In the past the use of rootstocks represented an important step in the fight against soil-borne diseases. But rootstocks offer many more possibilities: Improved vigour, setting, flower and fruit quality. And more resistance to stress or low temperatures. With respect to saving energy, the latter is an interesting option and is faster to achieve than by breeding.
Grafting allows you to combine the desired properties of the rootstock with those of the graft. When growing in soil, resistance to soil-borne diseases is an important reason to choose a rootstock. As growers have switched to substrates this argument no longer holds true for many crops and the use of grafted plants has decreased. However, time has shown that substrates are not as sterile as at first thought and that persistent soil-borne diseases can occur here too. Therefore resistance to microbial diseases remains a reason to choose a rootstock. