Root-zone salinity: II. Indices for tolerance in agricultural crops

This paper provides the tools for distinguishing levels of tolerance to root-zone salinity in agricultural crops. Such distinction rests on the response of a crop's product yield following the declining, sigmoid-shaped, modified compound-discount function {Y-r = 1/[1 + (C/C-50)(exp(SC50))]} for plants grown as crops exposed to increasing root-zone salinity. This nonlinear function relates relative yield (Y-r,) to root-zone salinity (C) measured in equivalent saturated soil-paste extract electrical conductivity with two nonlinear parameters, the salinity level producing 50% of the nonsaline crop yield (C-50) and a response curve steepness constant (s) equal to the absolute value of the mean dY(r)/dC from Y-r = 0.3 to 0.7. These discount parameters suggest the existence of a single-value salinity tolerance index (ST-Index) equal to the 50% reduction in crop yield from that of the nonsaline yield plus a tendency to maintain some product yield as the crop is subjected to salinity levels approaching C-50, i.e., ST-Index = C-50 + s(C-50). The explicit purpose of this study is to determine if the discount function using biophysically relevant parameters can be applied to historical data sets. Approximations for C-50 and s were identified in the threshold salinity (C-t) and declining slope (b) parameters of the well-known threshold-slope linear response function. Several procedures for converting C, to C-50 and b to s offer the linkage between these linear and nonlinear response functions. From these procedures, two regression equations, C-50 = 0.988[(0.5/b) + C-t] - 0.252 and s = 1.52b, proved the most appropriate for the eight representative field, forage, and vegetable crops tested. The selected conversion procedures were applied to previously published C-t and b values to obtain a list of the relative root-zone salinity tolerance in agricultural crops. In addition to C-50 and s, values for exp(sC(50)) and the ST-Index were computed for each crop. The revised list provides extension personnel and plant growth modelers the parameter values from a nonlinear analog of crop yield response to root-zone salinity.