Soil Compaction. A Role for Ethylene in Regulating Leaf Expansion and Shoot Growth in Tomato?

The role of ethylene in regulating growth in tomato (Lycopersicon esculentum Mill.) during compaction stress was examined using wild-type (cv Ailsa Craig) and transgenic ($\text{ACO}1_{\text{AS}}$) genotypes; the latter has a reduced capacity to produce ethylene. Ethephon or silver ions were applied to increase ethylene production or block its action. Shoot growth in both genotypes was comparable in uncompacted (1.1 g cm-3) and uniformly compacted soil (1.5 g cm-3). However, a 1.1/1.5-g cm-3 split-pot treatment invoked marked genotypic differences: growth was reduced in cv Ailsa Craig but was comparable to uncompacted control plants in $\text{ACO}1_{\text{AS}}$. As xylem sap abscisic acid levels were similar, abscisic acid was not responsible for inhibiting growth in cv Ailsa Craig. These genotypic differences in growth were accompanied by increased ethylene evolution in cv Ailsa Craig, suggesting that the ability of $\text{ACO}1_{\text{AS}}$ to maintain growth in the split-pot treatment reflected its lower ethylene levels, a view supported by the observation that excising the roots in the compacted compartment reduced ethylene evolution and restored shoot growth in cv Ailsa Craig. Treatment with silver restored shoot growth in cv Ailsa Craig, whereas treatment with ethephon reduced growth in $\text{ACO}1_{\text{AS}}$. Thus, ethylene apparently has a key role in determining growth when tomato plants encounter differential soil compaction.