There are two conflicting hypothesis to explain why chloroform improves tomato seed germination:
1. As a solvent, chloroform removes growth inhibiting substances from the seeds.
2. Residual chloroform in the seeds stimulates growth.
Many years ago, I tried a wide variety of other solvents, including carbon tetrachloride, without improvement of seed germination, thus, showing that hypothesis 1. is very unlikely. Now, to prove hypothesis 2., I performed the following experiment:
Using Ramapo tomato seeds, packed for 2015 and stored in a refrigerator, I placed 6 seeds in each of 4 Petri dishes, labelled 0, 1, 2 and 3, on 1/16/23. Then I added 2 ml of water solutions of chloroform to each Petri dish as follows:
0 - pure tap water (control)
1 - 1000 ppm choroform
2 - 500 ppm choroform
3 - 250 ppm choroform
Some of the seeds began germinating on 1/20/23. The following photo was taken on 1/21/23:
This photo shows that 1000 ppm of chloroform inhibited germination but 500 ppm and 250 ppm stimulated germination, thereby indicating that hypothesis 2. is correct. This is good news because it is more practical to apply choroform as a soil drench. Also, it indicates an auspicious opportunity to experiment with foliar sprays of chloroform to stimulate the growth of established tomato and possibly other plants.