Seed structure and germination of Dichrostachys cinerea

Transcription

1 S.Afr.J.Bt., 1993, 59(1): Seed structure and germinatin f Dichrstachys cinerea W.E. Bell and J. van Staden* UN/FRO Research Unit fr Plant Grwth and Develpment, Department f Btany, University f Natal, Pietermaritzburg, 3 Republic f Suth Africa Received January 199; revised 1 August 199 Dichrstachys cinerea (L.) Wight et Arn. (Legume: Mimsideae) is a majr cntributr t the bush encrachment prblem in Mkuzi Game Reserve. Seed characteristics were examined s that eclgical and managerial cncepts culd be evaluated. Drmancy is impsed by the smth, hard and impermeable testa. Scarificatin r cracks in the testa allw imbibitin thrugh the lens, which is fllwed by germinatin. The percentage germinatin f Dichrstachys cinerea is higher in light than in dark and there is a lss f temperature sensitivity with time. The behaviur f Dichrstachys cinerea seeds is nt as that typically reprted fr mimsid seeds, since germinatin imprved with strage, and light enhanced germinatin f scarified seeds. This has imprtant eclgical implicatins, since seeds wuld germinate easier where the canpy has been remved r the sil has beeh disturbed. Dichrstachys cinerea (L.) Wight et Arn. (Legume: Mimsideae) is 'n belangrike bydraer tt die bsindringerprbleem in die Mkuzi Wildreservaat. Saadkaraktertrekke is bestudeer sd at eklgiese en bestuursknsepte geevalueer kn wrd. Saadrus wrd verrsaak deur die gladde, harde en waterdigte testa. Skarifikasie en barste in die testa laat imbibisie deur die lens te sdat ntkieming kan plaasvind. Die persentasie ntkieming van Dichrstachys cinerea is her in die lig as in die dnker en daar is 'n verlies van temperatuursensitiwiteit met tyd. Dichrstachys cinerea sade reageer nie ss verwag wrd van mimsidsade nie, aangesien ntkieming met pberging verbeter het, en lig die ntkieming van geskarifiseerde sade verhg het. Hierdie aspek het belangrik eklgiese implikasies mdat sade beter sal ntkiem waar die plantegrei verwyder f die grnd versteur is. Keywrds: Dichrstachys cinerea, legume, Mimsideae, seed, drmancy, seed structure, germinatin. -T whm crrespndence shuld be addressed. Intrductin Bush encrachment is a majr prblem in Mkuzi Game Reserve (Gdman 1977). Dichrstachys cinerea (L.) Wight et Am. grws n a variety f sils within the reserve and is ne f the majr cntributrs t the bush encrachment prblem. It is a thmbush belnging t the Leguminsae subfamily Mimsideae. Due t climate, stcking densities and management practices, dense thickets f this plant have frmed, preventing grazing animals frm reaching available grass. While this plant can make a valuable cntributin as feed fr brwsing stck, it nevertheless reduces the carrying capacity f the veld. It is therefre necessary t determine the physilgical and eclgical factrs which cntribute t the spread f D. cinerea in its natural habitat if cntrl prgrammes are t be instituted. The ccurrence f hardseededness in members f the Leguminsae is well established (Cmer 1951; Bewley & Black 19; Van Staden et al. 199). At present, little is knwn abut the seed characteristics f D. cinerea. This must be remedied befre eclgical and managerial cncepts and strategies can be fully evaluated. In this paper we reprt in detail n the structure nd hardseeded characteristics f the seed. Materials and Methds All the seed used in this study was cllected in Mkuzi Game Reserve in nrthern Natal during May and June 199. Only dry mature pds were cllected frm the trees. Unctamaged seeds were remved frm the pds and these were stred in sealed glass bttles at 5 C in the dark. T minimize damage by bruchids (Cleptera: Bruchidae), Captab (N-trichlrmethylthi-cyclhex--ene-1,-dicarbximide) was included with the seeds. T determine the degree f hardseededness, ten batches f five seeds each were maintained at 5 C in the light and in the dark n mist filter paper in Petri dishes. At 1-hurly intervals the seeds were bltted dry using absrbent paper twelling and then weighed. Mechanical scarificatin was effected by nicking the seeds at the chalazal end with a sharp scalpel blade. These seeds were incubated at 5 C in the light and in the dark. Seeds were incubated in cntrlled incubatrs at, 5, 3 and 35 C in light and the rate f imbibitin and germinatin was recrded every tw days. The experiment was repeated and 3 mnths later n the same seed batch. Fr the germinatin experiments, radicle emergence was recrded daily. The rute f water uptake was studied by immersing the seeds in a clured slutin (aqueus fast green.1 % w/v) and tracing the path f water entry by examining the seed under a light micrscpe after varius time intervals. Tw methds f seed preparatin fr scanning electrn micrscpy (SEM) were cmpared. Seeds were sputter-cated and viewed as previusly described (Manning & Van Staden 197), r the E.M. SP sputter cry lw-temperature system was used where the temperature in the micrscpe was lwered t -175 C using liquid nitrgen, and the specimens were then examined.

