YIELD FORMATION IN RICE IN RESPONSE TO DRAINAGE AND NITROGEN APPLICATION

Four field experiments were carried out in Tamil Nadu, India, to deter mine effects of internal drainage (percolation rate) on growth and yie ld formation in rice, in interaction with nitrogen (N) management. Gra in yield response to drainage was positive at all N levels in all expe riments. Yields (averaged over N treatments) increased by 14% (Expt. 1 ), 10% (Expt. 2), 25% (Expt. 3) and 22% (Expt. 4) in response to drain age. Higher yield responses to N application were found under well dra ined than poorly drained soil conditions. In late-season crops under p oor drainage, grain yields decreased in response to high doses of N. T he positive yield response to drainage was always associated with incr eased number of filled grains per panicle (+13 to +17%), increased tra nslocation of stored reserves, increased root biomass at harvest (+20 to +36%), increased N concentration in roots and increased root activi ty (as measured by alpha-naphthylamine oxidation). Of the yield increm ent resulting from drainage, 25 to 35% could be accounted for by incre ased translocation of stem reserves in Expt. 1, 5 to 25% in Expt. 2 an d 30 to 40% in Expt. 3. Yield increase in response to drainage was not always associated with more green leaf area, leaf longevity, leaf N c ontent, crop N uptake, nor with a larger dry matter production at give n levels of the leaf N pool and global radiation, as quantified with t he help of a calibration factor f(sv). A literature review is presente d leading to two hypotheses to explain the effect of drainage on yield : (1) a possible potassium deficiency is expressed more in reduced (lo w-redox) soil due to a lowering of K availability per se; or to increa sed K demand associated with maintaining the root's 'oxidizing power'; (2) improved root condition resulting from an increase in soil redox potential induces a prolonged synthesis and transport of cytokinins in roots, resulting in an extended photosynthetic activity, enhanced sin k strength of grains and more translocation and deposition of carbohyd rates in the grains. These hypotheses are not mutually exclusive. They were not explicitly tested in this study. (C) Elsevier Science B.V.