Proc. Indian Acad. Sci. ( Sci.), Vol. 97, No. 6, December 1987, pp. 461-467. Printed in India.

Embryology of Apluda mutica ()

R K BHANWRA and PROMILA PATHAK Department of Botany, Panjab University, Chandigarh 160 014, India MS received 25 February 1987; revised 14 September 1987 Abstract. In 3 populations of Apluda mutica, a useful forage grass, the anther wall is 4- layered and its development corresponds to the Monocotyledonous type. Successive cytokinesis in microspore mother cells results in isobilateral tetrads of microspores. Pollen is shed at 3-celled stage. The female gametophyte is of the monosporic Polygonum type. Megaspore tetrads and female gametophyte degenerate at different stages of development and this results in poor seed set. The ovule is bitegmic, pseudocrassinucellate and hemianatropous. The integuments and the nuceUus get consumed after fertilization. The pericarp forms a thin-layer around the endosperm and mature embryo and the latter is of the panicoid type. Keywords. Embryology;Apluda mutica; Poaceae.

1. Introduction

Apluda Linn. belonging to the tribe is a useful forage grass. It is a taxonomically difficult and highly polymorphic distributed in India and extending to south-east and Australia (Bor 1960). Aposporic apomixis has been reported by Murty (1973) in diploid (2n=20), hexaploid (2n=60) and heptaploid (2n=70) cytological races of A. mutica var. aristata. Bor (1960) recognises two varieties in the genus i.e.A, mutica var. mutica and A. mutica var. aristata. Recently, Cope (1982) did not recognise these varieties in the genus. The populations of A. mutica have been reported to be diploid in the hills of north-west India but they are tetraploid in the plains (Mehra et al 1968; Sharma and Kumar 1985). The aim of the present study is to elucidate the reproductive behaviour in A. mutica and to have a detailed information regarding the embryology (sensu Davis 1966) of the awned and unawned forms in the genus. The current paper deals only with the 3 popula- tions of the awned form. The embryology of the unawned form and relationships between the two shall be presented elsewhere.

2. Materials and methods

The material was collected from the natural populations of A. mutica growing at Chandigarh and hills of Kalka and Kasauli. lnflorescences at different stages of development were ¡ in FAA. After the customary methods of dehydration and embedding, sections of the entire unit of inftorescence or of individual florets cut at 6-10/~m were stained in safranin-fast green combination or hematoxylin and fast green. Identification of the was confii'med by Dr T A Cope, Royal Botanical Gardens, Kew, England. Voucher specimens have been deposited in the herbarium of the University. 461 462 R K Bhanwra and Promila Pathak

3. Observations

3.1 Microsporogenesis and male 9ametophyte

The anthers are tetrasporangiate. The development of the anther wall is of the Monocotyledonous type (figures 1-9). The anther wall consists of epidermis, endo- thecial layer, a middle layer and tapetum (figure 11). The primary sporogenous cells divides radially to form 4-6 cells (figures 5, 7) and about 36 sporogenous cells in a vertical file of ah anther lobe. Meiosis in microspore mother cells is normal (figures 11, 13). Cytokinesis is successive resulting in isobilateral tetrads (figure 14). The pollen is shed at 3-celled stage (figures 15-17). The middle layer of the anther wall is ephemeral (figure 13). The tapetum is of the glandular type and its cells remain uninucleate (figure 13). Pollen fertility is about 98~.

3.2 Megasporogenesis and female gametophyte

A hypodermal archesporial cell which differentiates in the nucellus increases in size and behaves as megaspore mother cell(figures 18, 19). It divides meiotically to form a linear or T-shaped tetrad of megaspores (figures 20, 21). Occasionally the upper dyad cell fails to divide so that only a linear row of 3 cells is present (figure 22). The chalazal megaspore functions and it develops into a Polygonum type of embryo sac (figures 23-25). Degeneration of the megaspore tetrad or of the female gametophyte at various stages of development is a feature commonly observed in all the 3 popu- lations (figures 26-30). This results in a poor seed set. There is no indication of aposporic embryo sac development at any stage.

3.3 Ovary and ovule

The gynoecium consists of an ovoid ovary having a short stalk, two long styles and two feathery stigmas (figure 31). The ovule is bitegmic, pseudocrassinucellate and hemianatropous (figure 32). The outer integument is 2-1ayered in thickness and encloses only about one third of the ovule (figure 32). The inner integument is also 2- or 3-1ayered thick and it grows upto the level of the nucellus (figure 32) ora little beyond it. The nucellar epidermis in the vic!nity of micropyle undergoes periclinal divisions to forma parietal tissue 4-6-1ayered deep (figure 32). The apical cells and some of the hypodermal cells become conspicuous owing to large size and dense cytoplasmic contents. The dorsal ovary wall has an epidermis and ground parenchyma comprising 8 or 9-1ayers of cells in the proximal and distal regions but there are 5 or 6-1ayers of ceUs in the middle thinner portion (figures 33, 34). The layer of ground parenchyma cells lining the locule disorganises at the time of fertilization and the layer subjacent to it shows meristematic activity. These ceUs form palisade-like layer in a vertical section (figures 33, 34).

3.4 Pollination

A large number of stigmas were removed at anthesis and pollen tube growth was Embryology of Apluda mutica (Poaceae) 463

1 4~ (/

91 50pm ' 1,2,4,5,19,20,2~ "3 24 2 5p.m 1 3,7,9JU 3,V~ 7,18,21--23 Figures 1-24. Microsporangium, microsporogenesis, megasporogenesis, male and female gametophytes. 1. Anther primordium. 2. Differentiation of hypodermal archesporial cells. 3. Formation of primary parietal and primary sporogenous cell. 4. Concentric layer of parietal cells surrounding a primary sporogenous cell. 5. One of the anther tobes showing epidermis, outer secondary parietal layer and inner secondary parietal layer and 4 sporogenous cells. 6, 8, 10 and 12. Tetrasporangiate anthers. 7, 9, 11 and 13. Magnified view of the marked segments. 7. Epidermis, endothecial layer, dividing cetls of the inner secondary parietal layer and 5 sporogenous cells. 9. Four-layered anther wall and 2 sporogenous cells. 11. Anther wall and microspore mother cells. 13. Epidermis, endothecial layer, disorganised middle layer, uninucleate tapetum and microscope mother cells at metaphase-I. 14. Microspore tetrad. 15-17. Uninucleate, binucleate and trinucleate pollen grains. 18. Vertical section of ovule primordium with archesporial cell. 19. Megaspore mother cell. 20 and 21. Linear and T-shaped tetrad of megaspores. 22. A linear triad. 23 and 24. Two- and 4-nucleate embryo sacs respec- tively. 464 R K Bhanwra and Promila Pathak

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Figures 25-39. Gynoecium, ovule, female gametophyte and post-fertilization changes. 25. Organised embryo sac. 26--30. Disorganized tetrad, and female gameto- phyte. 31. Mature gynoecium (x 19). 32. Structure of a mature ovule. 33 and 34. Distal and middle regions of the dorsal ovary wall. 35. A segment of the dorsal pericarp, nucellus and endosperm. 36. Four-celled proembryo and cellular endosperm. 37. A young caryopsis. 38. Vertical section of caryopsis showing mature embryo and cellular endosperm. 39. A segment of the dorsal pericarp and endosperm. (ant, Antipodal cells; ate, aleurone transfer cells; cc, central cell; end, endosperm; e, egg; nuc, nucellus; per, perichrp; pem, proembryo; pn, polar nucleus; r, radicle; syn, synergid). Embryology of Apluda mutica (Poaceae) 465

observed following the technique of Ramming et al (1973). The germination of pollen was observed to be very poor. In a large number of mature ernbryo sacs which were apparently healthy, there was no evidence of the entry of the pollen tube. These embryo sacs and ovules even- tually degenerate (figure 30).

3.5 Endosperm

The endosperm is of the nuclear type. It becomes cellular at the globular stage of the proembryo. Figure 36 shows a 4-celled proembryo and cellular endosperm. The outermost layer of endosperm differentiates into the aleurone layer (figures 35 and 39). These cells differentiate into aleurone transfer cells at the chalazal end of the biconvex ovate caryopsis (figure 37). In the mature caryopsis (figure 38) the cells of the endosperm acquire thick walls.

3.6 Seed coat and pericarp

The integuments start to disorganize after the commencement of organogenesis in the embryo (figure 35). In mature caryopsis only a patch of nucellar ceUs survives at the chalazal end (figure 38). The epidermal and ground parenchyma cells of the pericarp grow;in length and circumference during early stages of caryopsis develop- ment. When the scutellum and coleoptile are differentiated in the embryo, only the epidermal layer and meristematic layer are traceable. In the mature caryopsis the epidermis and the me¡ layer become thick-walled (figure 39).

3.7 Embryo

The early development of the embryo could not be studied due to poor seed set. The mature embryo is about two thirds the length of the caryopsis (figure 39). The shield- shaped Scutellum is free from the coleorhiza, which encloses the radicle and the root cap. The coleoptile surrounds the shoot apex and embryonic leaves. The vascular supply to the shoot apex and scutellum does not diverge from the same point.

4. Discussion

The 3 populations of A. mutica studied here are amphimictic so lar as their mode of reproduction is concerned. This is in contrast to A. mutica var. aristata, where some diploid but aU hexaploid and heptaploid cytological races have been reported to be aposporic apomicts by Murty (1973). Degeneration of sporogenous tissue and female gametophyte at various phases of development results in a poor seed set in A. mutica but there was no indication of apospory. The anther wall development is of the Monocotyledonous type (Davis 1966), a feature of common occurrence in Gramineae. The tapetal cells remain uninucleate in A. mutica. Both uninucleate and binucleate condition of tapetum occurs in the tribe Andorpogoneae. This feature is not constant even at species level e.g. both uninuc- leate and binucleate condition has been reported in Cymbopogon (Choda et al 1982). 466 R K Bhanwra and Promila Pathak

Successive cytokinesis in microspore mother cells results in isobilateral tetrads of microspores, a feature reported also in other species of Andorpogoneae. Pollen is shed at 3-celled stage which seems to be a general condition in grasses. Supernume- rary male gametes have been reported in Sorghum vulgare (Artschwager and McGuire 1949). The inner integument grows almost up to the level of the nucellus or slightly beyond ir in A. mutica, a feature also reported in other taxa of the tribe Andropogo- neae investigated so far. The inner integument degenerates after fertilization. The inner integument is ephemeral in Bothriochloa odorata, Capillipedium huegelii, C. parviflorum, Eremopogon foveolatus, Cymbopogon nardus, C. parkeri, C. martinii, Imperata cylindrica, Iseilema prostratum, Saccharum bengalense and Vetiveria ziza- noides (Choda and Bhanwra 1977, 1980; Choda et al 1982; Bhanwra and Choda 1981; Bhanwra et al 1982), Coix lacryma-jobi (Weatherwax 1930), and Euchlaena mexicana and Zea mays (Randolph 1936; Cooper 1937) but it persists in rnature caryopsis in Saccharum officinarum (Artschwager et al 1929) and Sorghum vulgare (Artschwager and McGuire 1949). The outer integument is poorly developed and its upper part encloses only about one third of the ovule. This feature is constant in the tribe as is indicated by its occurrence in about 14 species investigated so far (Artschwager et al 1929; Randolph 1936; Cooper 1937; Artschwager and McGuire 1949; Chandra 1963; Venkateswarlu and Devi 1964; Choda and Bhanwra 1977, 1980; Choda et al 1982; Bhanwra et al 1982). The nucellar epidermis in the vicinity of micropyle undergoes periclinal divisions so to form a parietal tissue 4-6 layers in thickness in A. mutica, a feature reported in the species studied earlier. The mature embryo sac is oriented parallel to the longitudinal axis of the ovule in A. mutica. This feature is consistent in all the andorpogoneae so far studied. The mature embryo is about two-thirds the length of the caryopsis and is of the Panicoid type as has been previously reported in 16 species of the tribe Andorpogo- neae investigated by Reeder (1957). The important point to be noted is that A. mutica is a facultative apomict due to the absence of apospory in diploid and tetraploid populations from Chandigarh, Kalka and Kasauli and its presence in one of the diploids and all the hexaploid and heptaploid populations investigated by Murty (1973) from a number of localities from other parts of India.

Aeknowledgement

We are extremely grateful to Dr T A Cope, Royal Botanical Gardens, Kew, England for confirming the identification of the plant under investigation.

References

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