Considerable increase in net photosynthetic rates ribulose-l-5-bisphosphate carboxylase (RuBPC) and phosphoenolpyruvate carboxylase (PEPC) activities, and pigment contents was observed in Helianthus annuus L. plants affected by different types of N-fertilization, but no cultivar variability was found. and water use efficiency (WUE) were enhanced strongly when the plants were treated with a combined compost (12N-12K-12P) or urea while manure induced a rather weak effect. The carboxylase activities changed wifli the N treatment and they were higly correlated with and WUE (r > 0.9). The RuBPC/PEPC ratios fell drastically in urea and 12N-12K-12P treated plants.