39
and Sinclair, 1985). The nodule gas permeability can then be calculated once the total nodule surface area has been estimated such that
P = k / Narea (4.3)
where Narea is the total nodule surface area (mm 2). The nodule surface area can be estimated by the method of Weisz et al. (1985) using measurements of the cross-sectional dimensions of the nodules.
Use of the above approach to determine P is, however, dependent on the use of the acetylene-reduction assay which as described in Chapter III may inhibit nitrogenase activity and thus result in substantial under estimations of Vmax (Minchin et al., 1983; Witty et al., 1984; Minchin et al., 1985). To avoid these acetylene effects, Denison et al.
3 -3
(1983b) used a series of acetylene concentrations below 0.01 mm mm which is an order of magnitude below those at which Minchin et al. (1983) reported the acetylene effects. While this approach avoids the confounding effects of acetylene on the parameters being assayed, it limits the range of acetylene concentrations over which V can be assayed. Thus the regression procedure to solve for the three unknown parameters in Equation 4.2 uses data obtained from only a very small portion of the total acetylene response curve. Furthermore, if only four concentrations of acetylene are used as done by Weisz et al. (1985) only one degree of freedom for the error term in a non-linear regression is left and the ability of the procedure to estimate the parameters with a significant degree of reliability is severely restricted. To improve upon this situation, Denison and Sinclair (1985) assumed that the KM was