Physiological changes in developing pea seeds

Abstract

Physiological changes in developing pea seeds have been investigated in relation to the development of the ability of seeds to withstand desiccation and to germinate as succulent seeds when taken directly from the pod. In the developing garden pea seed the ability to withstand desiccation was found to be preceded by a fall in respiration rate and seed moisture content, both of which followed a sharp decline in ethanol soluble sugars. When harvested seeds were kept in high humidity conditions respiration was found to decline even though the moisture content was maintained. The fall in respiration was always associated with a fall in the level of sugars and seeds whose respiration rate had fallen in humid storage could be induced to respire more rapidly by the addition of sucrose. Further, in defoliated plants kept in the dark the sugar content of the seeds still attached to the plant was reduced resulting in a reduced respiration rate. It is suggested that seeds can only withstand rapid desiccation after a decrease in physiological activity assessed by 02 uptake, following a fall in the supply of respiratory substrate in the form of sugars. During development, the ability of seeds, desiccated over calcium chloride under vacuum, to retain solutes improved with age and with storage under high humidity conditions. This finding was investigated further by looking at the changes in phospholipid levels of succulent seeds during normal development and after three days storage under 100% rh. It appeared that improved solute retention was associated with an increase in the phosphatidyl choline levels in succulent seeds before they were dried. The desiccated seed on the other hand was characterised by a loss of phospholipid phosphorus and by an increase in the number of unidentified phospholipids as compared with undried seeds of the same age. The effects of the respiratory inhibitors potassium cyanide and salicylhydroxamic acid were examined at various stages of development. It was concluded that the TCA-terminal oxidase system is the main respiratory pathway in developing pea seeds. During the time period studied there appeared to be no transition from one aerobic respiratory pathway to another. During the course of this study it was noticed that immature seeds were capable of germinating before developing the ability to withstand desiccation. When immature succulent seeds were set to germinate, germination percentage was improved by the removal of the testa; however the early growth of immature seeds remained low but increased with age. It is suggested that premature germination of seed is prevented by a combination of a reduction in O2 supply and the strength of the testa. The low vigour of immature seeds is not influenced by these factors but by the poor ability of seeds to mobilize their storage reserves.