Photosynthetic determinants of growth in maize plants: Effects of nitrogen nutrition on growth, carbon fixation and photochemical features

Maize {Zea mays L.) plants were grown in a greenhouse with different levels of nitrate-N (2 to 20 millimolar). Nitrogen nutrition had dramatic effects on plant growth and photosynthetic char-acteristics of mature leaves. Increasing nitrogen resulted in greater biomass production, shoot/ root ratios, and rates of leaf expansion during the day. The elongating zone of high-N plants had higher activities (per gram fresh weight) of sucrose synthase and neutral invertase than low-N plants, suggesting that increased leaf growth was related to a greater biochemical capacity for su-crose metabolism. Mature leaves of high-N plants had higher rates of photosynthesis and assimilate export (su-crose formation), and partitioned more carbon into sucrose relative to starch. Increased photo-synthetic rates (leaf area basis) were associated with higher levels of ribulose-l,5-bisphosphate carboxylase, phosphoenolpyruvate carboxylase and pyruvate, phosphate dikinase (determined im-munochemically). In addition, N-nutrition affected the functional organization of chlorophyll in the leaves. Large increases in the number of PS I reaction centers were observed which fully ac-counted for increases in leaf chlorophyll content with increasing nitrate supply. Collectively, the results suggest that increased growth of maize plants at high light and op-timal nitrogen nutrition is related to greater capacity for photosynthesis and translocation in mature leaves, and possibly increased capacity for sucrose metabolism in expanding leaves. Key words: Assimilate export — Maize, biomass production — Phosphoenolpyruvate carboxyl-ase — Photosynthetic unit density — Sucrose metabolism — Sucrose-phosphate synthase