EFFECTS OF N-SUPPLY ON THE RATES OF PHOTOSYNTHESIS AND SHOOT AND ROOTRESPIRATION OF INHERENTLY FAST-GROWING AND SLOW-GROWING MONOCOTYLEDONOUS SPECIES

Are there intrinsic differences in the rates of photosynthesis, shoot- and root-respiration between inherently fast- and slow-growing monoco tyledons at high and low nitrogen supply? To analyze this question we grew 5 monocotyledons, widely differing in their inherent relative gro wth rate at high and low nitrogen supply in a growth room. Nitrate was exponentially added to the plants, enabling us to compare inherent di fferences in plant characteristics, without any effect of species diff erences in the ability to take up nutrients. At high nitrogen supply, the fast-growing species from productive habitats had a higher photosy nthetic nitrogen use efficiency and rate of root respiration than the slow-growing ones from unproductive habitats. Only minor differences w ere observed in their rates of photosynthesis and shoot respiration pe r unit leaf area. At low nitrogen supply, the rates of photosynthesis and shoot- and root respiration decreased for all species, even though there were no longer any differences in these processes between inher ently fast- and slow-growing species. The photosynthetic nitrogen use efficiency increased for all species, and no differences were found am ong species. Differences in the photosynthetic nitrogen use efficiency among species and nitrogen treatments are discussed in terms of the u tilization of the photosynthetic apparatus, whereas differences in res piration rate are discussed in terms of the energy demand for growth, maintenance and ion uptake and their related specific respiratory ener gy costs. It is concluded that the relatively high abundance of slow-g rowing species compared to fast-growing ones in unproductive habitats is unlikely to be explained by differences in rates of photosynthesis and respiration or in photosynthetic nitrogen use efficiency.