Rezk, Malak R.

English

Rezk Malak R.

University of Szeged

Ac'.a Biologica Szeged. 20 (1—4), pp. 89 — 123 (1974) S E N S I T I V I T Y T O L I G H T I N P L A N T A G E S E E D S A S R E L A T E D T O S E E D C O A T S T R U C T U R E M A L A K R . R E Z K Department of Botany University of Alexandria (Received July 21, 1973) Abstract The seeds of three different species of Plant age. namely P. erassifolia. P. major and P. squarrosa, were studied with regard to their varying sensitivities to light. It was found that the seeds of P. major were the smallest in size among the three species studied and exhibited the highest degree of sensitivity to light during germination, being light-favoured. Plantage crassifolla and P.squarrosa had larger seeds and showed lesser sensitivities to light during germination. The cause of such differences was sought in the seed coat structures of the different species. The seeds of Plantage major contained a well-defined, pigment-full cellular palisade layer, which is not present in the other two species. Phytogenetic relations among the three species under investigation were discussed in the light of their behaviour during germination. Introduction In a previous investigation ( T A I I R O S and R E Z K , 1966), it was shown that the seeds of some species of Plantago exhibit varying degrees of sensitivity to light during germination. Thus, in complete darkness, the seeds of the species studied showed low germination percentages, that differed from one species to another. The results of the above-mentioned investigation showed that the seeds of Plantago major gave 2% germination in the dark, and 66% in bright daylight: those of P. crassifolia gave 12')., and 38% respectively, in the two treatments; while those of P. squarrosa gave 50% and 92%, Further work ( R E Z K 1967a, 1967b; R E Z K et H O R V A T H , 1968) was carried out with the seeds of P. major, in which the phenomenon was studied from the points of view of the cffects of different chemical and physical factors on the breaking of the dark-induced dormancy. It was concluded that the behaviour of P. major seeds differed considerably from that of Lactuca saliva v. Grand Rapids, experi-mented on in similar investigations by many other authors (e.g. B O R T H W I C K et al.. 1954). For Plantago major seeds it was observed in common practice in our laboratory that scratching them with sand paper resulted in a rapid onset and a high percentage of germination in the absence of the brinht daylight usually needed. This has drawn attention towards a possible role of the seed coat in previnting the seeds from ger-mination in the dark. Seed coat dormancy in general is a wellknown phenomenon previously investigated by C R O C K E R et BARTON (1953) and others. 90 KEZK. R. MALAK The aim of the present investigation is to answer the questions: a) Is there any probable correlation between the varied sensitivities to light in the seeds of the species of Plantago studied and their morphological structures, b) Can this variation in sensitivity to light be correlated the their previously suggested interspecific phylo-genetic relationships ( T A D R O S et R E Z K , 1970). Materials and Methods Seeds of Plantage crassi/olia FORSK., P. major L. and P. squarrosa MURR. v. brachystachys Boiss. were collected from naturally-occurring plants in the vicinity of Alexandria, Microtome sections (20 ft thick) were prepared, which were stained with crystal violet (saffranin. Free-hand sec-tions mounted in water were also prepared for comparison, as the colouring material was not leached •out in the latter. Observations The seeds of the three species showed considerable morphological variation, Plantago crassi/olia and P. squarrosa seeds are lightbrown in colour with a smooth testa, while those of P. major darker sculptured testa. Further, the general shape of the seed differs from one species to another. P. squarrosa seed is nearly ellipsoidal in outline and concave-convex in T. S. measuring about 1.5—2 mm long. P. Crassifolia seed is narrower and more or less plano-convex in T. S. reaching 1.0—1.5 mm in length. Plantago major seed is triquetrous to irregular in outline, and nearly triangular in T. S.; it measures about I mm long. Examination of the median transverse sections of the seeds of the three species of Plantago reveals basic differences both in shape and in the structure of the testa. They all agree in possessing a thin, delicate, mucilageproducing epidermal layer. This layer has previously been studied in considerable detail for two species of Plantago. D O D D S ( 1 9 5 3 ) described it in