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As every commercial tomato grower knows, the germination rate of tomato seeds can drop by up to 50 percent if the seeds are kept longer than a year. Freshly extracted tomato seeds, on the other hand, can have a germination rate of over 90 percent. To increase the number of plants, many growers increase the number of seeds they plant and/or pretreat seeds with potassium nitrate or tripotassium phosphate. Either one of these approaches increases the cost of production, but preliminary research done at the University of Calcutta in Kolkata, India, could offer growers an alternative.
Regulation of seed germination in tomatoes has been studied using juice from several fruits, including apple, guava and tomato. When the tomato seeds were exposed to these juices, the results backed up the well-known fact that seeds typically will not germinate inside the fruit. Early germination is usually delayed until the fruit is gone. The next question: What would happen if tomato seeds were exposed to different dilutions of various fruit juices?
Debmalya Barh, H.C. Srivastava and B.C. Mazumdar at the university's Institute of Agricultural Science set out to answer this question. They used 17 different fruits, including pear, plum, grape, pineapple, black palm, mango, lemon and carambola (star fruit), as well as those listed above. The juices from the fully ripened fruits were extracted and diluted down to 2, 4, 6 and 8 percent with distilled water.
To examine the potential effects of vitamin C, four solutions were prepared, with 0.2, 0.3, 0.4 and 0.5 milligram of ascorbic acid per 100 milliliters of distilled water.
Tomato seeds were soaked in each of the four dilutions of the fruit juice extract and the four dilutions of ascorbic acid. As a control, tomato seeds were also soaked in distilled water. The seeds were soaked for six hours at 80.6 degrees Fahrenheit.
After the seeds had been soaked, they were placed in petri dishes lined with blotting paper. The seeds were then visually examined every 12 hours for root development, an indication of germination.
While it's important to have a high germination rate, it's just as important to have healthy seedlings. So, in addition to tracking germination, the researchers measured seed vigor every 24 hours by placing the germinated seed on graph paper and recording the root and shoot length for seven days.
A baseline by which to measure whether a treatment increased germination rate or not was created by counting the number of hours it took for 50 percent of the seeds that were presoaked in distilled water to germinate. In this study, it took 131 hours for 50 percent of those seeds to sprout.
While many of the fruit juice extracts at a low concentration did improve the germination rate of the seeds compared to the distilled water group, nothing beat the tomato juice and ascorbic acid.
The tomato juice concentrations of 2 and 4 percent showed a 50 percent germination rate at 92 hours, compared to the 131 hours it took for the distilled water group. Surprisingly, the ascorbic acid at low concentrations came in a close second, and there was an additional benefit. Tomato seeds treated with an ascorbic acid solution of 0.2 mg/100 ml took 95 hours to achieve a germination rate of 50 percent, while those treated with the 0.3 mg/100 ml solution took 98 hours.
While the 0.2 ascorbic acid solution sped up the time it took for 50 percent of the seeds to germinate, it also created stronger, more vigorous seedlings. While the groups of seedlings treated with distilled water were white to transparent in color when they first emerged, the seedlings in the 0.2 ascorbic acid group were green. Leaf development also differed between the two groups. Looking at the first leaves, the ascorbic acid group's vegetation was larger and the growth was more uniform when compared to the distilled water group.
What does this study mean for commercial tomato growers? It all boils down to money. While the chemical treatment noted earlier does work, it's costly. Another approach is to buy more tomato seed than you really need to pick up the slack due to the poor germination rate. Again, this approach adds to the cost of production, so you can either increase the cost of your product or eat that additional cost. In the past, many growers have taken the latter approach in order to stay competitive, and this study could change that.
In the future, there's the potential for production of commercial treatments that would simplify this process. A faster germination rate, seedling vigor and uniform growth are all important to growers, and research like this may lead to simpler, more cost-effective methods of achieving them.
Mindy S. McIntosh-Shetter has a B.S. in agricultural education with a minor in biology from Purdue University and an M.A. in interdisciplinary studies from the University of Louisville. In the past, she shared her love for agriculture in the classroom, and now teaches through blogs and how-to videos.