Dynamics of cell production and loss
Mechanical impedance causes a decrease in the root elongation rate, an effect which persists for several days, even after the impedance is removed (Goss & Russell, 1980; Croser et al., 2000). This slowing of root elongation is associated with both a decrease in final cell length and a slower rate at which new cells are produced and added to the cell files that comprise the meristem (Croser et al., 1999). In our experiments, compaction tended to result in a decrease in cell production in the root meristem. By contrast, there was an opposite tendency towards an increased rate of cell division in the cap meristem of compacted roots. These findings support results of Brigham et al. (1998) who suggested that the cap and main root meristems operate largely independently.
The duration of the mitotic cell cycle in the maize root cap meristem has been estimated as between 12 h (by thymidine labelling and by accumulation of colchicine-metaphases; Clowes, 1961) and 14 h for the columella region of the meristem, and 22.5 h for the lateral region (by thymidine labelling; Barlow & Macdonald, 1973). In our experiments, the relatively greater cell production rate from the lateral part of the cap meristem compared with the tip region (28 h and 40 h, respectively) may have resulted from the abrasive nature of the sand compared with the less abrasive sphagnum moss or hydroponics used by Clowes (1961) and Barlow & Macdonald (1973), respectively. However, the cell production rate from the lateral cap meristem region (compared with the central columella zone of the cap) is inherently greater because of the larger number of proliferative cells in this zone.
Steady-state cell kinetics in the root cap predicts that, in a given period of time, the number of cells released from the periphery of the cap should be equal to the number of cells produced by the cap meristem. The total number of cap cells should therefore remain constant throughout that period. However, there was some diminution of the size of the cap and the cell production was not in steady state (Table 3). The accumulated cells correspond to the release of approx. three layers of cells per day from the cap periphery in the loose treatment, and about four layers per day in the compact treatment. These estimates are similar to those of Barlow (1977) for the rate of cap cell loss from maize roots grown in solution culture.
The detached cell population, including broken cells, was estimated as 4960 cells in compacted sand and 3540 cells in loose sand (Table 3). These cells surrounding the cap would be sufficient to cover 103% of the area of the root cap in the compacted sand treatment, but only 11% in the loose sand treatment (see Discussion in Iijima et al., 2000). This suggests that in compacted sand the whole of the root cap is covered in a layer of detached cells. The surface area of the elongation zone of the root will be more sparsely covered with detached cells; 20% of the 1 mm of cylindrical surface area of this zone of the root will be covered in the compacted sand treatment compared with 4% in the loose sand treatment. Thus, the maximum lubricating action of the detached cells will be at the root cap itself, where the stress is greatest (Kirby & Bengough, 2002). This finding would accord with the observations of Bengough & McKenzie (1997) and those of Read et al. (1999) who showed that detached cap cells played some role in modulating the viscosity of maize root cap mucilage, allowing root tips to glide with less effort over a solid supporting medium.
Cap renewal times have been estimated in the seminal roots of maize seedlings grown either in hydroponics or in damp sphagnum moss (Clowes, 1971; Barlow, 1974). In later work (Barlow, 1978), it was shown that cap cells require about 7 d to reach the tip of the columella following their displacement from cap meristem, whereas they required 2–3 d to reach the flank of the cap. In the present experiment, the time taken for all of the nonmeristematic cells in the root cap to be replaced by new cells from the cap meristem was estimated to be 4.4 d in the loose sand treatment and 2.9 d in the compacted sand treatment (Table 3). These estimates rely on the assumption that cap cell production balances cell loss and that caps therefore maintain a constant size. This would imply that cell production and cell loss are coordinated events, as opposed to being regulated independently. Comparison of cell production rates of 2010 and 1570 cells d−1 in compacted and loose sand treatments, with the respective observed losses of 2950 and 1970 cells d−1, suggests that cell release from the cap periphery proceeds faster than cell production from the cap meristem. This accords with earlier results (Barlow, 1977) where meristematic reduplication was slower than the rate of shrinkage of the cap due to cell loss.
The experiments of Brigham et al. (1998) suggest that removal of border cells from the outside of the root cap might stimulate cell division in the cap meristem. They found, for example, that washing cells from the periphery of the cap of pea roots rapidly increased the fraction of cells undergoing mitosis in the cap meristem. In our experiment, the rate of cell production by the cap meristem may have been similarly disturbed by transfer to colchicine solution (a step whereby cells could potentially be washed from the cap), in which case it would not be possible to estimate rates of cap renewal as the values obtained from solution culture might not be comparable with those occurring in a sandy medium. However, the linear nature of the plots in Fig. 3 suggests that entry into mitosis was constant during the course of the colchicine treatment, and that no transient increase of cell production occurred as a result of the exposure to the solution. The slight depression in the percentage of prophase cells during the colchicine treatment suggest that, if there was any effect on cell production, it is likely to have been on the rate of entry into mitosis and, hence, cell production rates obtained by this method could have been slightly underestimated. With regard to the results of Brigham et al. (1998), it is possible that the observed elevation of mitotic index following the washing-off of released peripheral cells represented a transient inhibition of cell division, with cells accumulating in prophase and metaphase before completing anaphase, rather than being an indication of accelerated entry into mitosis. In this respect, the washing process temporarily mimicked the effect of the colchicine in our experiment.
In summary, mechanical impedance does not persistently decrease the production of new cells in the root cap meristem, even slightly enhanced that in the lateral region. The rate of release of border cells may be partly related to the rate of cell production by the cap meristem. An increased rate of release of cells from the periphery of the cap would assist root penetration into compacted soil by decreasing the frictional resistance to elongation growth.