P. coronopus seed from the point of view of its ecological value to the plant as a cementing material, that fixed the seed in the soil and allows the growing radicle to penetrate it easily. With regard to the development H Y D E ( 1 9 7 0 ) examined and described a similar layer in the related species Plantago oval a. Below the epidermis in Plantago squarrosa and in Plantago crassi/olia there is a layer of undifferentiated compact cells that contain a brown colour-ing material, responsible for the seed colour. This is the pigment layer. In the former species this layer becomes thinner on the hylar side of the seed. In Plantago major on the other hand, below the transparent epidermis lies a definite layer of cells whose radial and inner tangential walls are densely thickened and pigmented. This is the palisade or the malpighian layer. It is to this layer that the dark colour of the testa and its sculptured appearance are due. This colouring material seems to be lost during dehydration and staining techniques, so that the thickened walls are colourless in the permanent microtome sections. In the free-hand section mounted in water, it retains the colour and shows a definite dark-coloured cellular layer. The thickening is shown to be deposited in the from of radial striations. This palisade layer tightly ensheathes the contents of the seed below the epidermis. SENS IT IV ITY TO L IGHT IN PLANTAC.O SEEDS 9 1 Similar testa structures in the seeds of some crucifers have been reported by E D W A R D S ( 1 9 6 8 ) , and V A U G H A N ( 1 9 5 6 and 1 9 7 0 ) . This is especially met with in Brassica spp. and Si nap is spp., which are reported to exhibit some type of dor-mancy. Discussion and Conclusions It is clear from the above study that there are basic differences in the structures of the three species of Plantago studied, parallel to their degrees of sensitivity to light. As the pigmentcarrying layer increases in thickness, the need for a greater amount of light to induce germination also increases, or in other words, the germina-tion percentages in the dark decrease. This attains a maximum in the seed of Plantago major, where this layer is quite definite and obvious. Thus, it can safely be supposed that a direct relationship may exist between the thickness of this pigmentcarring layer and the germination light requirements of the seeds. The interference of the seed coat with the germination has been interpreted as taking place through any of the following routes: a) Mechanical, preventing the enlargement of the embryo. b) Hindering light from entering the embryo. c) Preventing some sort of inhibitor that may be present inside the seed from being leached out ( R O B E R T S , 1 9 6 9 ) . d) Limiting certain oxidation processes from being completed. I K U M A and T H I M A N N ( 1 9 6 3 ) have attributed the promoting effect of red light on the germination of lettuce seeds to the influence of this factor on the production of a set of enzymes that enable the radicle to elongate outwards through the seed tissue. G U G L I A D A et al., ( 1 9 6 7 ) working with the photoblastic seeds of Datura ferox, stated that the oxidised products with whose production the seed coat interferes would permit the photomechanism to become operative or, once this has been completed, to overcome the blockage imposed by the "inhibitor". E D W A R D S (1968) investigated the dormancy of charlock (Sinapts arvensis L.) seeds, which are shown to have a testa structure similar to that of Plantago major seeds, and attributed this phenomenon to the phenol and mucilage content of testa hindering the diffusion of oxygen into the tissue of the embryo. That the seed coat may completely block the germinationpromoting effect of red tight was demonstrated by S A N C H E Z et al., 1967. Then, it can be stated that in the light-sensitive seeds of Plantago and similar species, the seed coats may interfere either directly, preventing light from entering the embryo, or indirectly, preventing some light-sensitive oxidationinhibiting products from being leached out, therebly retarding germination in the dark. W A R E I N G (1966) reports that it is difficult to see what adaptive value arises from light-sensitivity in seeds and that the more frequent light requirements mean that germination can only occur on the soil surface. He adds that although it is difficult to see any very marked adaptive in light-sensitivity, its ecological importance is un-questionable, especially in (he germination of weed seeds following agricultural disturbance of the soil, T A D R O S and R E Z K ( 1 9 7 0 ) have put forward a working