How much leaf does a tomato plant need to produce a good yield? That’s the main question behind a research project, The New Crop. The trial at Wageningen UR Greenhouse Horticulture, the Netherlands, produced a surprising result because ‘bullying’ the plant by removing extra leaves led to an increase rather than a decrease in yield.
Removing leaves to produce the optimal Leaf Area Index (LAI) is ingrained within tomato growers but how far can you go? Should you primarily remove the old leaves or indeed remove more leaves at a very young stage? A plant with a small leaf surface area will transpire less and that could save energy. In the winter it helps if artificial or natural light is allowed to penetrate deeper into the crop. It certainly seems time to pay more attention to ensuring a good ratio between leaf and fruit.
Wageningen UR Greenhouse Horticulture has been running a project called, Het Nieuwe Gewas’ (‘The New Crop’) as part of the program, Kas als Energiebron (Greenhouse as Energy Supply). Researcher Arie de Gelder gives an update.
Just one leaf between bunches
Commercial nurseries with artificial lighting usually have a LAI of 3 to 4. In this trial the researchers cut it back to a LAI of 2 to 3. This was achieved by removing more leaves at an early stage. This produced a more open crop structure which they hoped would lead to better light utilisation during the winter. The trial in Bleiswijk had three sections that were planted with the variety Brioso in the second week of October. Stem density at the beginning was 2.5 per m2. Extra stems were maintained in week 50 and in week 4 so that the final density was 3.75 stems per m2. The roof covering is diffuse glass with a high haze and high light transmission.
A tomato plant produces three leaves between each bunch. In the control area one leaf was removed (33%) between each bunch so two leaves remained. In the area with the ‘open’ crop one leaf and then two leaves were removed alternatively (44%). In the area with the ‘very open’ crop firstly one leaf was removed and then two leaves were removed twice (55%). Therefore just one leaf was often present between the tomatoes.
From the bottom of the crop leaves were picked at the same bunch height in all three areas. As a result the plants kept 14, 12 and 10 leaves respectively. Brioso is well known as a vegetative variety so it was a good choice for this experiment.
Before the trial started the researchers and growers thought that such rigorous removal of the leaves (55%) would be risky. However, they were able to apply and sustain this treatment even during the transition from winter to spring. Leaf length, leaf width and plant length were accurately recorded, as were the thickness of the top, plant weight and dry matter.
It soon became apparent that the extreme pruning had a big impact on plant length. By April for example, the difference had risen to a metre. The plants in the very open section were the shortest. The leaves remained likewise shorter. De Gelder: “That isn’t a complete reflection on leaf area because it’s the width, not the length, which determines that.”
In general the tops of the stems were thickest on the plants in the standard area but up until week 12 there were many moments that the open plants were ahead. The top was usually the thinnest in the very open section There was also a large variation in LAI. For example, in the standard treatment this was 2.6 in December while in the very open section in January it was just 1.8.
Watering and fertilisation were in line with that used in commercial practice, although some growers think that the content of nitrogen in this trial is a bit high. The plants in the very open section took up less water and fertilisers. That’s logical if the plants have significantly fewer leaves. Also, the very open section had more drain.
It is necessary to mention that during the research the leaves were picked from underneath, in all sections at the same time. “It will be interesting in follow-up research to keep the leaf on the plant in the very open section for longer so all sections have approximately the same assimilating area,” says De Gelder.
Steer crop by leaf picking
Of course all eyes are focused on yield. De Gelder expected the best yield to be in the middle treatment, in the open section. After all the very open section was a very extreme treatment. However, it was somewhat different.
At the beginning, yield in the very open section took the lead while the average fruit weight, shelf life and taste were no different from the others. The difference in yield, compared with the standard section, rose to 0.9 kg/m2 early in the spring. In week 17 yield in the standard section was 19.4 kg/m2 compared with 20.3 kg/m2 in the very open section. This result is, in the very least, striking.
“We concluded, even though this is not a practical situation, that by removing leaves we can steer the crop considerably more than we thought. It might be possible to pick an extra kilo in winter,” says de Gelder.
The research began with the aim of saving energy by more efficient light utilisation and less transpiration. If the plants transpire less by removing leaves that should be reflected in energy consumption. The researchers found that differences did occur. When energy consumption was 20 m3/m2 of gas the difference between the standard and very open crop was 1 m3/m2.
Up until the end of April the crop received 1,650 hours of artificial lighting with SON-T lamps. The light intensity is 210 µmol/m2.s. The length of the lighting period is clearly under that used in practise and the light sum is partially compensated for by the intensity. The light was able to penetrate deeper into the crop due to the open nature of the crop.
The interesting results so far are an invitation to do further research into leaf removal. De Gelder already has ideas for a new research project. In addition, he would like to know if these measures also work in a normal crop without lighting. In any case, the first step to better steering the growth and production of tomatoes has been made.
Research into the extreme removal of leaves from tomato plants has yielded surprising results. Not only does it seem possible to bring forward production by removing extra leaves but it also saves energy. The challenge now will be to see if the results achieved in research units at Wageningen UR Greenhouse Horticulture can be reproduced in practise.
Text and images: Pieternel van Velden