Optimal use of leaf nitrogen explains seasonal changes in leaf nitrogen content of an understorey evergreen shrub.

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Understorey evergreen species commonly have a higher leaf nitrogen content in winter than in summer. Tested here is a hypothesis that such changes in leaf nitrogen content maximize nitrogen-use efficiency, defined as the daily carbon gain per unit nitrogen, under given temperature and irradiance levels. The evergreen shrub Aucuba japonica growing naturally at three sites with different irradiance regimes in Japan was studied. Leaf photosynthetic characteristics, Rubisco and leaf nitrogen with measurements of temperature and irradiance monthly at each site were determined. Daily carbon gain was determined as a function of leaf nitrogen content to calculate the optimal leaf nitrogen content that maximized daily nitrogen-use efficiency. As is known, the optimal leaf nitrogen content increased with increasing irradiance. The optimal leaf nitrogen content also increased with decreasing temperature because the photosynthetic capacity per Rubisco decreased. Across sites and months, the optimal leaf nitrogen content was close to the actual leaf nitrogen content and explained the variation in actual leaf nitrogen by 64 %. Sensitivity analysis showed that the effect of temperature on optimal nitrogen content was similar in magnitude to that of irradiance. Understorey evergreen species regulate leaf nitrogen content so as to maximize nitrogen-use efficiency in daily carbon gain under changing irradiance and temperature conditions.