Abstract
This study was conducted to clarify the interspecific difference of the relationship between nitrogen content of leaf blade and leaf photosynthesis in sveral tropical and temperate grasses, and legumes. Experimental plants were grown in pots with various levels of nitrogen fertilizer. The apparent photosynthetic rate of the youngest fully expanded leaf was measured in assimilation chamber. Leaf nitrogen was analysed by the semimicro-Kjeldahl method and percentage of contents were determined on the dry weight basis. The results are summarized as follows: 1) Tropical grasses (Echinochloa utilis, Eleusine corocana, Chloris gayana, Panicum maximum), native grasses (Eragrostis ferruginea, Pennisetum alopecuroides) and weeds (Digitaria adscendens, Echinochloa Crus galli), which are all possessing C_4 cycle showed 1.5 to 2 times higher photosynthetic rates per unit leaf nitrogen content (PN) than those of the temperate grasses (phleum pratense, Festuca arundinacea, Dactylis glomerata, Lolium multiflorum) and about 2.5 times higher than those of the legumes (Medicago sativa, Trifolium repens). Temperate grasses were about 1.3 times higher PN values than those of legumes. Leaf nitrogen concentration for the optimum PN values varied among species ; 2 to 3% for the temperate grasses and about 5% for the legumes. On the other hand, those for C4 species were about 4.5% for weeds, about 1.7% for native grasses and 2.5% to 3.5% for tropical grasses in leaf nitrogen concentration, respectively. 2) In all species, a linear relationship was observed between leaf photosynthetic rate and leaf nitrogen percentage in lower level of leaf nitrogen concentration. However, as nitrogen concentration in leaf became higher, photosynthetic rates of most species were saturated, followed by decline according with further increment of nitrogen percentage. The values of leaf nitrogen concentration which brought the maximum photosynthetic rates were different among species ; 4 to 6% for temperate grasses, about 6% for legumes and 4 to 5.5% for C4 species, respectively. 3) Inclination of light-photosynthesis curves were different among species as well as among nitrogen levels of leaf. In the case of similar levels of nitrogen percentage in leaf, light-photosynthesis curves of C4 species were much steeper than those of C3 species. In both C3 and C4 species, at high light intensities, leaves with higher nitrogen percentage exhibited higher photosynthetic rates than those with lower nitrogen percentage. There were, however, little difference at low light intensities. When the leaves contained less than about 2% leaf nitrogen, light-photosynthesis curves leveled off at relatively low light intensity.
