This study relates the leaf CO2 assimilation and leaf dynamics of nitrogen catch crops to environmental conditions. Winter rye (Secale cereale) and fodder radish (Raphanus sativus) were grown as catch crops in an outdoor pot experiment at two rates of nitrogen supply (N2 higher than N1) in Wageningen, Netherlands, from August 1993 until April 1994. Biomass increased with increasing N supply. There was no net growth after mid-November. The number of tillers in rye and of appeared leaves in radish were higher in N2 than in N1. Leaf appearance rate increased with temperature in fodder radish and rye. Tillering in rye ceased in mid-October. Leaf lifespan was related to the temperature sum between leaf emergence and leaf death. Leaf lifespan was 478 or- 68 degrees C day in fodder radish. In rye, the leaf lifespan gradually decreased from 592 or- 66 to 389 or- 25 degrees C day and from 545 (1 observation) to 401 or- 64 degrees C day in N1 and N2, respectively. In young leaves, Amax (light-saturated CO2 assimilation rate) was approximately 1.2 mg CO2 m-2 leaf s-1 in September and 0.5 mg CO2 m-2 leaf s-1 later on, independent of species and N supply, N supply affected the organic N concentrations only in the older leaves. Amax was not dependent on temperature at measurement (range: 12-19 degrees C in September, 5-15 degrees C from November until March), but, in contrast, strongly related to temperature and irradiance during the preceding growth period. Leaf nitrate concentrations increased with N supply. Water-soluble carbohydrate concentrations were higher in N1 than in N2 and higher in rye than in fodder radish. They fluctuated during the season. It is concluded that overall growth rates were limited by process rates other than that of leaf CO2 assimilation per unit leaf area in both N1 and N2.