Germination of stress-tolerant Eucalyptus pollen.

Earlier reports have indicated that the pollen of Eucalyptus is mechanically robust and unusually resistant to the osmotic stress imposed by immersion in water. We have investigated some of the features of the germination mechanism in the pollen of E. rhodantha with a view to clarifying the role of pollen-wall specializations in determining this resistance. Cultured in vitro, the pollen showed erratic germination, with a scatter of germination times up to 24 h. This was associated with variation between individual grains in the rate of hydration and dispersal of the pectins of the oncus, the thickened outer component of the intine present at each aperture. The oncus is itself differentiated, with a refractive outer layer lying within a sporopollenin operculum and itself overlying the protein-bearing layer of the intine. The outer layer, interpreted as a compacted pectin, undergoes only slow dissolution in aqueous media after the lifting of the operculum, and it is this that apparently protects the grain from the effects of short-term osmotic stress. The rate of dissolution varies between grains, possibly as a consequence of minor differences in developmental rate in the final stages of differentiation in the anther, and this contributes to the wider scatter of germination times. The dehydrated pollen gave one-third of the potential germination after 24 h exposure to 60 degrees C, and a small proportion survived 24 h at 70 degrees C. This degree of heat tolerance must primarily reflect properties of the protoplast of the vegetative cell, not examined in the present study; but the wall specializations may well provide a guard against extreme desiccation, and it is noteworthy that the function of the germination mechanism is not prejudiced by exposure to high temperatures.

Surfaces and secretions in the pollen-stigma interaction: a brief review.

Interactions between pollen and pistil are key events in angiosperm reproduction, and for this reason they have been the target of an increasing volume of research in recent years. The regulation of fertilization has proved to be remarkably complex, involving controls of various kinds imposed at several different levels. Pollen hydration, governance of germination and stigma penetration, guidance and nutrition of the pollen tube in the style-as well as various, still enigmatic, direct interactions between male and female gametophytes - may all be concerned, individually or in combination, in determining whether fertilization will or will not be effected. The controls differ in degree of efficiency and specificity. Depending largely on the taxonomic remoteness of the parents, interspecific incompatibility may result simply from lack of physiological co-adaptation between the partners, or from rejection mechanisms of a more specific kind. Intraspecific incompatibility, although generally having a deceptively simple genetic basis, has proved to be surprisingly diverse in its physiological manifestations. The paper will present a brief conspectus of the present status of research in the field, and review some recent interpretations of the specific recognition events that may be involved in some of the interactions.

Wide hybridization: pollination of Zea mays L. by Sorghum bicolor (L.) Moench.

Foreign pollen tubes in the stigma of Zea mays can be prevented from reaching the ovary cavity by the unusual length of the pollen tube pathway. A simple and rapid procedure is described for overcoming this difficulty by pollinating the basal parts of the stigmas without removing the ensheathing bracts ("husks"). The method maintains high humidity in the vicinity of the ovaries, and by conserving photosynthetic tissues probably also ensures a more normal O2 /CO2 balance in the neighbourhood of the stigmas than do bagging procedures. It is shown that Sorghum pollen tubes readily reach the ovary after pollination by the method. Their presence induces some of the characteristic post-pollination effects caused by Zea pollen tubes, but they frequently also stimulate premature enlargement of the nucellus and lysis of nucellar cells. Although Sorghum tubes have been traced across the inner ovary wall, they have not been seen to enter the micropyle, and hybrid embryos have not yet been obtained.

The evaluation of pollen quality, and a further appraisal of the fluorochromatic (FCR) test procedure.

Methods currently available for evaluating pollen quality in vitro include, (a) tests of germinability; (b) tests of the stainability of the vegetative cell contents; (c) tests for enzyme activity, and (d) the fluorochromatic procedure (FCR), which tests principally the integrity of the plasmalemma of the vegetative cell. Using germinability in vitro as a standard, a comparison has been made between histochemical methods of classes (b), (c) and (d) in application to various pollens, immature, mature, and treated in ways known to affect viability and membrane state. Predictably, the lowest correlation was obtained with tests of stainability. The highest was given by the FCR, which generally provided an excellent guide to potential germinability. The FCR procedure is subject to various limitations, however, (a) A high correlation between FCR and germinability can only be expected when mature, ripe pollen is used; with immature pollen, the FCR will predict excessively high potential germinability. (b) The FCR may also predict a higher potential level of pollen function than in vitro germinability when the germination medium is sub-optimal. In this situation, however, it will generally give a better guide to fertilising capacity, (c) The FCR is not a test of pollen viability. Like germinability in vitro, it can yield a negative score with pollen which is nevertheless capable of functioning. For example, false negatives will be obtained with some species if the pollen is not properly pre-conditioned by rehydration before testing, an important point in monitoring stored pollen. The paper includes a brief discussion of the rationale of pollen testing.

Pollen-wall proteins: Exine-held fractions associated with the incompatibility response in cruciferae.

During maturation of the pollen in species ofIberis (Cruciferae) materials from the anther tapetum are transferred to cavities of the exine. These sporophytically-derived materials are responsible for the rejection response induced in the stigmatic papillae within 4-6 h. following incompatible pollination, a response which leads to the occlusion of the stigma papillae by the polysaccharide, callose. The rejection reaction can be induced with agar or agarose gels into which pollen-wall materials have been allowed to diffuse, and also with isolated fragments of the tapetum itself taken from anthers of incompatible plants before transfer of its contents to the pollen. Partial fractionation of the exine-held materials using thin-layer gel filtration suggests that the incompatibility reactions are mediated by one or more "recognition" proteins or glycoproteins in a group with molecular weights in the range 10,000-25,000 daltons.The rejection response has been found to be a rapid and sensitive assay for incompatibility in several Cruciferae.

A cytochemical study of the leaf-gland enzymes of insectivorous plants of the genus Pinguicula.

Cytochemical methods have been used to study the distribution of acid phosphatase, esterase, ribonuclease, amylase and protease activity in the stimulated and unstimulated leaf glands of Pinguicula grandiflora, P. vulgaris, P. lusitanica, and P. caudata. Two gland types are present, stalked and sessile. The stalked glands bear a muco-polysaccharide secretion droplet, and are concerned with capture of the prey; the sessile glands are specialised for digestion. In unstimulated glands of both classes, acid phosphatase, esterase and ribonuclease activity is associated with the anticlinal walls of the head cells, which have a characteristic spongy inner surface, comparable with that of transfer cells. Acid phosphatase and esterase activity was also detected in the vacuoles of the head cells of the sessile glands. Substrate film tests showed that amylase is readily released from the stalked glands but not from the sessile ones, while in contrast proteolytic activity is mainly associated with the sessile glands.On stimulation by suitable nitrogenous materials, the glands begin to sectete fluid onto the leaf surface within 1 hr. During the process the enzymes held in the spongy walls are discharged, and activity is also lost from the intracellular sites in the sessile glands.Digestion on the leaf surface and resorption of the products has been followed autoradiographically after feeding of (14)C-labelled protein. Within 2 hr, digestion products enter the leaf, and move towards the margin in the vascular system. Movement out of the leaf begins within 12 hr. Microautoradiographs showed a concentration of products around the bases of the sessile glands and in the cells of the gland head, showing that these glands are involved in resorption as well as secretion.A possible mechanism of gland function is discussed.

Scanning electron microscopy of fresh leaves of pinguicula.

Moist surfaces of leaves of Pinguicula grandiflora Lamck. can be observed directly by scanning electron microscopy, without metal coating. Samples dry out rapidly in the instrument, but during the first few minutes images can be obtained which must represent the natural state of the leaf surface.