Does root-sourced ABA have a role in mediating growth and stomatal responses to soil compaction in tomato (Lycopersicon esculentum)?

Isogenic wild-type (Ailsa Craig) and abscisic acid (ABA)-deficicnt mutant ( flacca) genotypes of tomato mere used to examine the role of root-sourced A BA in mediating growth and stomatal responses to compaction. Plants were gr own in uniform soil columns providing low to moderate bulk densities (1.1-1 .5 g cm(-3)), or in a split-pot system, which allowed the roots to divide b etween soils of the same or differing bull; density (1.1/1.5 g cm(-3)). Roo t and shoot growth and leaf expansion were reduced when plants were grown i n compacted soil (1.5 g cm(-3)) but leaf,vater status was not altered. Howe ver, stomatal conductance was affected, suggesting that non-hydraulic signa l(s) transported in the transpiration stream were responsible for the obser ved effects, Xylem sap and foliar ABA concentrations increased with bulk de nsity for 10 and 15 days after emergence (DAE), respectively, but a ere the reafter poorly correlated with the observed growth responses, Growth was re duced to a similar extent in both genotypes in compacted soil (1.5 g cm(-3) ), suggesting that ABA is not centrally involved in mediating growth in thi s severely limiting 'critical' compaction stress treatment. Growth performa nce in the 1,1/1.5 g cm(-3) split-pot treatment of Ailsa Craig was intermed iate between the uniform 1.1 and 1.5 g cm(-3) treatments, whereas stomatal conductance was comparable to the compacted 1.5 g cm(-3) treatment, In cont rast, shoot dry weight and leaf area in the split-pot treatment of flacca w ere similar to the 1.5 g cm(-3) treatment, but stomatal conductance was com parable to uncompacted control plants. These results suggest a role for roo t-sourced ABA in regulating growth and stomatal conductance during 'sub-cri tical' compaction stress, when genotypic differences in response are appare nt, The observed genotypic differences are comparable to those previously r eported for barley, but occurred at a much lower bulk density, reflecting t he greater sensitivity of tomato to compaction. By alleviating the severe g rowth reductions induced when the entire root system encounters compacted s oil, the split-pot approach has important applications for studies of the r ole of root-sourced signals in compaction-sensitive species such as tomato.