Propagators from all over the world visited Grodan’s fifth international seminar, which was this year held in the Van Nelle factory in Rotterdam. In this historic building, which has featured on the world heritage list since June 2014, they listened to a presentation on the theoretical background of plant blindness given by Steven Groot, a seed quality researcher at Wageningen university. That was followed by a short practical. Ard Rijlaarsdam, quality manager at Monsanto, explained the strict procedures that his company uses to prevent problems such as plant blindness.

Although plant blindness, the dying of the apical meristem, was already the topic of the international seminar in 2015, the release of research data of Steven Groot and others was considered a good reason to focus on this issue once again.

The first research project, which took place between 2009 and 2013, was a joint effort of seed companies and propagators that was partly financed by the Top Consortium for Knowledge and Innovation in Horticulture and Propagating Material. Fewer seed companies are involved in the second project, which will be concluded in 2018.

Plant blindness due to high temperatures
Many different factors were considered in the search for the cause of plant blindness, such as the type of cultivar used (more or less susceptible to the problem), the priming of seed, high, or on the contrary low light intensities in combination with high or low temperatures during germination, and damage to the cotyledons.

Part of the first research project involved a series of tests during propagation. A high light intensity during propagation was found to increase the risk of plant blindness. Groot presented a diagram showing how 600 µmol light results in 70% plant blindness. This effect was most evident in the first three days after germination. The temperature was also found to be an influential factor. The effect was greater at 25°C than at 20°C. As the temperature still had a strong effect in the dark, the researchers concluded that temperature is the decisive factor.

“If seedlings are exposed to a high temperature for the first two days there will already be a risk of the formation of blind plants. We have developed tests with which seed companies and propagators can test seed samples themselves, to find out whether the seed is susceptible to high temperatures during germination,” says Groot.

Priming of seed was also found to have an effect. Every company has its own, secret procedure that apparently has different effects in terms of plant blindness. Another factor is hereditariness: some varieties are more susceptible to plant blindness than others. In the second project that is being carried out now the researchers are studying hereditary factors, conditions during seed production and the applicability of the developed tests.

There are many measures that propagators can take to prevent plant blindness, but the question is how effective they will be. “Closer study of plants has shown that plant blindness may often already have occurred without it being visible with the naked eye. By the time that the first truss becomes visible the second and third trusses, possibly afflicted by plant blindness, have already been formed, and there is no sense in taking any measures at that stage.”

The researcher showed photos to explain how a tomato plant grows. After a few leaves have been formed the growing point starts to form a cluster of flowers. The plant then continues to grow, forming a new cluster and shoot every three leaves. Such a young shoot looks beautiful under an electron microscope. It’s still quite visible under a light microscope. The people who tried separating the various parts of a plant under a microscope during the practical found that a rather difficult task. It is an acquired skill that requires quite a bit of practice. According to Groot it is certainly worthwhile to regularly check your plants’ development and look for signs of plant blindness.

Restriction and communication
Ard Rijlaarsdam, quality manager at Monsanto, talked about his company’s efforts to restrict plant blindness in the context of its general aim to supply healthy, well-germinating seed. The company uses various quality and certification systems to secure its processes, such as GSPP (Good Seed and Plant Practices) and ESTA (European Seed Treatment Assurance). This means that they inspect various quality characteristics of the seed in every step, from the seed’s production in the field up to and including its cleaning and packaging. Is the seed of the right variety and free of contaminants? Does the seed have a good germination capacity? Does it produce good plants or does something go wrong in the division of the cells in the apical meristem, resulting in plant blindness?

According to Rijlaarsdam, plant blindness may develop in different stages – in a young plant before the grafting or in larger plants, between the second and third clusters of flowers. ”Of course we make every effort to prevent plant blindness. We adjust the production of seed of varieties that are susceptible to it. And priming (a pre-germination treatment) ensures that seed will germinate faster and more uniformly,” Rijlaarsdam explained. Random tests are carried out of lots of seed before delivery, to determine the percentage of plant blindness. This is done using a UT (Usable Transplant) test. “We inspect trays of tomato seedlings in various stages. We grow the plants under controlled conditions in climate chambers and then inspect them for plant blindness and other defects. We have found that the light intensity and temperature - and an imbalance of the two - may cause plant blindness.” The company always communicates the results of its tests to its customers, the plant propagators. Seed lots that are found to have a high percentage of plant blindness are not supplied to customers.

Economic damage
In a brief calculation, Hans van Herk, propagation specialist at Grodan, showed how much economic damage plant blindness occurring after the second leaf may cause in practice, taking the variety Merlice as an example. “Let’s assume a total of 25000 plants per ha and 10% plant blindness. If each plant is missing two trusses we arrive at 5000 trusses of five fruits each, which together weigh 0.9 kilos. The total loss then amounts to 4500 kilos at € 0.50/kg. The total costs consequently amount to 10% of the total plant costs per ha. So there is every reason to take this problem seriously.”

According to Van Herk another cause of plant blindness is excess assimilates at an early stage. He is of the opinion that removing some of the second leaves results in more balanced shoots, less elongation of the part of the stem between the leaves and also less risk of plant blindness because there are no excess assimilates.

Latest developments
A Grodan seminar would not be complete without an impression of the latest developments, such as the NG 2.0 technology in plugs, blocks and slabs. “This new technology makes the initial wetting of the stone wool faster and more efficient. The water is uniformly distributed right up to the top of the block or slab. And the resaturation capacity is also better. These characteristics make it easier to promote generative development, Van Herk explained.

He has noted an increase in the demand for larger planting blocks in the Netherlands, especially in September, from growers who grow their crops with lighting. In the winter the demand for the standard size is greatest. In summer, a larger volume is used to ensure a greater buffer, to minimise the vegetative effect of the water dose. Other risks, such as those of the plants falling over and less firm stakes, must also be minimised.

The blocks with smaller planting holes have great potential due to the better connection between the plugs and the blocks, promoting faster root growth into the blocks.

A visit to Monsanto and Beekenkamp
The next day the foreign propagators visited the breeding and seed company Monsanto in Bergschenhoek and the nursery Beekenkamp Plants in Maasdijk.

After a short introduction to Monsanto the propagators were given an impression of the cleaning, disinfection, priming and coating of seed, and how germination tests are carried out in order to evaluate the seed’s health and germination capacity.

Dressed in hygienic protective clothing and wearing safety spectacles they were granted a look behind the scenes. A few seeds are selected at random from every lot and they are then sown and propagated under standard conditions. It’s important to obtain the most accurate possible impression of the seedlings during propagation. Monsanto has invested in 2D and 3D imaging equipment in order to be able to screen the young plants more accurately and more efficiently during their growth. This development is still in a test phase.

The Beekenkamp nursery is a family business comprising three divisions. One of the divisions focuses on propagating young plants for the production of vegetables and ornamental plants, another concentrates on breeding chrysanthemums and the third one on packaging. The company has several branches in the Netherlands and other countries. It produces around 350 million young vegetable plants a year. One of the branches is in the Dutch town of Maasdijk. Like the other branches it has concrete floors, insect screens in the ventilation windows, lighting and sunscreens. The temperature is controlled to reduce the risk of plant blindness. Almost literally amongst the young plants the visitors engaged in a lively discussion about light intensities, moisture deficit and the EC of the irrigation water and the blocks. Next, the group visited the new build at ‘s-Gravenzande, where a greenhouse with an area of 6 ha is being built, which will be taken into operation on 1 November.

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