A COMPARISON BETWEEN METHODS USED TO CONTROL NUTRIENT SUPPLY

Experimental methods to supply nutrients to culture solutions in order quantitatively to control plant nutrition are compared, In experiment s with tomato and birch plants, for which the data are available in da tabases (Ingestad et al., 1994a, b), the nutrients were supplied at co nstant relative addition rates (R(A)) over sufficiently long periods o f time to achieve acclimated plants and reliable measurements of plant responses. The plants were maintained under steady-state conditions, i.e. the internal nutrient concentrations (c(i)) remained constant, as a result of a numerical equality between the relative uptake rate (R( U)) and the relative growth rate (R(G)). These results are compared to experiments with pea plants (Macduff et al., 1993), In one series (a) , R(A) was applied, but without strict control of internal steady-stat e, and in the other series (b), the external concentration (c(e)) was maintained constant, With limiting nitrogen, in both series, there was a substantial deviation from equality between R(U) and R(G). In (a), c(i) changed during the experimental period and the purpose of the R(A ) approach was lost, In (b), a constant c(e) had little effect on nitr ogen uptake and plant growth, At the three highest concentrations, ste ady-states were obtained at non-limiting uptake rates, At the lowest c oncentration, the uptake rate of nitrogen was about the same, but ther e was a decrease of R(G), which apparently was not caused by reduced u ptake. Clear-cut relationships can not therefore be established betwee n treatment variables and plant responses and the conclusions reached by Macduff et al. (1993) have little support in their experimental res ults, This indicates an urgent need to update both theories and experi mental methods together: in particular, it is important to identify th e system under investigation and to distinguish between control of the medium and control of the plant.