THE EFFECTS OF DROUGHT AND NITROGEN ON LIGHT INTERCEPTION, GROWTH ANDYIELD OF WINTER OILSEED RAPE

The effects of drought and nitrogen on growth of winter oilseed rape w ere investigated during 1991-93 on a coarse sandy soil. Drought in the flowering phase or in the pod-filling phase or throughout the seasons was partly combined with three nitrogen levels of 200, 100 and 0 kg N ha(-1). During the dry years 1992 and 1993, at 200 kg N ha(-1), seed yield, pod number and seeds per pod were strongly decreased by drought , whereas the seed weight increased after drought during flowering. Th e straw yield was less affected by drought. Lower nitrogen levels redu ced seed yield in the fully irrigated treatments; in the unirrigated t reatments, however, the yield was slightly higher at the 0 N and 100 N level than at 200 N in both 1992 and 1993. Nitrogen and drought incre ased the nitrogen concentration of the seeds and decreased the oil con centration. The interception of photosynthetically active radiation (P AR) was estimated from spectral reflectance ratio measurements. The dr y-matter:radiation quotient (epsilon(t)) for top biomass growth was 2. 38 g MJ(-1) when growth was not limited by drought. Prolonged drought stress reduced both interception of PAR and epsilon(t) to the same deg ree. epsilon(t) was little affected by lower nitrogen levels, which ma inly reduced the interception of PAR. Seed yield was linearly related to PAR intercepted during the pod-filling phase with an apparent seed dry matter:radiation quotient of 2.47 g MJ(-1). However, in treatments severely drought stressed in the flowering phase, up to 65% of the as similates produced during pod filling ended up in the straw fraction, because the sink size of the seeds was limited.