lAWA Journal, Vol. 18 (2),1997: 197-201

OCCURRENCE OF SIEVE ELEMENTS IN PHLOEM RAYS by Kishore S. Rajput & K. S. Rao Department of Botany, Faculty of Science, Maharaja Sayajirao University of Baroda, Vadodara 390 002, India

SUMMARY

Solitary sieve elements or groups of sieve elements were encountered in the rays of secondary phloem of Erythrina indica, Guazuma tomentosa, Acacia nilotica, Azadirachta indica, and Tectona grandis trees. These elements were short and possessed simple and compound sieve plates on their transverse to slightly oblique end walls. Each sieve tube ele­ ment was associated with a single companion cell at their comers. Like axial sieve tube elements, the sieve tube elements of the rays showed slime (P-protein) plugs and cytoplasmic strands when functional and massive deposition of callose on sieve plates in nonfunctional sieve tube elements. The distribution pattern of these ray sieve elements differed among the species studied. The detailed structure and possible signifi­ cance of these elements are discussed. Key words: Sieve elements, phloem rays, Erythrina indica, Guazuma tomentosa, Acacia nilotica, Azadirachta indica, Tectona grandis.

INTRODUCTION

Occurrence of sieve elements in the rays of secondary phloem is a rare feature and has been reported in Cucurbitaceae (Fischer 1884) and Compositae (Chavan et al. 1983). They have also been reported in mesocarpic as well as endocarpic regions of fruits (Thanki 1978; Shah et al. 1983). However, in the aforementioned plants they are slightly oblique to the axial sieve tube elements and developed mostly in the regions of ray splitting and ray fusion. Meagre information is available on naturally occurring sieve tube elements in the rays of secondary phloem (Chavan et al. 1983). The present paper reports the occurrence and structure of sieve elements in the radial system of some tropical trees.

MATERIALS AND METHODS

Samples of phloem tissues from the main trunks of Erythrina indica, Guazuma tomen­ tosa, Acacia nilotica, Azadirachta indica and Tectona grandis were collected at breast height from trees growing in the Dangs forest of southern Gujarat, India. They were immediately fixed in FAA (Berlyn & Miksche 1976). Transverse, radial and tangential longitudinal sections of 12-15 !lm thickness were cut on a sliding microtome and

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Downloaded from Brill.com09/29/2021 05:34:07AM via free access Rajput & Rao - Sieve elements in phloem rays 199 stained with tannic acid-ferric chloride-lacmoide combination (Cheadle et al. 1953). Dimensional details were obtained directly either from tangential longitudinal or ra­ dial longitudinal sections with an ocular microscopic scale. The sieve tube elements that occurred among the ray cells are designated as ray sieve elements.

RESULTS

Radially arranged sieve tube elements were encountered either solitary or in groups of 1-5 in uniseriate as well as multiseriate rays of secondary phloem. They were short and each sieve tube element was associated with a single companion cell. In all the five species sieve plates were either simple (Fig. lA, B) or compound (Fig. 1C) on trans­ verse to slightly oblique end walls. The structure and behaviour of these ray sieve elements are similar to those of axial sieve elements. Massive deposition of callose (Fig. lD), collapse of companion cells, obliteration and loss of cell content are also noticed in ray sieve elements. These also showed accumulation of slime (P-protein) plugs against the sieve plate, cytoplasmic strands but no sieve areas on their lateral walls. The diameter of these elements was relatively larger than that of adjacent ray parenchyma cells, but more or less equal to that of axial sieve tube elements. The sieve pores were smaller, however. These sieve tube elements showed contact with axial sieve tube elements in all three planes (Fig. IE, F). Though the sieve tube elements were observed in both uniseriate and multi seriate rays, their distribution was characteristic for each plant studied. In Erythrina, they developed only in the centre of multiseriate rays but apparently not in uniseriate rays. In Guazuma they were found in both uniseriate and multi seriate rays and, when present in uniseriate rays, their position varied. They were located either at the extreme (outer) radial end or in the centre of the ray. In multi seriate rays they were present at the margin of the ray. The marginal ray sieve elements are often associated with axial sieve tube elements. However, in Acacia and Azadirachta the ray sieve elements occur only at the extreme radial ends or at the margin of rays. In Tectona they were found randomly among ray cells. The sieve plates of both the axial and ray sieve elements in Erythrina, Guazuma and Tectona are simple, while those of axial sieve elements are both simple and compound in Acacia and Azadirachta. Both simple and compound sieve plates occur in ray sieve elements of Acacia, but Azadirachta has only simple sieve plates.

Fig. 1. Sieve elements in phloem rays (A-0 tangential, E transverse, F radial view). - A: Group of sieve elements (arrow) in a phloem ray of Erythrina. - B: Sieve element in Azadirachta showing a sieve plate (arrow). - C: Sieve element at a ray margin with a compound sieve plate (arrowhead) in Acacia. - 0: Sieve plate (arrow) connecting ray and axial sieve elements in Tec­ tona. Note the massive callose deposition on the sieve plate. - E: Arrangement of radial sieve elements (arrow) between fibre bands in Tectona. Note the sieve plate between radial and axial sieve elements (arrowhead). - F: Continuation of sieve elements (arrow) from the axial to the radial system in Tectona. - Fig. A, x 390; Fig. B-E, x 380; Fig. F, x 300.

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DISCUSSION

In all the five species studied, radially arranged sieve tube elements occurred either solitary or in groups in the phloem rays, They tend to be shorter and narrower than the axial sieve tube elements, and shorter but wider than the parenchyma cells of the phloem rays, Ray sieve elements have been observed parallel or slightly oblique to axial sieve tube elements mostly in the region of ray splitting (Chavan et aL 1983), In the present study, however, the sieve tube elements are radially arranged like ray cells, Occur­ rence of simple as well as compound sieve plates has also been reported by Thanki (1978) and Chavan et aL (1983) and it seems true that even sieve plates of these ele­ ments show specialization in utilizing the maximum area on the sieve plate (Shah & Chavan 1980), Occurrence of well-developed sieve plates with slime plugs and their connection with the main phloem strands indicates that they are probably involved in the short-distance exchange of photosynthetic material with the ray parenchyma cells (Chavan 1981), The occurrence of ray sieve elements suggests that they develop as an additional translocatory pathway to cope with the rapid transport of photosynthates (Thanki 1978; Shah et aL 1983; Chavan et aL 1983), Their constant association with companion cells (Esau 1969), ability to deposit callose and slime plugs, and their contact with axial sieve elements also suggest their role in the translocation of photo­ synthates. Axial sieve tube elements which cease to function show massive deposition of callose on sieve plates as a sign of inactivation (Evert 1984; Lawton & Lawton 1971; Deshpande & Rajendrababu 1985; Vishwakarma 1991). In the present study ray sieve elements present in nonfunctional zones of phloem also showed sieve pores completely blocked by callose. The differentation of either solitary or grouped sieve tube elements in response to injury or chemicals, especially growth hormones, has already been reported by earlier workers (Eschrich 1953; La Motte & Jacobs 1963; Digby & Wareing 1966a, b; Aloni 1980). In the present study, the sampled trees were growing in deep forest and before sampling care was taken that the trees had not been injured earlier; therefore, the oc­ currence of these ray sieve elements was not in response to either injury or chemicals. This occurrence may be related, however, to incidental development to serve short­ distance transport of photosynthates between the axial and radial system.

ACKNOWLEDGEMENT

The authors are thankful to the University Grants Commission, New Delhi for financial support.

REFERENCES

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