1984. Turgor-sensitive sucrose and amino acid transport into developing seeds of Pisum sativum. Effect of a high sucrose or mannitol concentration in experiments with empty ovules. -Physiol. Plant. 61: 172-182.Sugar and amino acid transport into empty ovules of Pisum sativum L. cv. Marzia was examined. In frtiits containing 4-6 developing seeds, the embryo was removed from four ovules. After this surgical treatment, each empty seed coat was filled with a solution (pH 5.5) containing a low (0, 50 or 200 mM), medium (350,400 or 500^ mM) or high (0.7 or 1 M) concentration of sucrose and/or mannitol. In pulse-labelling experiments with sucrose and a-aminolsobutyric acid (AIB), transport of sucrose and AIB into an empty ovule filled with a solution containing a high sucrose concentration was the same as transport into an ovule filled with a mannitol solution of similar osmolarity, demonstrating that a high sucrose concentration in the seed coat apoplast affects phloem transport of sucrose and AIB into the seed coat only by the osmotic effect. The osmolarity of a given solution filling the seed coat cavity appeared to be important for phloem transport of sucrose and AIB into empty ovules. In our experiments, 350 mM appeared to be the optimal concentration for sucrose and AIB transport into the cavity within an empty ovule, giving results comparable with transport into intact ovules. A lower osmoladty of the solution induced less transport. Very high sucrose or mannitol concentrations caused a strong inhibition of sucrose and AIB unloading from the seed coat, so that transport into the empty ovules was inhibited. A low (strongly negative) but not too low osmotic potential of the solution in tlie seed coat apoplast seems necessary to maintain a normal rate of phloem transport into developing seeds. Apparently, the "sink strength" of developing seeds is turgor-sensitive.Additional key words -Garden pea, phloem transport, phloem unloading, pressureflow mechanism, seed development, sink strength. P. Wolswinket (reprint requests) and A.

Wolswinkel, P. 1985. Phloem unloading and turgor-sensitive transport: Factors involved in sink control of assimilate partitionhig. -Physiol. Plant. 65: 331-339.Additional key words -Phloem transport, pressure-flow mechanism, seed development, sink strength, source-sink relation.

After synthesis in the vegetative parts of the plant, assimilates are translocated to fruits through xylem and phloem. Research on factors controlling nutrient transport into developing seeds via the phloem has been stimulated by the development of the empty seed coat technique.There is a consensus that as assimilates are transported from maternal tissues to filial tissues they are delivered to the extracellular space (the apoplast) separating the two generations, prior to uptake from the apoplast into the tissues of the embryo or endosperm. The empty seed coat technique has been used for the study of several aspects of nutrient transport into seeds, e.g. metabolic control and turgorsensitive transport. The osmotic environment of seed tissues has a strong effect on assimilate transport into empty seeds. Several lines of evidence suggest that one of the main 'secrets' of the high sink strength of developing seeds, at least in many taxonomic groups of dicotyledons, is that the sink end of the phloem pathway is 'bathed' in an apoplast solution with a high concentration of osmotically active solutes. Data on maize do not fit this pattern. A turgor homeostat mechanism may help to maintain high solute concentrations in the seed apoplast. The apoplast environment of seed tissues may also stimulate synthesis of storage proteins and be involved in the prevention of precocious germination. In addition to the osmotic environment, other factors influencing sink strength are discussed. Some aspects of solute transformation during transport through seed tissues are described.

When luxuriantly growing Cuscuta parasitised Vida faba at the time of flowering, fruit setting of the host was generally completely inhibited. When the host was infected in a later stage of development Cuscuta seriously interfered with further growth of pods.The process of translocation of 1 4C-assimilates from a photosynthesising leaf to other parts of the host and to the parasite was analysed. The process of assimilate withdrawal by Cuscuta turned out to be so efficient that usually only negligible amounts of 14C-assimilates -or nothing at all -found its way into pods and seeds of the parasitised host.

By washing out 14C-labelled assimilates from the free space of stem segments of Vida faba L., considerably more label could be removed from a stem parasitised by Cuscuta species than from a normal one. This could mean that the moving of assimilates from the host phloem to the parasite at least partly occurs through the apoplast.The assumed unloading of the host phloem is apparently under metabolic control since it appeared to be inhibited at O°C and after addition of dinitrophenol.