Compartmental analysis of amino-acid release from attached and detached pea seed coats

Using the empty-seed-coat technique, we have studied the release of amino acids from pea (Pisum sativum L. cv. Marzia) seed coats, either attached to or detached from the mother plant, at various stages of development. During an experimental period of 8 h, about 70% of the amino-acid content of the seed coat was released into the solution with which the seed-coat cup was filled. Major components of the released amino-acid mixture were glutamine (25%), alanine (20%) and threonine (15%). At the end of the experimental period (t ≥ 6 h) the amino-acid efflux from attached seed coats was approx. 0.15 μmol·h-1·(seed coat)-1 greater than that from detached seed coats. This difference may be attributed to the import of amino acids into attached seed coats. It is equivalent to approx. 0.3 μmol N·h-1·(seed coat)-1, whereas the nitrogen demand of the embryo amounted to 0.50 ± 0.04 μmol N·h-1·embryo-1. The time course of the release from detached seed coats could be described by the sum of two exponentials with t1/2 = 0.4—0.7 h and t1/2 = 3.3—6.9 h, respectively, which probably represent the emptying of the cytoplasmic and vacuolar compartments. During development the vacuolar amino-acid pool decreased considerably (from 6.2 to 1.4 μmol·(seed coat)-1), whereas the cytoplasmic pool was much more constant (2.1—1.2 μmol·(seed coat)-1). For attached seed coats it will be shown that the time course of the amino-acid release could be fully accounted for by the sum of two exponentials and a linear term, where the parameters of the exponentials were, within error, the same as for detached seed coats and the linear term represented the import of amino acids. The results will be discussed with reference to prevailing models of phloem unloading in the seed coat, and in relation to the flux of amino acids from seed coat to the developing embryo.