Does an antagonistic relationship between ABA and ethylene mediate shoot growth when tomato (Lycopersicon esculentum Mill.) plants encounter compacted soil?

This study tested the hypothesis that antagonistic interactions between abs cisic acid (ABA) and ethylene mediate the effects of soil compaction on sho ot growth. Isogenic wild-type (Ailsa Craig), ABA-deficient (notabilis) and a transgenic (ACO1(AS)) tomato genotype with a reduced capacity to synthesi ze ethylene were examined. Exogenous ABA was also applied. Leaf area was co mparable when Ailsa Craig and ACO1(AS) were grown in uncompacted (1.1 g cm( -3)) or compacted (1.5 g cm(-3)) soil, but was lower in notabilis. However, a 1.1/1.5 g cm(-3) split-pot treatment invoked marked genotypic difference s, whereby leaf area was comparable to 1.1 g cm(-3) control plants in ACO1( AS) but was intermediate between the 1.1 and 1.5 g cm(-3) treatments in Ail sa Craig and notabilis. ABA may be discounted as the root-sourced signal re sponsible for reducing leaf area when the roots encountered compacted soil as Ailsa Craig and ACO1(AS) showed differing responses despite similar incr eases in xylem sap ABA concentration; leaf area was invariably lower in not abilis. These genotypic differences were correlated with ethylene evolution ; thus the greater leaf area in ACO1(AS) was associated with its reduced ab ility to synthesize ethylene, whereas the reductions in leaf expansion obse rved when Ailsa Craig and notabilis encountered compacted soil were accompa nied by increased ethylene production. Application of ABA had little effect on ACO1(AS), but promoted a recovery of leaf expansion in notabilis, and m ore surprisingly in Ailsa Craig. These results suggest that antagonistic in teractions between ABA and ethylene may regulate leaf expansion when the ro ot system simultaneously encounters uncompacted and compacted soil.