Changes of dictyosome ultrastructure during the cell cycle in onion root meristematic cells. A morphometric and stereological study

Summary Dictyosome ultrastructure changes during the cell cycle in onion root meristematic cells. Changes were mainly related to cisternae and intercisternal spaces morphology. Taking each dictyosome to be composed of three different regions (CIS, medial, and TRANS), several quantitative changes were detected in some of the compartments. Many of the planimetric parameters evaluated showed higher values in medical cisternae, while CIS and TRANS remained nearly constant. We have also found an increased activity of dictyosomes, as indicated by increase in the volume fraction of vesicular attached structures. This reaches maximum values at ana-telophase in coincidence with the onset and progression of cytokinesis.Abbreviations A anaphase - Ac mean area occupied by cisternae per stack section - C CIS - CCS cell cycle stage - D_A mean total dictyosome area - I_SA mean area occupied by intercisternal spaces per stack section - M metaphase - N mean number of cisternae per stack - P prophase - S.E. standard error - T telophase - T TRANS - V_v volume density     

Gradient and zonal analysis of understorey suppression by Eucalyptus wandoo

Gradients, zones and boundaries can be recognized on the basis of biotic, abiotic, field and experimentally-determined attributes along transects through isolated patches of Eucalyptus wandoo trees. The patch zone (corresponding to the tree canopy) could be defined on numerous environmental grounds. The suppression zone (with few shrubs) which embraced the patch zone, could only be distinguished from the surrounding scrub zone (with a dense shrub cover) on vegetation grounds. Site relationships, including identification of boundaries, were analyzed with the aid of a range-standardized dissimilarity measure and metric multidimensional scaling. Differences in availability of nutrients and phytotoxic substances from wandoo leaves, roots and litter had some effects on growth in pot trials but it is concluded that their role in explaining understorey suppression in the field is negligible. Competition for water at depth between the extensive lateral root system of wandoo trees and the roots of adult shrubs is shown to be the most likely explanation for genesis of the suppression zone and location of its boundary. Factors affecting seedling establishment, mainly surface water availability, are more concerned with maintenance of the suppression zone once it is formed.     

Membrane transport and cytokinin action in root hairs of Medicago sativa

In an effort to develop a cellular model for studying cytokinin action in higher plants, we investigated the effect of cytokinins on growth and membrane transport in root hairs of alfalfa (Medicago sativa␣L.) seedlings. Alfalfa seedlings grown for 24 h in the presence of cytokinins showed increased root hair length and formation of root hairs close to the root cap. Increased growth of root hairs was observed within 10 min of cytokinin application and was accompanied by a hyperpolarization of the plasma membrane. The ion-transport inhibitor diisothiocyanostilbene-2,2′-disulfonic acid (DIDS) blocked both cytokinin-induced root hair growth and hyperpolarization but did not, by itself, alter growth or membrane potential. Finally, hyperpolarization was induced by extracellular cytokinin but not by injection of cytokinin into the cytoplasm. These findings show that root hairs undergo several rapid responses to cytokinin, including changes in membrane transport and growth. We conclude that multiple cytokinin response in root hairs may be mediated by events involving perception and ion transport at the plasma membrane. Received: 18 July 1996 / Accepted: 1 October 1996     

Ultrastructural study of the maize epidermal root surface. I. Preservation and extent of the mucilage layer

Summary This study clarifies the relationships between the root mucilage and the epidermal cell-wall in axenically grown maize roots. Mucilage appears to overlay the epidermal cell-wall which has two distinct layers. This mucilage is well preserved after cationized ferritin fixation and PATAg staining. It extends all over the entire root epidermis.     

Comparative root system structure of post-fire Pinus halepensis Mill. and Cistus monspeliensis L saplings.

