Summary
Potential pathways for sucrose unloading in the potato tuber were examined by light and electron microscopy. Abundant plasmodesmata connected sieve elements with surrounding parenchyma elements and also sieve elements with companion cells. Plasmodesmata were rarer, however, between companion cells and parenchyma elements. These observations suggest that sucrose may leave the sieve elements and enter the storage parenchyma cells directly via the symplast and that transport through the companion cell may not be a prerequisite for unloading. Plasmodesmata, grouped together in primary pit fields, were also abundant between storage cells, and isolated storage cells, separated enzymically, showed considerable variation in plasmodesmatal distribution between cells and also on different faces of a single cell. Deposition of starch was found to occur in the tuber cortex while an endodermis with Casparian strip was present external to the phloem, suggesting that assimilates initially enter the cortical storage cells by an entirely symplastic pathway. The possible involvement of ATPase in the unloading process was examined cytochemically, using a lead-salt precipitation method. By contrast with previous findings for phloem no evidence was found for ATPase activity that was unique to the sieve element-companion cell complex. The present observations favour the view that phloem unloading in the potato tuber is a symplastic and passive process.
Similar content being viewed by others
References
Artschwager EF (1924) Studies on the potato tuber. J Agric Res 24: 809–835
Bentwood BJ, Cronshaw J (1978) Cytochemical localization of adenosine triphosphatase in the phloem ofPisum satium and its relation to the function of transfer cells. Planta 141: 111–120
Beringer H, Haeder HE, Lindhauer M, (1983) Water relationships and incorporation of14C assimilates in tubers of potato plants differing in potassium nutrition. Plant Physiol 73: 956–960
Browning AH, Hall J, Baker DA, (1980) Cytochemical localization of ATPase activity in phloem tissues ofRicinus communis L. Protoplasma 104: 55–65
Crafts AS (1933) Sieve tube structure and translocation in the potato. Plant Physiol 8: 81–104
Cronshaw J (1980) Histochemical localization of enzymes in the phloem. Ber dtsch bot Ges 93: 13–28
Cutter EG (1978) Structure and development of the potato plant. In:Harris PM (ed) The potato crop, the scientific basis for improvement. Chapman and Hall, London, pp 70–152
Eschrich W (1983) Phloem unloading in aerial roots ofMonstera deliciosa. Planta 157: 540–547
Evert RF, Eschrich W, Heyser W (1978) Leaf structure in relation to solute transport and phloem loading inZea mays L. Planta 138: 279–294
Fakarai H, Hall JL (1984) Changes in calcium-ATPase activity associated with the washing (ageing) of potato tuber discs. J Plant Physiol 117: 69–79
Fisher DG, Evert RF (1982) Studies on the leaf ofAmaranthus retroflexus (Amaranthaceae): ultrastructure, plasmodesmatal frequency and solute concentration in relation to phloem loading. Planta 155: 377–387
Geiger DR, Sovonick SA, Shok TL, Fellows RJ (1974) Role of free space in translocation in sugar beet. Plant Physiol 54: 892–898
—,Fondy BA (1980) Phloem loading and unloading: pathways and mechanisms. What's New in Plant Physiology 11: 25–38
Giaquinta RT (1977) Sucrose hydrolysis in relation to phloem translocation inBeta vulgaris. Plant Physiol 60: 339–343
—,Lin W, Sadler N, Franceschi VR (1983) Pathway of phloem unloading of sucrose in corn roots. Plant Physiol 72: 362–367
Hughes JE, Gunning BES (1980) Glutaraldehyde-induced deposition of callose. Can J Bot 58: 250–258
Ho LC, Baker DA (1982) Regulation of loading and unloading in long distance transport systems. Physiol Plant 56: 225–230
Mares DJ, Marschner H (1980) Assimilate conversion in potato tubers in relation to starch deposition and cell growth. Ber dtsch bot Ges 93: 299–313
O'Brien TP,McCully ME (1981) The study of plant structure. Principles and selected methods. Blackwell Scientific Publications
Oparka KJ, Gates P (1981) Transport of aassimilates in the developing caryopsis of rice (Oryza sativa L.). Ultrastructure of the pericarp vascular bundle and its connections with the aleurone layer. Planta 151: 561–573
— — (1984) Sink anatomy in relation to solute movement in rice (Oryza sativa L.): a summary of findings. Plant Growth Regulation 2: 297–307
Peterson CA, Peterson RL, Barker WG (1981) Observations on the structure and osmotic potentials of parenchyma associated with the internal phloem of potato tubers. Am Pot J 46: 361–373
Reeve RM, Hautula E, Weaver ML (1969) Anatomy and compositional variation within potatoes. 1. Developmental histology of the tuber. Am Pot J 46: 361–373
—,Timm H, Weaver ML (1973) Parenchyma cell growth in potato tubers. II. Cell divisions vs cell enlargement Am Pot J 50: 71–78
Sabnis DD, Hart JW (1974) Studies on the possible occurrence of actomyosin-like proteins in phloem. Planta 118: 271–281
Tomiiyama K, Lee HS, Doke N (1974) Effect of hyphal homogenate ofPhytophthora infestans on the potato-tuber protoplasts. Ann Phytopath Soc Jpn 40: 70–72
Turgeon R, Webb JA (1976) Leaf development and phloem transport inCucurbita pepo. Maturation of the minor veins. Planta 129: 265–269
Wareing PF (1978) Hormonal regulation of assimilate movement. In:George BF (ed) Opportunities for chemical plant growth regulation. British Crop Protection Council, Monograph 21: 105–111
Author information
Authors and Affiliations
Rights and permissions
About this article
Cite this article
Oparka, K.J. Phloem unloading in the potato tuber. Pathways and sites of ATPase. Protoplasma 131, 201–210 (1986). https://doi.org/10.1007/BF01282983
Received:
Accepted:
Issue Date:
DOI: https://doi.org/10.1007/BF01282983