at fruit level); 12-hr nights of 15 C.

Between 6 and 30 days after anthesis, pod photosynthesis resulted in small gains of CO₂ from the external atmosphere, and assimilated most of the CO₂ respired by the fruit during the day. From then until maturity (40 days) the fruit lost CO₂ during the day. Night losses of CO₂ increased with fruit age.

The gas cavity of the fruit contained 0.15 to 1.5% (v/v) CO₂. Lower levels were maintained in the day than at night. CO₂ levels were influenced by fruit age, radiant flux, and temperature. Labeled CO₂ injected into the gas cavity was fixed by the pod but not by seeds in the light, and by neither pod nor seeds in darkness. Dark-to-light or light-to-dark transfer of a fruit promoted rapid changes in CO₂ and O₂ levels of the gas space, consistent with a shift in the assimilation-respiration balance of the pod.

The fruit transpired 27.6 cm³ H₂O per gram dry matter accumulated. Daytime ventilation was greatest 12 to 15 days after anthesis and declined as pod photosynthesis became increasingly involved in the retrieval of CO₂ respired by pod and seeds. Most, 69% by weight, of the translocate from the parent plant was converted to dry matter of seeds; nearly half, 45%, to useful seed reserves (sugar plus starch-protein-oil, 45:20:1). Illumination resulted in a fruit requiring 16% less translocate than if laying down an equal amount of dry matter in darkness.