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Summary. Using energy-dispersive X-ray microanalysis of mature, unpollinated wheat ovaries fixed by freeze-substitution, we show here that the synergid cells store a relatively high concentration of calcium. Based on our results and other indirect evidence, we suggest that supraoptimal levels of calcium in synergids may regulate: (1) correct orientation of the pollen tube, by forming a calcium gradient in the vicinity of the synergids, and (2) arrest and rupture of the pollen tube to release the sperm near the egg.

1991. Visualization, adhesiveness, and cytochemistry of the extracellular matrix produced by urediniospore germ tubes of Puccinia sorghi. Can. J. Bot. 69: 2044-2054. Adherence of germinating urediniospores of the common maize rust fungus (Puccinia sorghi Schw.) to substrata was studied by ultrastructural and cytochemical examination of extracellular matrix produced by germ tubes in conjunction with measurements of adhesion to plastic and glass surfaces. Copious amounts of extracellular matrix on germ tubes could consistently be visualized by scanning and transmission electron microscopy only when (i) a cationic detergent (cetylpyridinium chloride, polydiallyldimethylammonium chloride) or a cationic stain (ruthenium red, alcian blue, cuprolinic blue) was added to the fixation solutions, (ii) germ tubes were fixed by rapid-freezing and freeze-substitution and observed with a scanning electron microscope, or when (iii) germ tubes were observed in a frozen-hydrated state by low-temperature scanning electron microscopy. Incubation of germinated spores with dilute alkalies (NaOH, KOH), pronase E (nonspecific protease), and laminarinase ((3-1,3 (1,3; 1,4-glucanase) removed the extracellular matrix and detached germ tubes from surfaces. Treatments with water, dilute acids, ionic and neutral detergents, organic solvents, hydrocarbons, and several polysaccharide-degrading enzymes did not remove the extracellular matrix and also did not detach germ tubes. These results, together with staining patterns obtained with lectins and other polysaccharide-specific reagents, indicate that the extracellular matrix is composed mainly of glycoproteins rich in acidic amino acids and (3-1,3-glucan polymers, and that it is probably responsible for the adhesion of the rust germ tubes to the host leaf surfaces. matrix produced by urediniospore germ tubes of Puccinia sorghi. Can. J. Bot. 69 : 2044-2054. Les auteurs ont CtudiC I'adhCsion aux substrats des urCdiniospores en germination du champignon de la rouille commune du mais (Puccinia sorghi Schw.); ils ont examin6 les ultrastructures et la cytochimie de la matrice extracellulaire produite par les tubes germinatifs, tout en mesurant leur adhCsion 21 des surfaces de plastique et de verre. Ils ont pu continuellement observer de grandes quantitCs de matrice extracellulaire sur les tubes germinatifs en utilisant la microscopie Clectronique par balayage et par transmission, uniquement quand (i) un dCtegent cationique (chlorure de ~Ctylpyridinium, chlorure de polydiallydimCthylammonium) ou encore une teinture cationique (rouge de r u t h h u m , bleu d'alcian, bleu cuprolinique) est ajoutCe aux solutions de fixation, (ii) les tubes germinatifs sont fixCs par congClation rapide et cryo-substitution avant d'Ctre observes par microscopie Clectronique par balayage, ou quand (iii) les tubes germinatifs sont observes 2I 1'Ctat hydratC et conge16 en microscopie Clectronique balayage B basse tempCrature. Une incubation des spores germCes en prtsence de bases diluCes (NaOH, KOH), de pronase E (protCase n...

Sister synergids of wheat were examined for post-pollination calcium distribution using freeze-substitution fixation, scanning electron microscopy, and energy-dispersive X-ray microanalysis. At 15-30 min postpollination (before pollen-tube penetration and discharge), the sister synergids were similar in size and in elemental concentrations. Both synergids measured about the same as the egg and contained relatively high concentrations of calcium, potassium, and magnesium. The sister synergids remained identical even in an aberrant case where a pollen tube had penetrated, but not discharged into a synergid. After 30 rain post-pollination (after pollen-tube discharge), typically one of the sister synergids was reduced in size and devoid of high concentrations of elements. The other synergid initially remained unaltered and only later underwent slight size reduction; however, it retained high concentrations of elements. The results indicate that high concentrations of calcium, potassium, and magnesium are rapidly lost from a synergid following pollen-tube discharge. A hypothesis is proposed to explain the regulation of entry by only a single pollen tube into an embryo sac.