Functional Ecology 2004 18, 419 – 425 © 2004 British Ecological Society 419 Blackwell Publishing, Ltd. Allocation of nitrogen to cell walls decreases photosynthetic nitrogen-use efficiency Y. ONODA,† K. HIKOSAKA and T. HIROSE Graduate School of Life Sciences, Tohoku University, Aoba, Sendai, 980-8578, Japan Summary 1. Nitrogen (N) is an essential limiting resource for plant growth, and its efficient use may increase fitness. We investigated photosynthetic N-use efficiency (photosynthetic capacity per unit N) in relation to N allocation to Rubisco and to cell walls in Polygonum cuspidatum Sieb. et Zucc. which germinated in May (early germinators) and August (late germinators). 2. There was a significant difference between early and late germinators in photo- synthetic capacity as a function of leaf N content per unit area. Higher photosynthetic N-use efficiency in late germinators was caused primarily by a larger allocation of N to Rubisco. 3. Nitrogen allocation to cell walls was smaller in late germinators. The shorter growth period in late germinators was associated with higher photosynthetic capacity, which was achieved by allocating more N to photosynthetic proteins at the expense of cell walls. 4. The trade-off between N allocation to photosynthesis and to structural tissues suggests that plants change N allocation to increase either the rate or duration of carbon assimilation. Such plastic change would help plants maintain themselves and cope with environmental changes. Key-words: cell-wall proteins, germination time, leaf life span, nitrogen allocation, ribulose-1,5-bisphosphate carboxylase/oxygenase Functional Ecology (2004) 18, 419– 425 Introduction Nitrogen availability often limits plant growth in natural ecosystems and its efficient use may increase fitness (Chapin 1980; Aerts & Chapin 2000). Leaves accumulate most of the N in the plant, and about half the total leaf N is used for photosynthetic activities. Thus the photosynthetic apparatus is the largest sink of N in the plant (Evans & Seemann 1989; Poorter & Evans 1998). Ribulose-1,5-bisphosphate carboxylase/ oxygenase (Rubisco) constitutes ≈ 50% of photosynthetic N, and is the most abundant and important enzyme of photosynthesis (Evans 1989). Consequently, photo- synthetic capacity correlates strongly with leaf N con- centration (Field & Mooney 1986; Evans 1989). It is less clear how much leaf N is allocated to components involved in activities other than photo- synthesis. In addition to many nitrogenous compounds required for cellular metabolism, cell walls are a major N sink (Lambers & Poorter 1992). Cell walls contain proteins that function in defence, growth, development, signalling, intercellular communication and environ- mental sensing, and as selective exchange interfaces (Showalter 1993). Lamport (1965) showed that primary cell walls have a substantial amount of N, ranging from 0·4–2·2% in several species. If cell walls account for 30–50% of leaf dry mass (Merino, Field & Mooney 1984), they constitute a considerable amount of leaf N. Leaves with a long life span tend to have a high leaf mass per area (LMA) and low photosynthetic N-use efficiency (PNUE, photosynthetic capacity per unit N). LMA, which is positively correlated with struc- tural toughness, is often used as an index of structural (cell-wall) biomass (Reich et al . 1991; Wright & Cannon 2001). On the other hand, LMA is negatively correlated with PNUE (Poorter & Evans 1998). These studies sug- gest that leaves with a long life span invest more N in cell walls and consequently less in the photosynthetic apparatus. Thus a trade-off between N allocation to cell walls and to Rubisco is expected across leaves with different life spans. Leaf life span of a species changes with biotic and abiotic factors. Germination time is one of the factors that determine leaf life span in deciduous plants. Even when they germinate at different times, they shed all their leaves at the end of the growing season. Con- sequently late germinators have a shorter leaf life span, †Author to whom correspondence should be addressed. E-mail: onoda@mail.tains.tohoku.ac.jp