Carbohydrate Reserves, Translocation, and Storage in Woody Plant Roots

Abstract

All of the perennial organs of a woody plant may serve a storage function, but the highest concentrations of carbohydrate reserves are usually found in root tissues. These root reserves change dramatically throughout the year, decreasing rapidly with budbreak and early vegetative and reproductive development, and then increasing late in the growing season, usually after cessation of vegetative growth and fruit maturation. Accumulation of these reserves is very sensitive to late-season stresses and management practices, and decreased accumulation can profoundly affect a tree’s performance the following year. These root reserves apparently play important and specific roles in supplying substrates for shoot respiration and growth, especially in woody species that flower and begin fruit development before substantial canopy development. Although phloem transport may be involved in root-to-shoot transport, considerable xylem transport occurs in some early flowering and fruiting species. Regulation of mobilization of root reserves remains unclear, but both gibberellins and auxins are possibly involved. Unfortunately, despite their apparent importance, mobilization, transport and the specific functions of root reserve carbohydrates are only superficially defined for any woody plant. nearly fully expanded before anthesis, and much of fruit development occurs late in the season, long after canopy development. Hansen (1971) reported that apple flowers depend on reserves only during their earliest stages of development, or until the first five or six leaves have formed. Once petals emerge from the sepals, photosynthesis from the leaves becomes the major source of carbohydrates for flower and fruit growth. Most of the apple reserves are apparently used in respiration rather than for new building materials (Hansen and Grausland, 1973), and, in young apple rootstocks, < 20% of [ 14 C]-labeled reserves assimilated in fall was fixed in new growth the following season (Hansen, 1967; Kandiah, 1979b). Tromp (1983) concluded that, while apple roots do certainly supply aboveground parts with carbohydrates early in the spring, it was doubtful whether such contributions are significant, and it was likely that reserves are used mostly in respiration.