This article reviews the responses of plant roots to elevated rhizosphere cadmium (Cd) concentrations. Cadmium enters plants from the soil solution. It traverses the root through symplasmic or apoplasmic pathways before entering the xylem and being translocated to the shoot. Leaf Cd concentrations in excess of 5-10 μg g(-1) dry matter are toxic to most plants, and plants have evolved mechanisms to limit Cd translocation to the shoot. Cadmium movement through the root symplasm is thought to be restricted by the production of phytochelatins and the sequestration of Cd-chelates in vacuoles. Apoplasmic movement of Cd to the xylem can be restricted by the development of the exodermis, endodermis, and other extracellular barriers. Increasing rhizosphere Cd concentrations increase Cd accumulation in the plant, especially in the root. The presence of Cd in the rhizosphere inhibits root elongation and influences root anatomy. Cadmium concentrations are greater in the root apoplasm than in the root symplasm, and tissue Cd concentrations decrease from peripheral to inner root tissues. This article reviews current knowledge of the proteins involved in the transport of Cd across root cell membranes and its detoxification through sequestration in root vacuoles. It describes the development of apoplastic barriers to Cd movement to the xylem and highlights recent experiments indicating that their maturation is accelerated by high Cd concentrations in their immediate locality. It concludes that accelerated maturation of the endodermis in response to local Cd availability is of functional significance in protecting the shoot from excessive Cd loads.

Root responses to cadmium in the rhizosphere: a review

Multifunctional surface structures of plants: An inspiration for biomimetics

Cohesion-tension transport of water is an energetically efficient way to carry large amounts of water from the roots up to the leaves. However, the cohesion-tension mechanism places the xylem water under negative hydrostatic pressure ( P x ), rendering it susceptible to cavitation. There are conflicts among the structural requirements for minimizing cavitation on the one hand vs maximizing efficiency of transport and construction on the other. Cavitation by freeze-thaw events is triggered by in situ air bubble formation and is much more likely to occur as conduit diameter increases, creating a direct conflict between conducting efficiency and sensitivity to freezing induced xylem failure. Temperate ring-porous trees and vines with wide diameter conduits tend to have a shorter growing season than conifers and diffuse-porous trees with narrow conduits. Cavitation by water stress occurs by air seeding at interconduit pit membranes. Pit membrane structure is at least partially uncoupled from conduit size, leading to a much less pronounced trade-off between conducting efficiency and cavitation by drought than by freezing. Although wider conduits are generally more susceptible to drought-induced cavitation within an organ, across organs or species this trend is very weak. Different trade-offs become apparent at the level of the pit membranes that interconnect neighbouring conduits. Increasing porosity of pit membranes should enhance conductance but also make conduits more susceptible to air seeding. Increasing the size or number of pit membranes would also enhance conductance, but may weaken the strength of the conduit wall against implosion. The need to avoid conduit collapse under negative pressure creates a significant trade-off between cavitation resistance and xylem construction cost, as revealed by relationships between conduit wall strength, wood density and cavitation pressure. Trade-offs involving cavitation resistance may explain the correlations between wood anatomy, cavitation resistance, and the physiological range of negative pressure experienced by species in their native habitats.

https://www.ualberta.ca/~hacke/pdf/Perspectives2001.pdf

Functional and ecological xylem anatomy

Trichome diversity and development

The Influence of the Mineral Composition of the Medium on the Growth of Planktonic Algae: Part I. Methods and Culture Media

DR. TH. HERZOG has produced a monograph of Io8 pages with ninety-three text figures upon the anatomy of the liverworts, and in this space it would appear possible to give a very complete report of progress in this very specialised field. There is, how ever, no mention of mycorhiza, and some important American work is not discussed. The work is divided into three sections. In the first there is a description of the various types of differentiated cell reported in the group, and it is striking how frequently these differentiated elements are to be found in thalloid forms. This section is followed by two further sections, dealing with the anatomy of gametophyte and sporophyte respectively, in which each group of the liverworts is passed in review in turn.Handbuch der Pflanzenanatomie.Prof.K.Linsbauer. Abteilung 2, Teil 2: Bryophyten. Band VII/1: Anatomie der Lebermoose. Von Prof. Dr. Th. Herzog. Pp. iv + 112. (Berlin: Gebrüder Borntraeger, 1925.) 8.70 gold marks.

Karl Schnarf

Papers

Handbuch der Pflanzenanatomie

Vergleichende Embryologie der Angiospermen

Embryologie der Angiospermen

Embryologie der Gymnospermen

Anatomie der Gymnospermen-Samen