Science - September 15, 2011

In search of the 24/7 tomato

Tomato plants grow for about 12 hours of the day under natural sunlight, which sets off photosynthesis.

Aeron Velez-Ramirez researches the impact of light on tomato plants.
Dutch horticulturalists are boosting tomato production by exposing greenhouse tomatoes to extra light. Additional lamps are stretching the growing time to as much as 18 hours per day. But that's the limit - stronger lights or longer periods under light damage the tomato plants, causing yields to go down again. Even tomatoes need some rest at night, at least six hours.
A few years ago, the De Ruiter Seeds company, owned by the seed giant Monsanto, decided to research whether that nocturnal rest really is so essential. Because there are some tomato plants that survive under continuous lighting, namely wild tomato plants. If it were possible to build in these characteristic in the company's own varieties, horticulturalists could get harvest increases of 25 percent.
Wageningen researcher Aeron Velez-Ramirez worked on this research during an internship at Monsanto. He researched the genetic basis of the 24-hour tomato in order to find out how the continuous growth process works, and now he knows which chromosomes cause the light tolerance. The project on which Velez-Ramirez worked was successful: Monsanto managed to develop a light-tolerant tomato in which half a chromosome - the tomato has 12 - is exchanged for that of a wild tomato.
Unclear reason
But that is not enough to create a light-tolerant tomato, because it is not known under which conditions this characteristic is expressed. Moreover, Monsanto still has no idea why bred tomatoes and other crops need rest at night, whereas various wild plants do not. That is what Velez-Ramirez, who comes from Mexico, is investigating now in his PhD research at the Plant Physiology and Horticultural Chains chair group in Wageningen.
Why does a plant go into meltdown when it is subjected to continuous light for two weeks? ‘Maybe the photosynthesis centre in the plant gets damaged and switches off', says Velez-Ramirez. The higher the light intensity, the more damage is done to the tomato. We know that. But we also know that tomato plants get damaged by a lighting schedule of six hours of light followed by six hours of darkness. So it doesn't seem to be a question of the duration of the light.'
Sugars
So Velez-Ramirez is also experimenting with the colour of the light, the spectrum. He exposes the plants to blue light, red light and plasma light, knowing that the plants have multiple light receptors which each recognize part of the spectrum. ‘Do one or more light receptors go out of control when there is continuous light? We are looking into that now with experiments.'
Another possibility is that the plants manufacture too many sugars through the continuous photosynthesis. Those sugars have to get from the leaves to the fruits. If there are excessive amounts of sugars in the leaves, the plant gets sick, is the theory. ‘The sugars play a role in the explanation', says Velez-Ramirez. ‘But it doesn't seem to be the decisive factor, as the tomatoes which are light-tolerant have to process a lot of sugar too, but they don't die.'
Velez-Ramirez will soon obtain two large datasets with RNA sequences of the 24-hour tomato and a conventional variety. By comparing the two, he hopes to discover the mechanism which causes the light tolerance in the plant. He also plans to do practical research on the 24-hour tomato in the experimental greenhouse belonging to Wageningen UR and the GreenQ company in Bleiswijk.
Will he find the secret? ‘I think so but my supervisor is realistic. He says: do experiments and check what the results are step by step. And don't forget your publications.'
 

Re:act