hypothesis as regards the interspecific phylogenetic relations in four species of Plantago. From that hypoth-esis it was proposed that Plantago major by virtue of its mesophytic character moist 9 2 R E Z K , R \ 1 A L A K habitats and subcosmopolitan distribution, may be regarded as the ancient as com-pared with the other non-mesophytic species of Plantago. e.g. P. squarrosa (annual psammophite) and P. crassifolia (perennial ha I ophite ). If we non try to correlate the evolutionary tendencies of the species in the light of the working hypothesis mentioned above, il can be said that there is a parallel decrease in ihe sensitivity to light of the seed wiih the progress of evolution. The seeds of P. major the more ancient mesophytic species, proved to be acutely light-sensitive, containing a light-impervious layer, the palisade or malphighian layer, that is assumed to be the site of preventing light from entering the seed. As evolution proceeds and new species develop from the more ancient, a decrease in the light-sensitivity is manifested. This may be supposed to have proceeded through (he fol-lowing parallel lines: a) Change of seed shape from the small closed triquetrous ( P. major) to the larger, more open piano-convex (P. crassifolia) to the concave-convex boat shaped (P. squarrosa) seed. b) Decrease of pigmentation of the seed coat and the disappearance of the palisade light-impervious layer, paralleled by the decreased sensitivity to light. These conclusions, however, need further verification and evidence through other cytotaxonomic, morphologic and genetic studies. References B O R T H W I C K , H , A., S. B . H E N D R I C K S , F . H . TOOLE and V. IC. T O O L E ( 1 9 5 4 ) : Action of light on lettuce seed germination. — Bot. Gaz. 115, 205—225. C R O C K E R , W , and L. B A R T O N (1953): Physiology of Seeds. — Chron. Bot. Co. U.S.A. D O D D S . J . G . ( 1 9 5 3 ) : Plantage coronopus L . — Jour, ECOL. 41. 4 6 7 — 4 6 8 . E D W A R D S . M. M. (1968): Dormancy in seeds of charlock. 11 The influence of the seed coat. — Jour. Exp. Bot. 19. 583—600. " G U Ü L I A D A , M. L., A, S O R I A N O and B I J R K A R I (1967): The seed coat effect in relation to the photo-induction of germination in Datura ferox L. — Canad. Jottr. Bot. 45, 377—381. HYDE. B. B. (1970): Mucilage-producing cells in the seed coat of Plantage mata. Developmental fine structure. — Amer. Jour. Bot. 57. 1197—[206. IKIJMA, H . and K, V. T H I M A N N (1963): The role of the seed coats in the germination of photosensi-tive lettuce seeds. — Plant et Cell Physiol. 4, 169—185. R E Z K , M . R . (1967/a): Studies on the light sensitivity of Plumage major L . seeds. I. The effect of gibherellin, kineiin and potassium nitrate. — Acta Biol. Szeged. 1.1, 31—37. R I Z K , M . R.(1967/b): Studies on the light sensitivity of Plantage major L. seeds. U . The effect of red light alone and mediated by chamicals. — Acta Biol. Szeged. 13. 39—44. R F Z K , M . R . and I. H O R V Á T H ( 1 9 6 8 ) : Studies on the light sensitivity of Plantage major L. seeds. III. The effect of light colour and intensity. - Acta Biol. Szeged. 14, 29—33. ROBERTS, E. H . ( 1 9 6 9 ) : Seed dormancy and oxidation processes. — Symp. Soc. Exp. Biol. 23. 161 1 6 4 — 1 9 2 . SANCHEZ, R. A., S O R I A N O and S . SLABNIK (1967): The interaction of seed coat and gibberellic acid in the germination of Datura ferox L. — Canad. Jour. Biol. 45, 371—376. T A D R O S , T. M, and M. R . R F Z K (I966I: Ecological studies on the distribution of four species of Plantage in the northwestern desert Of Egvpi. IT. Micro enviromental factors. — Bull. Fac. Sc. Alex. 7, 179—200. T ADR os, T. M. and M. R. RtZK (1970): Ecological studies on the distribution of four species of Plantage in the northwestern desert of Egypt. V. Effect of element dificiency. — Bull. Fac. Sc. Alex. 10. 369—390. SENSIT IV ITY TO L I G H T IN PL A NT AGO SEEDS 93 VAUGHAN, J. G. ( 1 9 5 6 ) : The seed coat structure of Brassica integrifolia (WEST O . E , SCHULZ var. earinata) (A. BR.) — Phytomorphology 6, 363—367. VAUGHAN, J. G . ( 1 9 7 0 ) : The structure and utilization of oil seeds. — Chapman et Hall. London. WAREINIG, P. F , ( 1 9 6 6 ) : Ecological aspects of seed dormancy and germination. — 8. S. B . I . Con-ference N o . 9. 103—121. Address of the author: MALAK R . REZK Department of Botany, University of Alexandria 

Sensitivity to light in Plantago seeds as related to seed coal structure

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Sensitivity to light in Plantago seeds as related to seed coal structure

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