Mobilization of Phosphorus in the Cotyledons of Young Seedlings of the Garden Pea (Pisum sativum L.)

Changes in the levels of various phosphorus fractions and of phytase activity in the cotyledons of young pea seedlings grown in the light have been studied. It is shown that from the onset of germination there is a lag of several days in the hydrolysis of phytic acid and that this is associated with a low level of phytase activity in cotyledon extracts. Rapid development of phytase during the next few days is accompanied by a rapid increase in the rate of phytic acid break-down and both reach maximum levels after 6–7 days from soaking the seed. The amount of phytic acid in the cotyledons becomes negligible after about 15 days and at the same time phytase activity declines markedly. At this point protease activity is at a maximum and the water content of the cotyledons begins to fall. Removal of the shoot 4 days after soaking the seed caused an immediate decrease in export of phosphorus from the cotyledons but did not affect the level of phytic acid for several days. Subsequently there was a small, but significant reduction in the rate of phytic acid hydrolysis in de-shooted seedlings as compared with intact plants in spite of the fact that phytase activity was not affected for several days. Similar effects were observed when excised cotyledons were cultured on moist filter-paper. Control mechanisms for phytic acid hydrolysis are discussed and it is concluded that regulation by the axis of the inorganic phosphate concentration at the sites of phytase activity may be a means of controlling phytic acid hydrolysis.