Importance of Cycling and Recycling of Mineral Nutrients within Plants for Growth and Development

Abstract

Abstract: Cycling of mineral nutrients, i.e. retranslocation in the phloem from the shoot to the roots, and recycling, i.e. translocation of cycled nutrients back in the xylem to the shoot can contribute substantially to the fluxes of phloem‐mobile nutrients between roots and shoot. Cycling and recycling of nutrients serves several well defined functions. These include supplying the root with nutrients assimilated in the shoot (nitrate and sulphate reduction), maintenance of cation‐anion balance, providing additional driving force for solute flow in the xylem and phloem, and acting as a shoot signal to convey nutrient demand to the roots. Cycling of mineral nutrients like K is also required to cover the demand for growth of apical root zones and to smooth out fluctuations that occur spatially and with time in the external nutient supply of soil‐grown plants. Cycling and recycling of mineral nutrients is also closely related to the process of phloem loading and export of photosynthates from source leaves. This is particularly true for potassium, magnesium and phosphorus. Nutrient deficiency‐induced shifts in dry matter partitioning between shoot and roots are therefore closely related to the solute flow in the phloem not only of photosynthates but also mineral nutrients from source leaves to roots. More research is needed, however, to elucidate in greater detail the contribution of cycling and recycling of mineral nutrients in the integration of growth processes at the whole plant level.