IS COORDINATION OF LEAF AND ROOT-GROWTH MEDIATED BY ABSCISIC-ACID - OPINION

Leaf growth is more inhibited than root growth when the soil is nitrog en-deficient, dry, saline, compacted, or of restricted volume. Similar differential responses in leaf and root growth occur when ABA is appl ied to plants in well-watered and well-fertilised conditions, and oppo site responses are often found in ABA-deficient mutants. ABA levels in crease in plants in dry or saline soils, suggesting a regulating role in leaf and root growth in soils of low water potential. In nitrogen-d eficient or compacted soils, or soils of restricted volume, ABA only s ometimes increases, and in these situations its accumulation may be of secondary importance. Use of ABA-deficient mutants has so far indicat ed that ABA influences leaf and root growth in unstressed plants, and plants in dry soils, but not in soils that are compacted, of restricte d volume, or are nitrogen-deficient. For ABA to determine the relation ship between the rate of leaf growth and the rate of root growth, ther e must be long-distance transport of either ABA itself or a compound t hat controls ABA synthesis in the growing cells of leaves and roots. A BA invariably increases in xylem sap as the soil becomes dry or saline , and sometimes when it becomes nitrogen-deficient or compacted, howev er the ABA is of too low a concentration to affect leaf growth. There may be a compound in xylem sap that controls the synthesis of ABA in t he leaf, but no such compound has been identified. ABA accumulates in phloem sap of plants in dry or saline soil, but its function in contro lling root or leaf growth is unknown. We conclude that ABA affects the ratio of root growth to leaf growth via its independent effects on ro ot and leaf growth, and may regulate the ratio of root to leaf growth via feedforward signals in xylem or phloem, but there is no satisfacto ry explanation of its mechanism of control.