SEASONAL-CHANGES IN THE PHOTOSYNTHETIC CAPACITY OF WINTER RAPE PLANTSUNDER DIFFERENT NITROGEN REGIMES MEASURED IN THE FIELD

Winter rape (cv. 'Falcon') grown under different nitrogen regimes (N-0 , N-120; 0 and 120 kg.ha(-1), respectively) in northern Germany was in vestigated over the 1996 spring-summer season. Using a CO2 H2O diffusi on porometer, diurnal courses of net photosynthesis and respiration we re measured in situ and were related to microclimatic conditions and l eaf water relations. Photosynthesis was modelled and daily CO2 gain wa s calculated. In contrast to the N-120 plants, plants of the low nitro gen plot (N-0) grew less densely and their leaves behaved more like su n leaves. Increased nitrogen supply had little influence on photosynth etic capacity but it increased productivity through higher leaf area i ndex and an extended period of photosynthetic activity. N-120 plants a lso appeared to be better acclimated to hot, summer conditions. Higher nitrogen supply substantially increased seed production with the yiel d of the N-120 plants being 16% of the N-0 plants.