Egzogena dormantnost semena

Egzogena dormantnost semena je oblik dormantnosti semena, a prema terminologiji koju je predložio Lang sa saradnicima, ona je paradormantnost što predstavlja mirovanje izazvano fiziološkim faktorima ili biohemijskim signalima poreklom van dormantne strukture. Kontrola aktivnosti embriona dolazi iz neke od okružujućih struktura: perikarpa, semenjače pa i endosperma. Ovaj oblik dormantnosti potvrđuje se normalnim klijanjem embriona eksplantiranog iz semena. Egzogena dormantnost obuhvata tri tipa: fizičku, hemijsku i mehaničku dormantnost, kao i njihove kombinacije.

Makrosklereidne ćelije semenječe leguminoza.

Seme gvozdenog drveta (levo), trnovca (gore), kelreuterije (dole) i ruja (desno) ima nepropusnu semenjaču za vodu i gasove.

Seme jednosemene žutilice u pupčanoj brazdi ima "higroskopni ventil" kojim se sprečava prodor vlage kroz mikropilu.

Sočni hipantijum šipka sadrži inhibitorne materije koji prodorom u seme blokiraju klijanje.

Lactuca test: seme salate zaliveno rastvorenom pulpom šipka ne klija (desno) i kontrola zalivena vodom intenzivno klija (levo).

Seme jele u semenjači sadrži depoe terpentina koji izazivaju dormantnost.

Fizička dormantnost

Fizička dormantnost prouzrokovana je nepropustljivošću omotača (semenjače, perikarpa (ili njegovih delova - endokarpa, najčešće) i/ili endosperma) za vodu i ponekad za kiseonik. Seme sa ovim tipom dormantnosti očuva klijavost mnogo godina, čak i na visokoj temperaturi. Najčešće je izazivaju (makrosklereidne) ćelije semenjače debelih zidova slične palisadnim (svetla linija pod mikroskopom) i sloj kutikule. Za ove vrste se obično kaže da imaju "tvrdo seme" koje ne bubri ni posle 10 dana močenja u vodi. Klijanje započinje tek po omekšavanju ili oštećivanju omotača. Čim se sloj probije voda prodire u unutrašnjost i dolazi do bubrenja. Mnoge vrste iz familija Mimosaceae, Cesalpiniaceae, Papilionaceae, Anacardiaceae, Ericaceae, Rhamnaceae i Sapindaceae imaju ovaj oblik dormantnosti: Acacia spp, Albizia spp, Cytisus spp, Gleditsia spp, Gymnocladus spp, Koelreuteria spp, Laburnum spp, Petteria spp, Rhus spp, Robinia spp, Sophora spp.

Proučavanje propustljivosti omotača posebno je značajno za one vrste kod kojih setveni materijal, pored semenjače sadrži još neke omotače, kao što je slučaj kod jednosemenih ili lomljivih mahuna (lomentum). Pitanje dorade i uklanjanja ovih struktura presudno je za klijanje. Istraživanja sa setvenim materijalom bagrenca (Amorpha fruticosa L.), jednosemene žutilovke (Genista monosperma Lam.), lespedeze (Lespedeza bicolor Turcz.) i šibike (Coronilla emerus L.) ukazuju na različitu ulogu perikarpa, semenjače i endosperma kod ovih vrsta.

Kod nekih leptirnjača (Genista monosperma Lam., Lupinus arboreus Sims) u pupčanoj brazdi, iznad mikropile nalazi se vrsta higroskopnog ventila, koga čine palisadne ćelije koje bubre ako je vlaga van semena veća od one u semenu ne dozvoljavajući da ona dopre u seme. Pri obrnutoj situaciji, "ventil" se otvara zbog smanjenja zapremine palisadnih ćelija koje gube vlagu i embrion može da diše. Kod drugih (Albizia lophantha (Willd.) Benth.) mali otvori strofiole (strophiolae) u blizini hiluma zatvorene su plutastim čepovima, koji otpadaju intenzivnim protresanjem, pritiskom ili izlaganjem suvoj toploti (požaru) i omogućuju upijanje vode.

Stepen tvrdoće zavisi od klimatskih uslova za vreme sazrevanja semena (suša) i faze zrelosti (nepropustljivost kao posledica prosušivanja ili stajanja karakteristična je za Symphoricarpus spp.), kao i od individualnih osobina roditeljskih stabala (kod bagrema procenat tvrdih zrna u zavisnosti od roditeljske individue varira od 13-84%). U prirodi fizička dormantnost prevazilazi se prolaskom semena kroz crevni trakt životinja ili požarom, radom mikroorganizama, smenjivanjem niskih i visokih temperatura uz obilje vlage, što sve može da potraje i više godina.

