Induction of air embolism in xylem conduits of pre-defined diameter.

A new method is presented that enables the induction of embolisms in a fraction of all xylem vessels, based on diameter, at one cut end of a stem segment. The method is based on the different capillary characteristic of xylem vessels of different cross-sectional size. To verify the method, air embolisms were induced in cut xylem vessels of chrysanthemum (Dendranthemaxgrandiflorum Tzvelev cv. Cassa) stem segments at different xylem tensions and compared with the distribution of gas-filled vessels as visualized by cryo-scanning electron microscopy (Cryo-SEM). At -6 kPa xylem pressure, air-entrance was only induced in large diameter vessels (>30 microm), while at -24 kPa embolisms were induced in almost all xylem vessels (>10 microm). Although the principle of the embolization method worked well, smaller diameter vessels were observed to be embolized than was expected according to the calculations. The role of cross-sectional shape and contact angle between xylem sap and vessel wall at the menisci are discussed. After correction for the observed (diameter independent) deviation from circularity of the cross-sectional vessel shape the contact angle was calculated to be approximately 55 degrees. Hydraulic resistance (Rh) measurements before and after embolization showed that the effect of embolizing only large diameter cut xylem vessels had only a small influence on overall Rh of a stem segment. Embolizing all cut xylem vessels at one cut end almost trebled overall Rh. The difference was discussed in the light of the networking capacity of the xylem system.

Xylem hydraulic conductivity related to conduit dimensions along chrysanthemum stems.

The stem xylem conduit dimensions and hydraulic conductivity of chrysanthemum plants (Dendranthema x grandiflorum Tzvelev cv. Cassa) were analysed and quantified. Simple exponential relations describe conduit length distribution, height dependency of conduit length distribution, and height dependency of stem hydraulic conductivity. These mathematical descriptions can be used to model the xylem water transport system. Within a chrysanthemum stem of 1.0 m, the conduit half-length (the length within which 50% of the conduits have their end) was 0.029 m at soil surface and decreased by half at a height of 0.6 m. With each 0.34 m increase in height up the stem, the hydraulic conductivity decreased by 50%. The resistance calculated from conduit lumen characteristics was 70% of the measured resistance. The remaining unexplained part of the hydraulic resistance is at least partly caused by inter-conduit connections.

Molecular characterization of the CER1 gene of arabidopsis involved in epicuticular wax biosynthesis and pollen fertility.

The aerial parts of plants are coated with an epicuticular wax layer, which is important as a first line of defense against external influences. In Arabidopsis, the ECERIFERUM (CER) genes effect different steps of the wax biosynthesis pathway. In this article, we describe the isolation of the CER1 gene, which encodes a novel protein involved in the conversion of long chain aldehydes to alkanes, a key step in was biosynthesis. CER1 was cloned after gene tagging with the heterologous maize transposable element system Enhancer-Inhibitor, also known as Suppressor-mutator. cer1 mutants display glossy green stems and fruits and are conditionally male sterile. The similarity of the CER1 protein with a group of integral membrane enzymes, which process highly hydrophobic molecules, points to a function of the CER1 protein as a decarbonylase.

A Seed Shape Mutant of Arabidopsis That Is Affected in Integument Development.

A seed shape mutant of Arabidopsis was isolated from an ethyl methanesulfonate-treated population. Genetic analysis revealed that the heart-shaped phenotype was maternally inherited, showing that this is a testa mutant. This indicated the importance of the testa for the determination of the seed shape. This recessive aberrant testa shape (ats) gene was located at position 59.0 on chromosome 5. A comparison was made between ovules and developing and mature seeds of the wild type and of the mutant using light and scanning electron microscopy. We showed that the mutant seed shape is determined during the first few days after fertilization, when the embryo occupies only a very small part of the seed. The integuments of ats ovules consisted of only three rather than five cell layers. In double mutants, the effect of ats was additive to other testa mutations, such as transparent testa, glabra (ttg), glabrous2 (gl2), and apetala2 (ap2). The ats mutation resulted in a reduced dormancy, which was maternally inherited. This effect of a testa mutation on germination was also seen in ttg seeds, in which the outer layer of the testa was disturbed. This indicated the importance of the testa as a factor in determining dormancy in Arabidopsis.