2 S.-Afr.Tydskr.Plantk., 1993,59(1) 1 Results Seed structure The dry mature seed f D. cinerea is a typical mimsid (Gunn 191) with a pleurgram, hilum, lens and micrpyle (Figure la). The surface f the testa was smth fllwing sputter-cry-cating (Figure la) and cracked after sputtercating (Figure lb). The sputter cry methd was adpted fr all further SEM wrk. The seed cat is cmpsed f the cuticle, macrsclereids with a clearly visible light-line, stesclereids and parenchyma (Figure lc). The vascular bundle is visible belw the macrsclereids twards the lens where it lies beneath the epidermal palisade (Figure ld). The macrsclereid cells in the lens regin are shrter than thse surrunding the hilum (Figure Id). tim Figure 1 The lens end f the seed f Dichrstachys cinerea shwing the lens (L), micrpyle (M) and hilum (H). These scanning electrn micrgraphs were prepared using: (a) sputter cry, and (b) sputter-cating. The latter methd f cating results in cracks (arrw) in the testa. (c) A transverse sectin thrugh the testa f Dichrstachys cinerea shwing the cuticle (C), the macrsclereids, apparently divided by a light line (LL), stesclereids () and the parenchyma (P). (d) The vascular system (arrw) in the seed f Dichrstachys cinerea at the lens end. It starts at the hilum (H), where it enters the seed and extends int the mesphyll tissue befre cntinuing twards the lens (L).

3 11 S.AfrJ.Bt.,1993,59(1) Imbibitin Fresh undamaged seeds f D. cinerea did nt imbibe r germinate unless scarified. Once the seed had been mechanically scarified, imbibitin and germinatin tk place prviding that water was available. Water uptake was rapid during the first h (Figure ). N difference was nted between seeds imbibed in the light r dark (Figure ). Seeds which were unscarified did nt germinate in the light r dark (Figure 3). Once seeds were scarified, germinatin was significantly higher in the light than in the dark (Figure 3). tin (Figure 5c). Germinatin f scarified seeds at all temperatures was apprximately - 9%. S C r / Effect f temperature n the imbibitin and germinatin f Dichrstachys cinerea seeds The initial rate f imbibitin f scarified seeds incubated at, 5 and 3 C was similar (Figure ). Incubatin f the scarified seeds at 35 C reduced the rate f water uptake significantly (Figure d). Unscarified seeds did nt imbibe r germinate. Germinatin f the scarified seeds ccurred mre rapidly at 3 C than at, 5 r 35 C (Figure 5). Repetitin f this experiment n seed stred fr tw and three mnths, respectively, resulted in greater rates f germinatin fr seeds incubated at 5 and 35 C (Figures 5b,d), while seeds stred fr three mnths and incubated at 3 C resulted in similar rates and final percentage germ ina- C 1 C) E 1 3 C 3S C r I Figure The rate f water uptake f scarified seeds incubated at (a), 5 (b), 3 (c) and 35 C (d), respectively (n = 1 fr each trial). The vertical bars represent ± S.E..5 dark, light 1.C S C j 1 Figure The rate f water uptake f -seeds stred at SOC and subsequently incubated at a temperature f SOC in the light (a) and dark (b). Seeds were unscarified () r scarified (e). The vertical bars represent ± S.E. 3S C 1 light Jl dark ( ffi, (!) (!) t- : a Figure 3 The percentage germinatin f seeds which received n pretreatment and which were subsequently incubated at 5 C in the light (a) r dark (b). Seeds were unscarified () r scarified (e). The vertical bars represent ± S.E. Figure 5 The percentage germinatin f scarified seeds incubated at (a), 5 (b), 3 (c) and 35 C (d), respectively (n = 1 fr each trial). Brken lines represent repetitin f this experiment at a later date: the results fr 5 C were btained after tw mnths and fr 3 and 35 C after three mnths. The vertical bars represent ± S.E.

