Tomatoes are generally prone to blindness; since 1994 the problem has been seen every year at the beginning of the tomato-growing season and still no real solution has been found. Peter Reinders of the breeding and seed company Rijk Zwaan talked about blindness in relation to seed quality. According to Reinders, blindness is not usually seed-related; only if blindness is seen immediately above the seed leaf is this the case.
Reinders named the following factors as affecting blindness: choice of variety, the amount of light and temperature during propagation and the method used for pinching the plants. It is of course the case that choice of variety often depends, at least in part, on retailer demand.
Light and temperature play a particularly large role during the initial growth period. Un-pinched tomato plants should not receive too much light in the first three weeks of growth; this is four weeks for pinched plants. Also, the screen must provide about 250 watts in the initial growth period. Once the shoots of the pinched plants are longer than 2 cm, an extra 50 watts per day may be added. In the case of artificial light, 2000 to 3000 lux between 6 a.m. and 2 p.m. is enough during the first 21 days. After pinching, six hours of constant-intensity light per day is enough. This can be slowly increased in time (two hours per day) and intensity. Reinders advised a propagation temperature of 23 to 26°C for the first four days after sowing. This can be reduced to 21 to 23°C on the fifth day, with a pre-night temperature of 17–18˚C, for a more generative plant.
The moment at which the plant is pinched also has a large influence on blindness – this can be after the seed leaf, the second leaf or the third leaf. The general advice is that there are fewer problems with blindness if the plant receives less light when it is pinched.
According to Henry Bruggink of the seed technology company Incotec, seed priming also has an effect. Priming involves bringing the seed almost to the point of germination so that germination takes place quickly and uniformly and produces a better shoot, also under stressful conditions. However, such treatment does not always produce good results. Research carried out in 1997 showed that seeds treated with the alternative primer Senso were more resistant to extreme germination conditions than seeds treated using the more common primer Presto, or untreated seeds. Germination was however slower.
Between 2009 and 2013, research was carried out at PRI in Wageningen into the influence of genetic and environmental factors on the development of the meristem in shoots. This research investigated the effects of abiotic stress on blindness. A low germination temperature (19°C) results in less stress than a high germination temperature (32°C). Also, if seeds are kept at 32°C for 24 hours, there are more problems in plants that are sown one day later than in plants that are sown four days later.
Incotec hopes to follow-up on the results of this PRI research. This will require developing a test to screen sensitivity to the presence of blind plants. Bruggink believes it is possible, with the correct priming and optimal germination conditions, to reduce blindness to a minimum level.
Advisor Ferry Klap of Horti-Advies has plenty of experience in tomato growing and propagation in the Netherlands. He gave some practical advice on how to manage blindness through the climate and vegetative growth regulation. According to Klap, about 80% of plants in the Netherlands are pinched at the second leaf. He advises creating vegetative growth conditions for the first 12 days after pinching by adding chalk to reduce light levels in the spring, summer and early autumn and using screens to ensure a maximum of 400 W, by providing light at night and on rainy days and by creating the most stable climate possible. If young shoots develop too strongly, the advice is to use chalk and two screens, to reduce CO2 and to find the right light-temperature balance.
It is also possible to manage growth by reducing the amount of leaf (LAI), and Klap gave some concrete tips on how to do this. For a plant with two shoots: remove the third leaf part after the first leaf and remove the third and fourth leaf parts after the second leaf. For a plant with three shoots, also remove the third, fourth and fifth leaf parts after the third leaf. The trick is to achieve the right amount of leaf on each shoot. This has to be done carefully in the winter due to the lack of natural light, but in the summer it is a very effective method.
Very often pinching is done at the third leaf to obtain a plant with two good, uniform shoots. The lowest shoot is then removed when it is five cm long (seven to nine days old). Shoots two and three are then more stable and, according to Klap, so are the trusses; there are more likely to be three leaves between trusses two and three and there will be fewer large flowers. However, and just as important, there are also fewer blind plants. The screens are no longer needed once the shoots are 12 to 14 days old, unless it is extremely hot, dry and sunny outside.
There were plenty of questions for Klap: about the climate levels (the temperature-light ratio), CO2 levels, water management (preferably water once a day in the initial phase), the fertilization strategy and the greenhouse space-cost issue. Early planting out helps generative development, but too late and it results in vegetative development. However, this increases the shoot uniformity and is still very important.
Hans van Herk, propagation specialist at Grodan, quickly ran through some new developments, including larger-volume blocks. The advantages include a larger growth volume, but also a reduced watering interval, for generative growth. In a test using larger blocks for sweet peppers, this increased the plant volume. The result is more uniform shoots, which makes it easier to achieve generative growth. The blocks reacts a little drier, resulting in an improved water-air ration. This in turn reduces the risk of Fusarium. All these steps can result in an extra production volume of 2¬–4%.
It used to be that a large planting hole in a block was essential for the rootstock. However, because rootstocks are now of a better quality, and in particular the roots, it is now possible to use blocks with smaller planting holes. This allows the roots to grow more quickly in the blocks, resulting in a three-to-four day advantage and more uniform batches. This is particularly interesting for the propagation of plants in areas in which it is very warm in the summer.
Also new, and mainly for tomatoes and sweet peppers, are plugs with deeper seed holes, which are funnel-shaped so that the seed rolls into the centre of the plug. Plants anchor more easily into deeper plugs, and this helps improve the stability and anchoring of rootstock. This can also improve the strength of sweet pepper plants.
A visit to a seed company and a plant nursery
The next day, the participants visited the breeding and seed company Rijk Zwaan in De Lier and the plant nursery Van der Lugt in Bleiswijk. Rijk Zwaan currently holds fifth position in terms of turnover in a ranking of seed companies and the company has undergone considerable expansion in the last ten years. Annual growth currently totals around 10%. Rijk Zwaan presents itself as a company that contributes to the global food supply and to a healthy diet.
Rijk Zwaan has several production sites, but all the seed is sent to De Lier for a thorough quality control, drying, upgrading, packaging and long-term storage. The company has a wide range of tools it can apply to ensure that it produces uniform, high-quality batches of seeds with a high germination capacity. These seeds are packed in a modified atmosphere and stored for many years. A quality test is carried out after each step in the process.
The plant nursery Van der Lugt is a real family company, in which the fourth generation is now part of the management team. Van der Lugt has worked together with WPK (Westlandse Plantenkwekerij) since 2013, and one of the new spearheads in the organisation is the Plant Factory. The aim of this Plant Factory is a more international presence. Van der Lugt is specialised in the grafting and propagation of vegetable plants. The company has an area of 12 hectares, divided into units of 4 hectares each. As well as vegetable plants, the company also focuses on the propagation of pot plants, snack vegetables and the extended propagation of cut gerbera flowers.