"Low-cost" initial burst of root development in whole Fagus crenata seedlings: The key to survival?

Terrestrial plants are rooted in one place, and therefore their metabolism must be flexible to adapt to continuously changing environments. This flexibility is probably influenced by the divergent metabolic traits of plant organs. However, direct measurements on organ-specific metabolic rates are particularly scarce and little is known about their roles in determining whole-individual meatabolism. To reveal this on seedlings of Fagus crenata, which is one of the most widespread dominant genus in temperate deciduous broad leaf forests in the circum-polar Northern Hemisphere, we measured respiration, fresh mass and surface area for total leaves, stems and roots of 55 individuals in two years from germination and analyzed their relationships with individual metabolism. Proportion of roots to whole plant in mass increased from approximately 17% to 74%, and that in surface area increased from about 11% to 82% in the two years. Nonetheless, the increment of the proportion of root respiration to whole-plant respiration was from 9.2% to only 40%, revealing that the increment in mass and surface area of roots was much larger than the increment in energetic cost. As a result, only the roots showed a substantial decline in both respiration/surface area and respiration/mass among the three organs; roots had about 90% decline in their respiration/surface area, and 84% decline in their respiration/mass, while those in leaves and stems were relatively constant. The low-cost and rapid root development is specific to the two years after germination and would be effective for avoiding water and nutrient deficit, and possibly helps seedling survival. This drastic shift in structure and function with efficient energy use in developmental change from seeds to seedlings may underpin the establishment of F. crenata forests. We discuss significance of lowering energetic cost for various individual organisms to effectively acquire resources from a wide perspective of view.