To describe root system topology of Pinus halepensis and Cistus monspeliensis saplings co-inhabiting natural post-fire sites, 55 P. halepensis and 26 C. monspeliensis saplings were extracted by the total excavation method from a burnt pine stand. Seedlings were individually labelled when emerging after fire and extracted three years later, at the sapling phase. In order to evaluate the effect of inter-specific competition of C. monspeliensis on P. halepensis root system, a stratified sampling was carried out according to density and height of the saplings. Topological parameters considered in the analysis were magnitude, total external pathlength, and altitude of the root systems. Weight and length of roots were also measured in order to estimate the specific root length, an index commonly used in morphological studies. Results clearly evidenced greater variability in root system topology of P. halepensis than C. monspeliensis saplings. Herringbone architecture (i.e., the most ordered pattern possible, with branching confined to the main axis) characterised small pine saplings, regardless of competition from C. monspeliensis, which changed to random branching in large saplings. In medium sized saplings, the root system was affected by inter-specific competition, which delayed changes in root branching. In contrast, C. monspeliensis invariably adopted randomly branched architecture, regardless of intra-specific competition. It is concluded that such different topological patterns make C. monspeliensis more competitive during the early stages of post-fire succession, because its root system is much more transport-efficient in the nutrient-rich environment. Those pines which finally branch roots by random pattern will reach higher stem height and magnitude, a factor which allows them to successfully compete with C. monspeliensis for soil nutrients and water. The morphological analysis showed a significant increase in the specific root length with competition, both in P. halepensis and C. monspeliensis saplings, which could be interpreted as a consequence of the reduction of root diameter in response to nutrient depletion.     

Effects of applying stem-shortening plant growth regulators to leaves on root elongation by seedlings of wheat, oat and barley: mediation by ethylene

Several plant growth regulators (PGRs) commonly used in practicalfarming to restrict shoot height and control lodging were examined for theirimpact on root growth in naturally short or tall cultivars of barley (cvs.Kymppi and Saana), oat (cvs. Veli and Pal), and wheat (cvs. Mahti and Tjalve).The possible involvement of ethylene in the responses was also examined. Shootswere sprayed at the two-leaf stage with the gibberellin biosynthesis inhibitorsCycocel (chlormequat chloride) (CCC) or Moddus (Trinexapac-ethyl) (TE), or withthe ethylene-releasing agent Cerone (ethephon) (ETH) at 0, 0.1, 1, 10 or 50times the recommended agricultural rate (RR). Root elongation and ethyleneproduction by roots or shoots were unaffected by CCC at all application ratesorby TE at ×0.1 or ×1.0 RR. At ×10 and ×50 RR, TE wasinhibitory to root extension but did not increase ethylene biosynthesis bytheseroots or the shoots. ETH at ×0.1 or ×1.0 RR did not affect rootextension or ethylene production in roots or shoots. At all higher rates ofapplication ETH stimulated ethylene production strongly in shoots and roots ofall three species, while root elongation was retarded severely in barley,moderately in oat and only slightly in wheat. These differences in elongationresponse are attributed to differences in sensitivity to ethylene released byethephon. Accordingly, root elongation in wheat was only slightly affected whenethylene gas was supplied at concentrations up to 100 ppm for 3d. In contrast, root elongation in barley was strongly inhibitedbyethylene, with oat demonstrating an intermediate responsiveness.     

水培对羽裂蔓绿绒解剖结构的影响

采用石蜡切片法,对天南星科植物羽裂蔓绿绒(Philodendron pitfieri Engl.)水培和土培根的解剖特征进行观察,探讨蔓绿绒适应水生环境的形态学特点,并比较了水培根与土培根的形态解剖学差异.结果表明:水培后根冠不明显,淀粉体较少,多糖类物质较少而土培根根冠体积较大,淀粉体较多,多糖类物质多,土培根PAS反应强烈;水培根表皮厚度比土培根小,水培根根毛少或退化消失;水培根皮层薄壁组织发达,细胞大型,壁薄,细胞间隙中含有溶生性通气组织,土培根皮层未见通气组织.