THE PROCESSES OF ANTHER DEHISCENCE AND POLLEN DISPERSAL: I. THE OPENING MECHANISM OF LONGITUDINALLY DEHISCING ANTHERS.

The process of anther opening was followed in different species by means of light and electron microscopical techniques, stereo microscopical observation inside living tissues and micromanipulation. From these data and a review of the mainly very old literature on this subject, the opening process is outlined. It consists of the following stages: (1) Expansion of the epidermis and endothecium cells and deposition of U-shaped wall thickenings inside the latter. (2) Enzymatic opening of the septum between two locules. (3) Mechanical rupture of the tapetum. (4) Mechanical opening of the stomium by the centripetal force of the highly turgescent epidermis and endothecium. (5) Outward bending of the locule walls by the centrifugal force of the dehydrating epidermis and endothecium. Steps (4) and (5) clearly reflect the mechanical functions of the two latter tissues. The roles of the other anther tissues and the filament during the earlier stages are discussed.

THE PROCESSES OF ANTHER DEHISCENCE AND POLLEN DISPERSAL: II. THE FORMATION AND THE TRANSFER MECHANISM OF POLLENKITT, CELL-WALL DEVELOPMENT OF THE LOCULUS TISSUES AND A FUNCTION OF ORBICULES IN POLLEN DISPERSAL.

The development of the locule of Gasteria verrucosa (Mill.) H. Duval and Lilium hybrida cv. Enchantment, especially the border between the sporophyte and the gametophyte, is investigated by means of light and electron microscopy, histochemistry and micromanipulation and related to pollen dispersal. The pecto-cellulosic cell walls of both the middle layer and the tapetum and a part of the endothecium disappear, and the tapetum cells are covered with sporopollenin-containing tapetal membranes and orbicules. The cell contents of the tapetum turn into the hydrophobic pollenkitt. In the locule, similar changes are observed as the pecto-cellulosic and callose walls of the meiocytes disappear and the future pollen grains are covered with the sporopollenin-containing exine. Due to their non-wettability, these hydrophobic substances on both sides of the sporophyte-gametophyte surface are important in pollen dispersal. The transfer of pollenkitt from the tapetum to the locule appears to be due to capillary forces in the locule after the continuous expansion of the pollen, rather than to active flow or to direct attraction by the pollen grains.

Structure and function of the microtubular cytoskeleton during pollen development in Gasteria verrucosa (Mill.) H. Duval.

In a study of pollen development in Gasteria verrucosa, the changes in the spatial organization of microtubules were related to the processes of cell division, nuclear movement and cytomorphogenesis. Sections of polyethylene-glycol-embedded anthers of G. verrucosa were processed immunocytochemically to record the structure and succession of fluorescently labeled microtubular configurations. Using microspectrophotometric measurements the relative quantity of tubulin in microtubules per unit of cytoplasm was determined. Cell dimensions and nuclear positions were measured to relate changes in cell shape and nuclear movements to microtubular configurations. Microtubules were detected in the different cells during microsporogenesis and microgametogenesis. In microspore mother cells which are approximately isodiametric at interphase, microtubules were predominantly arranged in a criss-cross pattern. The microtubules probably function as a flexible cytoskeleton which sustains the integrity of the cytoplasm. Bundles of microtubules were observed in the microspores, in the generative cells and during nuclear division, where they functioned in establishing and maintaining cell and spindle shapes. Microtubules radiating from nuclear membranes appeared to fix the nucleus in position. In prophase of meiosis and after microspore mitosis, periods a high fluorescence intensity were distinguished indicating a variation in the quantity of microtubules.