KINETIC-PARAMETERS OF NITRATE UPTAKE BY DIFFERENT CATCH CROP SPECIES - EFFECTS OF LOW-TEMPERATURES OR PREVIOUS NITRATE STARVATION

The pollution of aquifers by NO3- in temperate environments is aggrava ted by farming practices that leave the ground bare during winter. The use of catch crops during this time may decrease nitrate loss from th e soil. Nitrate uptake by several catch crop species (Brassica napus L ., Sinapis alba L., Brassica rapa L., Raphanus sativus L., Trifolium a lexandrinum L., Trifolium incarnatum L., Phacelia tanacetifolia Benth. , Lolium perenne L., Lolium multiflorum Lam. and Secale cereale L.) wa s here studied in relation to transpiration rate and low temperatures applied to the whole plant or to roots only. The Michaelis constant (K (m)), maximum uptake rate (V(max)), time of induction and contribution s of inducible and constitutive mechanisms were estimated from measure ments of NO3- depletion in the uptake medium. There were large differe nces between species, with K(m) (muM) values ranging between 5.12 +/- 0.64 (Trifolium incarnatum) and 36.4 +/- 1.97 (Lolium perenne). Maximu m NO3- uptake rates expressed per unit root weight were influenced by ageing, temperature and previous NO3- nutrition. They were also closel y correlated with water flow through the roots and with shoot/root rat io of these species. The combined results from all species and treatme nts showed that V(max) increased with shoot/root ratio, suggesting a r egulatory role for the shoots in NO3- uptake. Overall, the results sho wed a great diversity in NO3- uptake characteristics between species i n terms of kinetic parameters, contribution of the constitutive system (100% of total uptake in ryegrass, nil in Fabaceae) and time of induc tion.