4 1 Site f water entry Only a few f the intact, unsrted seeds which had been saked in the fast green dye fr h imbibed. In these seeds the green dye was lcalized in patches n the seed cat and in the pleurgram (Figure a). Thse seeds which had nt imbibed were stained in the hilar regin (Figure b) when cmpared with thse seeds which had nt been placed in the S.-Afr.Tydskr.Plantk.,1993, 59(1) dye (Figure c). Permeable seeds which were left fr 1 h became swllen and green at the lens (Figure d). The tissue split slightly at the lens t reveal a cavity which became visible nce the cap was disldged (Figure e). Transverse sectins f the permeable seeds shwed that the dye entered the testa at the lens (Figure f), and accumulated in the vascular tissue between the lens and the hilum. Figure Untreated, unsrted seeds were used in experiments t trace the path f water entry int the seeds. (a) A seed f Dichrstachys cinerea left in fast green fr 1 h which imbibed. The dye penetrated prtins f the testa and pleurgram (P) (X37.5). (b) Seed f Dichrslachys cinerea which was saked in fast green dye fr 1 h but did nt imbibe. Dye was absrbed at the hilum regin (H), but nt at the lens (L) r micrpyle (M) regins f the seed (X75). (c) Dichrslachys cinerea seed which was nt placed in dye (cntrl). Lens (L), hilum (H), micrpyle (M) (X 75). (d): Seed f Dichrslachys cinerea, left in fast green dye fr 1 h. The seeds which the dye had penetrated were selected fr bservatin. The dye had penetrated the lens (L) which was swllen arund the edges (arrw), leaving a small split in the centre. Hilum (H), mi<..1pyle (M) (X75). (e) Seed f Dichrslachys cinerea left in fast green dye fr 1 h. The swllen tissue arund the lens (L) f permeable seed was scraped away with a scalpel t reveal a sub-lens area (arrw). Hilum (H), micrpyle (M) (X 75). (f) Transverse sectin thrugh the lens end f a permeable, imbibed Dichrslachys cinerea seed shwing the vascular trace (arrw). Lens (L), hilum (H), micrpyle (M) (X55).

