Spatio-Temporal Distribution of Cell Wall Components in the Placentas, Ovules and Female Gametophytes of Utricularia during Pollination

In most angiosperms, the female gametophyte is hidden in the mother tissues and the pollen tube enters the ovule via a micropylar canal. The mother tissues play an essential role in the pollen tube guidance. However, in Utricularia, the female gametophyte surpasses the entire micropylar canal and extends beyond the limit of the integument. The female gametophyte then invades the placenta and a part of the central cell has direct contact with the ovary chamber. To date, information about the role of the placenta and integument in pollen tube guidance in Utricularia, which have extra-ovular female gametophytes, has been lacking. The aim of this study was to evaluate the role of the placenta, central cell and integument in pollen tube pollen tube guidance in Utricularia nelumbifolia Gardner and Utricularia humboldtii R.H. Schomb. by studying the production of arabinogalactan proteins. It was also determined whether the production of the arabinogalactan proteins is dependent on pollination in Utricularia. In both of the examined species, arabinogalactan proteins (AGPs) were observed in the placenta (epidermis and nutritive tissue), ovule (integument, chalaza), and female gametophyte of both pollinated and unpollinated flowers, which means that the production of AGPs is independent of pollination; however, the production of some AGPs was lower after fertilization. There were some differences in the production of AGPs between the examined species. The occurrence of AGPs in the placental epidermis and nutritive tissue suggests that they function as an obturator. The production of some AGPs in the ovular tissues (nucellus, integument) was independent of the presence of a mature embryo sac.

Female Short Shoot and Ovule Development in Ginkgo biloba L. with Emphasis on Structures Associated with Wind Pollination

The orientation and morphology of the female cone are important for wind pollination in gymnosperms. To examine the role of female reproductive structures associated with wind pollination in Ginkgo biloba, we used scanning electron microscopy and semithin section techniques to observe the development of female short shoots and ovules in G. biloba before and during the pollination period. The ovule differentiation process was divided into six stages: undifferentiated, general stalk differentiation, integument differentiation, nucellus differentiation, collar differentiation, and mature stage. Before the pollination period, the integument tip generated the micropylar canal and the micropyle, while the nucellus tip cells degenerated to form the pollen chamber. During pollination, the micropylar canal surface became smooth, the micropyle split into several pieces and bore a pollination drop, and the pollen chamber directly faced the straight micropylar canal. The leaves and ovules were spirally arranged on the female short shoot, with the ovules erect and the fan-shaped leaves bent outwards and downwards. The ovules of G. biloba have differentiated some special architectural features adapted for pollen capture and transport. Together, these structures constitute a reproductive structural unit that may improve wind pollination efficiency at the female level.

Breaking physical dormancy of Cassia leptophylla and Senna macranthera (Fabaceae: Caesalpinioideae) seeds: water absorption and alternating temperatures

This study analysed the anatomical structure of the seed coats, identified the location of water uptake and evaluated the effects of alternating temperatures and heat treatment on the breaking of physical dormancy of two species of Fabaceae (Caesalpinioideae), Cassia leptophylla and Senna macranthera, from the Atlantic forest of Brazil. The seed coats of both species consisted of a cuticle (extra-hilar region) or remaining funicle region (hilar region), subcuticular layer, palisade layer with lignin, osteosclereids, sclerified parenchyma and white cells. The palisade layer was formed by elongated macrosclereids with a light line of callose. In thermally scarified seeds of C. leptophylla, water entered through the micropylar canal, and in S. macranthera the water entered through the lens. Alternating temperatures that ranged from 15 to 30°C did not break physical dormancy of either species; however, exposure to 50°C broke seed hardcoatedness, allowing the entrance of water in both species.

Ovular secretions in the micropylar canal of larches (Larix kaempferi and L. x eurolepis)

In Larix kaempferi (Lamb.) Carr. and L. × eurolepis A. Henry, a secretion fills the micropylar canal of the ovule during a period of archegonial development that ranges from central cell stage until fertilization. Dissection of the ovuliferous scales caused excess fluid to be exuded from the micropylar canal, forming a drop at the tip of the micropyle. This drop was collected, and its production was quantified. Drop volume was recorded, and the percentage of ovules with drops was counted. The maximum volume of 217 nL far exceeded the volume of the micropyle, which ranged between 18 ± 8 and 28 ± 7 nL (mean ± SD). Removal of drops resulted in further drops being secreted. No drops were produced in ovules that had aborted megagametophyte development.Key words: Larix, micropylar secretions, ovule development.

Prefertilization events in ovules of Pseudotsuga: ovular secretion and its influence on pollen tubes

A natural ovular secretion from the megagametophyte, essential to sexual reproduction, was examined in Pseudotsuga. This secretion began soon after egg formation or about 1 week before fertilization. Secreted fluid filled the micropylar canal. Morphological study showed that the fluid affected (i) dissolution of intine materials; (ii) pollen distortion that may relate to prezygotic selection; and, (iii) induction of pollen tubes. The origin of the secreted fluid is interpreted from an experiment using homogenates. Elongated pollen supplied with a homogenate of the megagametophyte shows immediate movement of starch grains and body cell. Some pollen formed pollen tubes. Homogenates of the nucellus or the integument elicited little or no response. If ovules are dissected before the natural secretion and kept in high relative humidity, the micropylar canal becomes filled with fluid. This fluid is an artifact resulting from dissection, but is also capable of inducing pollen tubes. Keywords: megagametophyte, ovule, pollen tube, Pseudotsuga, secretion.

The pollination mechanism and development after bud burst of cones of Larix laricina

Pollen-cone and seed-cone development, from bud burst to maturity, was investigated on Larix laricina (Du Roi) K. Koch in three young plantations. The pollination mechanism was emphasized. Pollen cones grew rapidly to shed pollen, shrivelled, and remained on the trees for a year or more. Pollen was directed to the ovular regions by the bracts of the seed cones. Pollen adhered among papillae on the larger of two integument extensions. Degeneration of the centre of the papillate integument tip caused a collapse that drew pollen in as the papillate rim grew inward. This ingrowth was joined by that of the smaller integument extension, resulting in a sealed tubular structure that enclosed a dry micropylar canal. Pollen was held by the ingrown plug of degenerated tissue as the nucellus tip expanded into the base of the canal. As this occurred, the ovules, with or without pollination, grew to ultimate seed size, and the initially small ovuliferous scales overgrew the bracts. First bract, then ovuliferous-scale growth was associated with a double-sigmoid form of cone elongation. In mature cones the bracts decreased and the ovuliferous scales (except near the tip) increased in size acropetally. Key words: bract, integument, ovuliferous scale, pollen cone, seed cone, tamarack or eastern larch.