Hemijska (inhibitorna) dormantnost

Karakteristična je za tropske i suptropske vrste. Klijavost je sprečena prisustvom različitih inhibitora kao što su fenolne i apscisinska kiselina. U prirodi pod uticajem intenzivnih padavina ovi inhibitori mogu biti isprani. Kod nekih planinskih eukaliptusa, hemijski inhibitori u semenjači mogu izazvati nenormalno i nisko isklijavanje, koje se može prevazići niskim temperaturama. U ogledima sa Camptonia peregrina, potvrđeno je da je dormantnost izazvana hemijskim inhibitorom (verovatno apscisinska kiselina) u semenjači koji se ne može isprati. Uklanjanjem perikarpa i semenjače postignuta je klijavost od 71%, dok kod kontrole nije bilo klijanja.

Iako se prema Nikolajevoj hemijska dormantnost vezuje samo za vrste sa suvim nepucajućim plodovima kod kojih je prisutan inhibitor u perikarpu, ovde se može svrstati i niz vrsta sa inhibitorima u omotačima ploda (najčešće sočnim) ili semenjači. Tip koji je od više autora nazvan inhibitornom dormantnošću.

Ako se seme vrsta kod kojih nema smetnji u klijanju stavi na filter papir natopljen sokom ili rastvorenom pulpom nekih vrsta bobica, koštunica ili drugih tipova sočnih plodova, često dolazi do blokade klijanja što jasno ukazuje na prisustvo inhibitornih materija u sočnom omotaču. Istraživanja sa semenom salate (Lactuca test) potvrdila su postojanje inhibitornih materija kod Cotoneaster horizontalis Decne., Ilex aquifolium L., Juniperus chinensis 'PFITZERIANA' i Rosa canina L. tako što je pulpa božikovine i kineske kleke potpuno blokirala klijanje, ruže u manjoj, a dunjarice u najmanjoj meri.

Köckemann je 1934. godine otkrio u tkivu ploda paradajza supstancu (kiselinu karakteristika sličnih apscisinskoj) rastvorljivu u etru koja je inhibirala klijanje semena paradajza. Nazvao ju je "Blastokolin". Ovoj supstanci pripisivana je glavna krivica za inhibitornu dormantnost. Kasnije je utvrđena uloga organskih kiselina: sirćetne (najjači inhibitor), oksalne, vinske, jabučne, ćilibarne i limunske (najslabiji inhibitor). U manjoj meri su inhibitori i trščani, grožđani i voćni šećer. U semenjači jele nalazi se terpentin kome se pripisuje inhibitorno dejstvo. Tokom zime on ispari tako da seme nesmetano klija u proleće. Klijanje semena nekih vrsta je blokirano pored ostalog i kumarinom, β-indolsirćetnom kiselinom, nezasićenim laktonima, nekim fermentima... koji mogu da se nađu u endospermu ili semenjači (ali i embrionu), u različitoj koncentraciji.

Niske koncentracije ovih istih supstanci, međutim, mogu da deluju i stimulativno. Danas se smatra da se apscisinskoj kiselini, koja podstiče starenje i opadanje asimilacionih organa i ima presudnu ulogu kod nekih tipova fiziološke dormantnosti, može pripisati glavni uticaj i kod inhibitorne dormantnosti.

Na stepen inhibitorne dormantnosti utiču individualne razlike materinskih biljaka, sadržaj vode u plodovima, stepen zrelosti, način dorade, starost semena...

Mehanička dormantnost

Kod opalog ploda crnog oraha pod uticajem mikroorganizama razlažu se veze koje spajaju polovine perikarpa.

Mehanički otpor rastu embriona stvara čvrsta semenjača perikarp i/ili endosperm, što usporava klijanje. Često tkivo koje pucajuće delove ploda drži zajedno mora da omekša, iako je seme ibibirano. Sama mehanička dormantnost je retka, i obično je udružena sa drugim oblicima. Neke od vrsta koje pokazuju ovaj tip dormantnosti su: Crataegus spp, Elaeagnus angustifolia L., Prunus avium L., Symphoricarpus spp, Juglans nigra L., Olea europaea L., Carya spp. Mehanička dormantnost može da bude smetnja i za isklijavanje semena nekih vrsta četinara. Kod semena Pinus cembra L. (sa kombinovanim egzo i endogenim oblicima dormantnosti) javlja se sprečavanje prodora kiseonika i mehanički otpor klici dok semenjača istovremeno propušta vodu (+ dormantan embrion). Kod lipa rožnat endosperm predstavlja mehaničku smetnju klijanju. Mehanička dormantnost kod nekih vrsta u prirodi se prevazilazi prolaskom kroz crevni trakt životinja koje se hrane plodovima tih vrsta ili radom mikroorganizama u zemljištu po opadanju ploda.

Kombinovana dormantnost

Kod nekih vrsta javljaju se različite kombinacije egzogenih tipova dormantnosti, a seme nekih drugih ima smetnje koje su kombinacija egzogenih i endogenih dormantnosti. Tako glogovi, lipe, tisa, grab, kleke, dren... imaju nepropustljive semenjače i fiziološku dormantnost embriona.

Vidi još

• Dormantnost semena
• Endogena dormantnost semena
• Prevazilaženje dormantnosti semena

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