Tip burn, or marginal necrosis, is a long-standing problem in tomato cultivation which occasionally rears its head in the new strategy of Next Generation Growing (NGG). What would be better than to solve this phenomenon with properly adjusted fertilisation? Dutchman Wim Voogt has investigated the influence of chlorine and iron polyphosphate on calcium uptake. To date, appropriate climate measures combined with the right variety choice have proved to be the best remedy.
Calcium, the mineral that is essential for good cell development, has an annoying trait: it can’t move through the plant of its own accord. This nutrient is completely dependent on water to transport it through the plant.
Calcium deficiency causes physiological defects such as internal browning in lettuce, tip burn in tomatoes and ornamentals, or even fruit problems such as blossom-end rot. These phenomena are directly associated with weak cells that have absorbed too little calcium as a nutrient during growth. Applying elements of NGG seems only to make the problem worse. That’s annoying, because an indirect consequence of this defect is that it makes the plant more susceptible to fungal diseases such as Botrytis.
Wim Voogt and his colleagues at Wageningen University & Research in the Netherlands have been working for many years on understanding the plant mechanism and the causes of deficiencies. Much is already known but new insights arise constantly.
Voogt draws on some basic principles that keep popping up in this context. “Calcium is a tricky element,” he explains. “The plant can only absorb it passively. Young, vigorous parts of plants need it to build cell walls and cell membranes. A plant cell is like a bicycle tyre. The cell wall is a rigid structure made of cellulose, pectin and hemicellulose. Calcium binds with the pectin chains in it. This makes the framework flexible.”
Calcium is transported to all parts of the plant via the tips of young roots and via the xylem vessels. This mainly happens during the day when the plant transpires, and to a lesser extent at night through root pressure. Young plant parts need calcium to strengthen their cell walls as the cells expand. But they are completely dependent on the calcium supplied via the xylem vessels. Once the mineral has arrived at its destination via the transpiration stream, it stays put. So parts that transpire the most get the most. That is why parts of the plant that are not (yet) transpiring enough don’t get enough calcium and suffer a deficiency, with the result that the newly formed cells are not strong enough.
Voogt: “So what you need to do is to stimulate xylem transport to growth points. You can do this in two ways: by facilitating transpiration or by ensuring sufficient root pressure.”
Weak cells are therefore the main problem, caused among other things by calcium deficiency. The real problems arise at times when root pressure is too high or when climatic conditions suddenly change. This will cause weak cell walls to burst or cell membranes to leak, resulting in the familiar physiological defects.
Last year Voogt took leaf samples from an artificially lit tomato crop to study the mineral balance in the varieties Komeett and Brioso. The first is susceptible to tip burn whereas the second isn’t. He took samples from a leaf at the top next to the youngest truss which had not yet flowered, a leaf next to the most recent fully set truss, and an old leaf at the bottom of the plant. These showed that there was hardly any difference in calcium levels between the two varieties, meaning that the susceptibility of varieties is not directly connected with these levels.
What was striking was the massive increase in calcium levels as the leaf ages. The researcher also noticed that the calcium levels in the youngest leaves (at the top) fell in winter, but also as the plant load increased. Every grower knows from experience that tip burn occurs mainly between January and March. The samples confirmed this, since calcium levels in the foliage at this time of year are a lot lower than in the late spring and summer. In June the youngest leaves contain more than twice as much calcium as in December. An old leaf contains ten times more calcium than a young top leaf, both in summer and in winter. And that’s quite a difference.
Modified nutrient solution
The study also focused on the question of whether calcium uptake could be stimulated by adjusting the nutrient solution. As it had previously been observed that chloride in the nutrients can improve calcium uptake, the NO3/Cl ratio was adjusted. In addition, a Fe-polyphosphate treatment was also applied because it had previously caused less leaf curl in cucumbers.
Compared with a standard nutrient solution, the effect both modifications had on tip burn was quite disappointing, with the researchers seeing no difference in the number of affected leaves.
With this in mind, Voogt decided to take another look at the climate settings, which have more effect on the development of tip burn than other measures. NGG sometimes leads to situations that tend to exacerbate the problem instead of preventing it.
NGG was originally conceived as a more energy-efficient method of growing. So minimum pipe is often dispensed with, RH can be higher and the humidity deficit is low. What’s more, greenhouse air contains a lot of CO2 so the stomata are not fully open. Due to the low levels of transport via the xylem vessels, not enough calcium reaches the new growth so the cells are weak. If this is followed in spring by a situation requiring ventilation, a high humidity deficit will suddenly occur. The leaves start to transpire and the root pressure can’t keep pace. This shock puts the plant under stress.
The trend of lighting the greenhouse for ever longer periods of time results in plants getting fewer hours of night-time rest. This allows less time for enough calcium to reach the new growth, as night-time transport happens partly via root pressure. As a result, the parts of the plant that don’t transpire much during the day get less calcium. These are the young shoots and leaves where new cells are made and where cell expansion occurs.
In addition to the humidity stress referred to earlier, internal moisture stress (root pressure) can also sometimes be so high that weak cells can’t take the stress and burst. This sounds like a contradiction in terms, since root pressure is supposed to prevent cell weakness by supplying the cells with sufficient calcium, but it happens because the period of weak cell development does not run in parallel with the occurrence of the symptom, i.e. cells bursting.
Voogt: “The main conclusion that can be drawn from this is that transitions in climate must take place very smoothly. Fortunately, the focus in NGG is turning increasingly towards optimising the climate, with plant balances and the water balance being the key criteria. That’s much better for the crop than simply aiming to save energy.”
Variety selection and climate are the main tools for preventing tip burn, since adjusting with nutrients hasn’t been shown to help improve calcium uptake. Young plant parts are particularly vulnerable, and using NGG can make a crop even more susceptible to physiological defects. Avoiding moisture shocks is the best remedy.
Text and image: Pieternel van Velden and Wageningen University & Research.