Originally published in Issue 9
Cornell University researchers developed a “fast crop” production schedule for greenhouse lettuce. But growers may have to alter cultural practices to avoid tipburn caused by calcium deficiency.
Tipburn is a physiological disorder of greenhouse-grown lettuce that can be a problem for growers who are trying to produce their crops in a short period of time. Tipburn can have a significant impact on the salability of a lettuce crop. The same disorder can manifest itself in tomato crops as blossom end rot.
“A challenge for greenhouse growers trying to produce their lettuce crops as fast as possible is ensuring that all of the nutrients are distributed to all the different parts of the plant in the right quantities,” said A.J. Both, associate extension specialist at Rutgers University. “In the case of lettuce, what sometimes happens is calcium cannot be transported fast enough in sufficient quantities to the quickly developing young leaf tissue. The plants’ cell walls cannot form properly and the cells collapse. This happens in the inner hearts of the lettuce heads.
“When the young leaves start to push out and grow larger these brown leaf edges appear. This is referred to as tipburn. Research showed that it is a calcium deficiency that causes tipburn.”
Both said Cornell University researchers encountered this disorder when they were developing a fast crop greenhouse production system for finishing lettuce in 35 days.
“This is a very fast crop—five weeks from seed for a 5-ounce head of lettuce,” he said. “The time between seeding and transplanting takes 11 days. The remaining 24 days the plants are in the greenhouse.
“The five-week production cycle is not the way most commercial operations grow their lettuce. Growers usually allow the growth rate to fluctuate depending on the amount of natural light the plants receive. In the summer when light levels are higher, growers can finish a crop in five to six weeks. But in the winter when natural light levels are lower, the crops take longer, as much as double the production time that occurs during the summer. If a lettuce crop can be grown at a slower rate, tipburn may not be an issue. At a slower growth rate, the nutrient uptake rate can better keep up with the plants’ demand.
“In the Cornell fast crop system supplemental lighting is used to ensure every crop finishes in five weeks. If the plants are pushed with supplemental light allowing this fast growth rate to occur, then tipburn can show up very quickly. A growing strategy was needed that allowed for a fast growth rate, but prevented tipburn from occurring.”
Both said the damage to the young leaves caused by calcium deficiency can happen within days. It may take a few more days after the damage occurs for growers to observe the symptoms.
“When the conditions for this disorder are right and there is not enough transport of calcium, the damage can start within a day,” he said. “Depending on how long the deficiency lasts will determine how severe the tipburn symptoms will be.”
Increasing plant transpiration rates
Both said researchers and growers have found that the rate of nutrient uptake, including calcium, can be increased by stimulating plant transpiration.
“Increasing the rate of air turbulence around the leaves leads to a higher level of transpiration from the leaves,” he said. “As a result, there is a higher rate of water uptake from the roots and translocation of the nutrients, including calcium, from the roots to the developing leaves.”
Both said different methods have been tried to raise the transpiration rate in plants in order to increase calcium uptake into the leaves.
“One of the solutions mentioned in research literature is to hang a small plastic tube above each head of lettuce and blow air through it so that air is delivered onto each individual head,” he said. “This might be feasible for a small growing operation, but for a large greenhouse with thousands of heads of lettuce that would be very difficult to do.”
Working with Cornell University researchers, Both turned typical greenhouse horizontal airflow fans 90 degrees so that the air from the fans was directed downward onto the crop, resulting in an increased transpiration rate of the plants.
“We used regular horizontal airflow fans and mounted them on a different bracket so that instead of moving air horizontally, they were moving air straight down,” he said. “If the fans were placed in a uniform pattern above the crop so that most of the plants received the air flow, we saw good results in preventing tipburn.”
Both said determining how many horizontal air flow fans need to be installed to raise the plants’ transpiration rate will require some trial and error. Traditional ceiling-type fans can also be used to create sufficient vertical air flow.
“The set up works, but on occasion conditions exist that despite our best efforts to increase air flow around the plant canopy, we still saw tipburn symptoms,” he said. “We were able to prevent tipburn under most conditions, but not all the time. There really wasn’t a satisfactory explanation for why sometimes we were able to prevent tipburn and other times it occurred.
“Although we have limited scientific data to back this up, the further away from the center of the fans there would be less air flow and the air movement may not be enough to sufficiently raise the transpiration rate to overcome tipburn. But where that location is in a particular greenhouse will require some experimentation with where the fans are placed, the distance between the fans and the plant canopy and how many fans are installed.”
Both said the fan set up was trialed with lettuce crops grown in troughs and in a floating production system.
“We have used the fans with both types of production systems and they worked equally well in preventing tipburn,” he said. “In a floating system, plants often experience a variety of conditions because the plants are usually pushed through the entire system from the seedling stage to the harvesting stage. In the case of troughs, the plants are usually stationary, but there could be more air movement between the plant rows because troughs are typically elevated. In either system, there is usually more air movement around the plants when they are smaller in size. Tipburn often becomes an issue when the plants have reached a larger size.”
Maximizing growth with supplemental light
In developing the fast crop production system for lettuce, Cornell researchers used both natural and supplemental light to maximize growth. High pressure sodium lamps were used to provide supplemental light.
“We tried to achieve a daily light integral of 16-17 moles per square meter per day during the entire production cycle,” Both said. “If the 16-17 moles were reached using natural light, then the lamps wouldn’t come on. If it was cloudy and we couldn’t achieve that light level with just natural light, then the HPS lamps came on to provide supplemental light.
“If we stayed at or just below this daily light integral number and provided vertical air flow, we were able, in most cases, to prevent tipburn. If we went above this daily light integral in order to try and push the growth of the plants even further, we were able to grow the plants, but tipburn occurred in many cases even though vertical air flow was used.
“We could have grown the lettuce at a lower light level and prevented tipburn, but it would have taken longer to finish the crop. This is also an economical consideration, because at a lower light level a grower wouldn’t be able to turn as many crops and thus would make less money.”
Substituting supplemental carbon dioxide for light
Another option that was studied to keep plants growing quickly was to use less supplemental light and to increase the amount of carbon dioxide in the greenhouses by a process called carbon dioxide enrichment.
“The grower would reduce the amount of supplemental light and increase the amount of supplemental carbon dioxide,” Both said. “A grower can easily manipulate the carbon dioxide level by releasing pure carbon dioxide gas. Using this technique, a grower might provide a daily light integral of 12-13 moles per square meter per day and increase the carbon dioxide concentration to 1,000-1,200 parts per million (approximately three times the ambient concentration) and would still be able to finish a crop in five weeks.
“A grower could choose whether it would be cheaper to pay for the electricity to run the supplemental lighting system or if it would be cheaper to add supplemental carbon dioxide. In our research, there wasn’t a difference in the amount of tipburn when plants were grown at a lower light level and a higher carbon dioxide level. The vertical air flow system would still need to be used when growing at higher carbon dioxide concentrations and lower supplemental lighting levels.
“I would expect that it would be cheaper for most growers to increase the carbon dioxide concentration than it would be to increase the daily light integral using supplemental lighting. However, if a grower decided to grow the plants at 16-17 moles per square meter per day and increase the carbon dioxide concentration, the plant growth rate would increase, but tipburn would occur sooner.”
For more: A.J. Both, Rutgers University, Department of Environmental Sciences, BioEnvironmental Engineering;
(848) 932-5730; firstname.lastname@example.org;
For more information on the production of controlled environment agriculture hydroponic crops, including lettuce, see http://www.cornellcea.com.