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Contents  Summary 647 Introduction 648 Overview of higher plant plasma membrane  anion channels 648 Anion channels in higher plant roots 655 Conclusions and prospects 661  Acknowledgements 662  References 662 Summary Recent years have seen considerable progress in identifying anion channel activities in higher plant cells. This review outlines the functional properties of plasma membrane anion channels in plant cells and discusses their likely roles in root function. Plant anion channels can be grouped according to their voltage dependence and kinetics: (1) depolarization‐activated anion channels which mediate either anion efflux (R and S types) or anion influx (outwardly rectifying type); (2) hyperpolarization‐activated anion channels which mediate anion efflux, and (3) anion channels activated by light or membrane stretch. These types of anion channel are apparent in root cells where they may function in anion homeostasis, membrane stabilization, osmoregulation, boron tolerance and regulation of passive salt loading into the xylem vessels. In addition, roots possess anion channels exhibiting unique properties which are consistent with them having specialized functions in root physiology. Most notable are the organic anion selective channels, which are regulated by extracellular Al3+ or the phosphate status of the plant. Finally, although the molecular identities of plant anion channels remain elusive, the diverse electrophysiological properties of plant anion channels suggest that large and diverse multigene families probably encode these channels.

The results suggest that different mechanisms of chromosomal radiosensitivity operate in G2 and G0 cells and that, in general, each chromosomally radiosensitive patient is defective in only one such mechanism, possibly via mutation (or polymorphism) of a single gene. Such mutations may confer cancer predisposition, of low penetrance, in a substantial proportion of patients.

In vivo bromodeoxyuridine (BrdUrd) labelling of the human large bowel was performed and a detailed histochemical localisation of label in' sections of crypts was undertaken using a monoclonal antibody to BrdUrd containing DNA. Flow cytometric studies on extracted nuclei were also performed (data presented elsewhere). The average crypt in the human large bowel (excluding the rectum) was 82 cells in height and 41 cells in circumference, with a total of about 2000 cells (assuming a topographical correction factor of 0.6). Ten per cent ofthe cells were replicating their DNA -that is, were in the S phase of the cell cycleand 0-4% were in mitosis. The median position for the labelling index versus cell position frequency plot is at the 20th cell position -at a quarter of the crypt height. The lower and upper limits of the cell proliferation are given by the 5th and 95th percentiles at cell positions 4 and 43 respectively. The peak labelling index is about 30% and it occurs at cell position 15. The labelling index at the crypt base, the probable stem cell zone, is about 14%, suggesting that these cells have a longer cell cycle.Taking a value of 8*6 hours for the duration of the S phase (deduced from the flow cytometric data) and assuming a growth fraction of 1 0 for the mid-crypt, these data provide an estimate of about 30 hours for the cell cycle time. The rectal crypts are about the same size but contain about 30% fewer S phase cells. The data also yielded a per cent BrdUrd labelled mitosis curve.