Testa Structure and Histochemistry Related to Water Uptake in Leucaena leucocephala Lam. (De Wit)

The testa structure and histochemistry of Leucaena leucocephalaLam. (De Wit) seed were investigated by bright-field and fluorescencelight microscopy, and scanning electron microscopy. The testaconsisted of several separate layers. Externally there was anon-cellular layer made up of two parts differing in histochemicalcharacteristics: an "outer strip" rich in hydrophilic substances,and a "thicker part" showing the occurrence of phenolics, involvedin water impermeability control. A second underlying thin layerwas formed by the palisade cell caps, joined one to another.This layer, which we called the "cap film ", became metachromaticreddish-blue with Toluidine O pH 4·4 and reacted positivelywith Alcian blue pH 2·5, revealing the presence of polysaccharidehydrophilic material. The palisade cells consisted of two parts,having different structures and histochemical features. Thefurrowed upper part revealed the simultaneous presence of hydrophilicand hydrophobic substances in the furrows and in the ribs respectively.The corresponding periclinal section showed a central daisy-likepattern (made up of hydrophilic material) with a small chainof tiny spots (made up of hydrophobic material) all around it.The inner part of the palisade cells was also furrowed, butshowed only hydrophobic substances of lipidic nature, detectedby Auramine O and Phosphine 3R. The light line was found tobe rich in callose as shown by the strong fluorescence inducedby Aniline Blue. These findings add supporting evidence of thecomplex structure and composition of the Leucaena leucocephalatesta.Copyright 1994, 1999 Academic Press Leucaena leucocephala, testa, structure, histochemistry, water entry     

Structure and Histochemistry of Embryo Envelope Tissues in the Mature Dry Seed and Early Germination of Phacelia tanacetifolia

The embryo envelope tissues in both mature dry seed and duringearly germination of Phacelia tanacetifolia were investigatedby bright-field and fluorescence light microscopy and scanningelectron microscopy. The ruminate seed had an irregularly reticulatesurface owing to the presence of polygonal areas, correspondingto the cells of the seed coat. The raised margins of these cellsjoined at the lobe tips, where radially arranged thickeningsoccurred. The unitegmic seed coat was made up of three distinctlayers: the frayed outer layer, the middle layer with portionsrising outwards to form the radial thickenings, and the innerlayer, the thickness of which was greatest in the micropylarzone. The endosperm tissue had two regions, the micropylar andthe lateral endosperm, which differed in polysaccharide composition,thickness and metachromasy intensity, and presence (in the lateralendosperm) or absence (in the micropylar endosperm) of birefringenceof the cell walls. Moreover, in the micropylar region, wherethe embryo suspensor remnant was found, Ca-oxalate crystalswere scarce or absent. The presence of a partially permeablecuticle covering the seed endosperm was observed. Incubationof seeds in Lucifer Yellow CH indicated that water was ableto penetrate quickly into the seed coat along the pathway formedby the radial thickenings, the raised margins of the polygonalcells and the middle layer. Afterwards, LY-CH readily infiltratedthe apical portions of the seed lobes and then the whole endosperm.Following imbibition, morphological changes were found in themicropylar endosperm, such as the initial digestion of proteinbodies. In addition, both in the seed coat and in the endosperm,a weaker fluorescence, probably due to leaching of polyphenolicsubstances, was observed. Once the seed coat was broken at themicropylar end of the seed, the endosperm cap surrounding theradicle tip had to be punctured by it so that complete germinationcould occur. Weakening and rupture of the micropylar endospermare briefly discussed. Copyright 2000 Annals of Botany Company Phacelia tanacetifolia, seed coat, micropylar endosperm, endosperm cap, early germination, structure, histochemistry     

The Hilar Region in Leucaena leucocephala Lam. (De Wit) Seed: Structure, Histochemistry and the Role of the Lens in Germination

Structural and histochemical features of the hilar region inthe seed of Leucaena leucocephala Lam. (De Wit) were investigatedby bright-field and fluorescence light microscopy and scanningelectron microscopy. In the dry seeds the lens appeared as anelliptical depression prevailing in size over the hilum andmicropyle. In the imbibed seeds, both naturally softened seedand those requiring softening with boiling water, the lens rosegradually to form a cavity between the palisade layer and themesophyll below. This apoplastic pathway was traced using RutheniumRed dye. In place of the hour-glass cells, the hilar regioncontained abnormal cells, called 'white cells' because theyremained unstained by Toluidine Blue O at pH 4·4. Theapparently opposing characteristics of the lens are discussed.It is hypothesized that the lens acts as a valve. By remainingclosed it hinders the entry of water because of the presenceof a high light-line rich in callose and in lipid-like substancesand to the almost total lack of hydrophilic polysaccharide material.Under favourable conditions for germination, the lens opens,allowing water to enter, through a thin palisade layer and withthe probable intervention of the 'white cells'.Copyright 1995,1999 Academic Press Seed germination, Leucaena leucocephala, hilar region, lens, structure, histochemistry