The physiological traits of tree roots have been considered associating with photosynthetic capacity; however, the linkages between diurnal patterns of root respiration and assimilation in individual leaves are poorly understood. Our objectives were to investigate the diurnal patterns of tree root respiration and the assimilation under natural luminous environments and to explore the potential environmental drivers of root respiration and the assimilation by relating trees' diurnal cycle to commonly measured variables. We simultaneously and continuously measured individual leaf assimilation and root respiration in 3-year-old seedlings of Quercus crispula and Fagus crenata, which are commonly the dominant tree species in deciduous broad-leaved forests in Japan, in full-sun and in the dark. Net assimilation rates followed a diurnal pattern in both species, with a marked decline in the morning that corresponded to a rapid increase in photosynthetic photon flux density, an increase in leaf temperature, and a vapor pressure deficit. Similarly, root respiration rates in both species decreased around noon in the sun treatment, although the soil temperature continued to increase. Seedlings in the sun treatment had significantly higher rates of root respiration than seedlings that were kept in the dark. Our results suggest that the midday depression of leaf assimilation could lead to downward transport of photosynthetic products, causing a midday depression in root respiration on a diurnal time scale. Furthermore, the effects of changes in the assimilated carbon supply on root respiration are likely to vary with light conditions experienced by leaves.
2014 The Society of Agricultural Meteorology of Japan