ROOT-GROWTH AS A FUNCTION OF AMMONIUM AND NITRATE IN THE ROOT ZONE

We examined the effect of soil NH4+ and NO3- content upon the root systems of field-grown tomatoes, and the influence of constant, low concentrations of NH4+ or NO3- upon root growth in solution culture. In two field experiments, few roots were present in soil zones with low extractable NH4+ or NO3-; they increased to a maximum in zones having 2 mug-N NH4+g-1 soil and 6 mug-N NO3-g-1 soil, but decreased in zones having higher NH4+ or NO3- levels. Root branching was relatively insensitive to available mineral nitrogen. Plants maintained in solution culture at constant levels of NH4+ or NO3- had similar shoot biomass, but all root parameters -biomass, length, branching and area - were greater under NH4+ nutrition than under NO3-. These results suggest that the size of root system depends on a functional equilibrium between roots and shoots (Brouwer 1967) and on the balance between soil NH4+ and NO3-.