Pieter Wolswinkel

Phloem unloading and ‘sink strength’: The parallel between the site of attachment of Cuscuta and developing legume seeds

Sink regions play a central role in determining assimilate distribution patterns. Two factors are discussed which have a strong effect on the ‘sink strength’ of a sink, viz. phloem unloading and turgor-sensitive transport. ‘Sink strength’ may be defined as the capacity of phloem in the sink region to import assimilates from other parts of the plants and to release the imported substances into the sink apoplast.A stem parasitized by Cuscuta represents a very strong sink. A review is presented of data on enhanced phloem unloading, at the site of attachment of Custuta. Recent data on metabolically controlled sucrose and amino acid unloading into the seed coat apoplast of developing legume seeds show a remarkable parallel with phloem unloading in a parasitized Vicia faba stem. Data on turgor-sensitive sucrose and amino acid transport into developing seeds are presented, which throw new light on the pressure flow theory of phloem transport.

Assimilate transport in developing seeds of sunflower (Helianthus annuus L.)

Summary Several aspects of sucrose and amino acid transport into developing seeds of sunflower ( Helianthus annuus L.) were studied. «Blocks» containing ca. 50 achenes (in some cases ca. 100 achenes) were excised from a sunflower head, leaving a layer of head tissue below the base of the achenes, and these blocks were placed in a horizontal position on a medium containing a mixture of [ 3 H]sucrose and [ 14 C]α-aminoisobutyric acid (AIB). This procedure permitted uptake of labelled solutes via the wound surface of head tissues, followed by phloem transport into developing seeds. The pattern of assimilate transport from tissues of maternal origin (seed coat) to embryonic tissues was similar to the pattern described for legumes. Sucrose or AIB release from the seed coat was, to some extent, inhibited by a treatment with p -chloromercuribenzenesulfonic acid (PCMBS) or KCN, but a very rapid efflux into the seed coat apoplast appeared to be an important characteristic of assimilate unloading from the sunflower seed coat. A treatment with PCMBS reduced efflux of labelled sucrose and AIB from cotyledons. The efflux of AIB from cotyledons was also reduced by a KCN-treatment. A high solute concentration in the bathing medium (a series of rising mannitol concentrations) reduced net sucrose or amino acid efflux from cotyledons or seed-coat halves. This inhibitory effect of a high osmolality on net solute efflux was largely «saturated» at an osmolality of 250 mM mannitol. Although some differences do exist between data on sunflowers and data reported for legumes, most data obtained support the view that the control of assimilate transport into sunflower seeds does not differ much from that of transport into developing seeds of legumes.

Effect of Inhibitors on Solute Efflux from Seed-Coat Halves and Cotyledons of Pisum sativum L., after Uptake from a Bathing Medium.: The Difference between Sucrose and Amino Acids

Summary Net sucrose and amino acid efflux from excised seed-coat halves and cotyledons of developing seeds of Pisum sativum L. into a bathing medium (pH 5.5) was measured over a period of 5 h, after uptake from a solution containing a mixture of 3 H - and 14 C-labelled solutes. Net efflux of amino acids (α-aminoisobutyric acid, valine and asparagine) from seed-coat halves and cotyledons was strongly inhibited by p -chloromercuribenzenesulfonic acid (PCMBS). Net efflux of labelled sucrose was not markedly inhibited by a treatment with PCMBS; under certain conditions it was enhanced by PCMBS. The effect of a low-temperature (0 °C) treatment did not differ much from the effect of a treatment with PCMBS. Net efflux of labelled solutes from seed-coat halves and cotyledons was enhanced by KCN, but this effect was much more pronounced in the case of sucrose than in the case of amino acids. The parallel between seed-coat halves and cotyledons illustrates the point that the seed coat contains tissues with ≪cotyledon characteristics≫ (e.g. intensive resorption of sucrose). After unloading into the seed coat apoplast from the seed-coat unloading sites, amino acids encounter less resorption activity than sucrose.

Sucrose Transport into, and Unloading from, the Seed Coat of Empty Ovules of Pisum sativum over Time-Spans Greater than Three Hours

Summary This paper discusses whether assimilate transport into seed coats of surgically modified ovules deteriorates over time-spans greater than about 3 h. In experiments with fruits of pea ( Pisum sativum L.), several hours before the start of a pulse-labelling procedure, two empty ovules were prepared in each fruit. Subsequently, after a time interval of several hours, two other empty ovules were prepared in each fruit. Immediately after this procedure, [ 14 C]sucrose was fed to the leaf subtending the fruiting node. During a pulse-chase period (80-360 min), transport of 14 C into ovules operated several hours before the start of a pulse-labelling procedure was only slightly reduced, in comparison with transport of 14 C into ovules operated directly before the start of a pulse-labelling procedure. Data on the release of 14 C-solutes from the seed coat also supported the view that, with respect to sucrose transport into developing ovules, surgically modified ovules can function almost normally over time-spans of at least 6-8 h.