5 13 S.AfrJ.Bt.,1993,59(1) Discussin Many authrs have described the testa surface f the mimsid seed as cracked (Dell 19; Manning 197; Bhattacharya & Saha 199). The testa surface f D. cinerea is smth and the cracks that were bserved apparently resulted frm the methd f seed preparatin fr scanning electrn micrscpy. The use f the sputter cry prcedure ensured that n artifacts ccurred during the preparatin f seed fr scanning electrn micrscpy. N anmalies were nted in the structure f the lens, micrpyle and hilum. The shrter macrsclereid cells that were bserved at the lens (Hamly 193; Aitken 1939; Ballard 1973), and the clse cntact between the vascular trace and lens (Newman 193; Tran 1979; Dell 19), suggests that this is the natural site fr water entry. The fact that the vasculature between the lens and the hilum became stained wuld suggest that the xylem is functinal (Dell 19) and nt blcked as was the case fr Acacia kempeana (Muell.) (Hanna 19). The precise rle f the hilum in imbibitin f D. cinerea seeds was nt established. N reprts have been published which indicate a sensitivity f legume seeds t light. Preece (1971) and Scifres (197) have shwn that fr Acacia aneura and A. farnesiana, genninatin results were identical in the light and the dark. D. cinerea appears t have a degree f light sensitivity, as the percentage genninatin was greater in the light than in the dark. This wuld suggest that there may be mre than ne dnnancy mechanism perative and that dnnancy may be brken in respnse t a number f envirnmental cues. Althugh light increased final percentage genninatin, damage t the seed cat remains a prerequisite fr imbibitin and germinatin. A sensitivity t light wuld ensure that seeds clser t the sil surface genninated preferentially cmpared t thse buried in the sil. There als appears t be a lss f sensitivity t germinatin temperature with time which wuld allw fr a number f seeds t germinate later in the summer seasn. Once again this type f behaviur has nt been reprted fr a legume seed. It wuld appear, therefre, that D. cinerea is nt behaving exactly as ther mimsid seeds, since the remval f the characteristic water impermeable cnditin did nt result in 1% genninatin. Instead, the germinatin f D. cinerea imprved with strage, while light appears t prmte germinatin nce seeds have been scarified. The imprved respnse t light may be an imprtant factr fr seedling establishment in areas where fire is f frequent ccurrence r used as part f the veld management prcedure. Canpy remval results in higher light intensities and may well increase seedling establishment and subsequent bush encrachment. Acknwledgements The Fundatin fr Research Develpment and the Univer- sity Research Fund are thanked fr financial supprt. References AITKEN, Y The prblem f hard seeds in subterranean clver. Prc. R. Sc. Vict. 51 : BALLARD, L.A.T Physical barriers t germinatin. Seed Sci. Technl. 1: BEWLEY, J.D. & BLACK, M. 19. The Physilgy and Bichemistry f Seeds in Relatin t Germinatin, V!.. Springer-Verlag, Berlin. BHATTACHARYA, A. & SAHA, P.K Ultrastructure f seed cat and water uptake pattern f seeds during germinatin f Cassia sp. Seed Sci. Technl. 1: CORNER, EJ.H The leguminus seed. Phytrrwrphlgy 1: DELL, B. 19. Structure and functin f the strphilar plug in seeds f Albizia lphantha. Am. J. Bt. 7: GOODMAN, P.S Sil types prne t bush encrachment. In: Prceedings f Veld and Bush Management fr Beef and Game Prductin, pp Hlabisa Sil Cnservatin Cmmittee. GUNN, J.R Seeds f Leguminsae. In: Advances in Legume Systematics (11), eds. R.M. Plhill & P.H. Raven, pp Ryal Btanic Gardens, Kew. HAMLY, D.H Sftening f the seeds f Meliltus alba. Bt. Gaz. 93: HANNA, PJ. 19. Anatmical features f the seed cat f Acacia kempeana (Mue1!.) which relate t increased germinatin rate induced by heat treatment. New Phytl. 9: 3-9. MANNING, J.C The legume seed as a multifunctinal rganism (with particular reference t the Papilinideae). Dctral thesis, Department f Btany, University f Natal, Pietermaritzburg. MANNING, J.C. & VAN' STADEN, J The rle f the lens in seed imbibitin and seedling vigur f Sesbania punicea (Cav.) Benth. (Leguminsae: Papilinideae). Ann. Bt. 59: NEWMAN, LV Studies in the Australian Acacias:. The life histry f Acacia baileyana F.V.M. Prc. Linn. Sc N.S.w. 59: PREECE, P.B Cntributins t the bilgy f mulga. Aust.1. Bt. 19: SCIFRES, C.J Salient aspects f Huisache seed germinatin. SW Naturalist 1: TRAN, V.N Effects f micrwave energy n the strphile, seed cat and germinatin f Acacia seeds. Aust. J. PI. Physil. : VAN STADEN, J., MANNING, J.C. & KELLY, K.M Legume seeds - the structure : functin equatin. In: Advances in Legume Bilgy, eds. C.H. Stirtn & J.L. arucchi (eds). Bt. Mngr. Syst. M. bt. Gdn. 9: 17-5.