| Abscisic acid (ABA), a plant hormone, has been regarded as a growth inhibitor. Paradoxically, ABA-deficient mutants have substantially impaired growth, supposedly due to poor water balance. However, it was recently demonstrated that wild-type levels of ABA are required for normal shoot growth in well-watered tomato independently of plant water balance and that it acts, in part, by restricting ethylene production. Studies have suggested this relationship may exist under drought conditions, leading to the question: What is the influence of ABA accumulation on shoot growth in water-stressed tomato?; Using a partially-complemented notabilis line of tomato that grew like the wild type when well-watered but did not accumulate ABA under drought, along with the experimental strategy to control plant water balance and remove the water status variable, it was found that accumulated ABA was not a cause of growth inhibition under water stress. Unexpectedly, neither was ABA necessary to maintain shoot growth during drought. This was so regardless of leaf water potential. In addition, in the absence of soil hardening and nutrient deficiencies, ethylene did not increase during soil drying and was, therefore, not a cause of growth inhibition under water deficit. These results suggest that the shoot growth reduction under mild drought conditions is due to metabolic limitations that are independent of ABA, ethylene and leaf water status. |
