ROOT SYSTEMS AND ROOT-MASS RATIO - CARBON ALLOCATION UNDER CURRENT AND PROJECTED ATMOSPHERIC CONDITIONS IN ARABLE CROPS

Roots of annual crop plants are a major sink for carbon particularly d uring early, vegetative growth when up to one-half of all assimilated carbon may be translocated belowground. Flowering marks a particularly important change in resource allocation, especially in determinate sp ecies, with considerably less allocation to roots and, depending on en vironmental conditions, there may be insufficient for maintenance. Stu dies with C-14 indicate the rapid transfer belowground of assimilates with typically 50% translocated in young cereal plants of which 50% is respired; exudation/rhizodeposition is generally <5% of the fixed car bon. Root:total plant mass decreases through the season and is affecte d by soil and atmospheric conditions. Limited water availability incre ased the allocation of C-13 to roots of wheat grown in columns so that at booting 0.38 of shoot C (ignoring shoot respiration) was belowgrou nd compared to 0.31 in well-watered plants. Elevated CO2 (700 mu mol C O2 mol(-1) air) increased the proportion of root:total mass by 55% com pared with normal concentration, while increasing the air temperature by a mean of 3 degrees C decreased the proportion from 0.093 in the co ol treatment to 0.055 in the warm treatment.