ROOT BIOMASS FRACTION AS A FUNCTION OF GROWTH DEGREE DAYS IN WHEAT

Root, underground and above-ground biomass were measured on various wh eat cultivars from 1986 to 1988 in the south-east of France. The resul ts are expressed as root: total (f(r)) or underground: total (f(u)) bi omass fractions. Observed f(r) and f(u) values are in good agreement w ith previous results. f(r) and f(u) decrease steadily from emergence t o maturity, with an exponential tendency. When using cumulative growth degree days since emergence relative to cumulative growth degree days until ear emergence (theta) as time scale, f(r) and f(u) can be expr essed as simple functions of theta f(r)(theta*) = 0.05 + 0.58e-1.48-t heta f(r)(theta*) = 0.12 + 0.88e-2.28-theta* The incremental root bio mass partitioning coefficient, alpha(r) = (dW(r)/dt)/(dW(t)/dt), which describes the net increase in root biomass dW(r) over time dt relativ e to the increase in total biomass (dW(t)) over the same time period, has been derived from f(r) and the relative growth rate. Its time cour se is accurately represented by alpha(r)(theta) = -0.15 + 0.63e-0.98- theta Under our experimental conditions, with no severe water stresse s or nutrient deficiencies, and for our sampling frequency, around 2 w eeks, the development scale theta, is the main factor governing the t ime courses of f(r), f(u) and alpha(r).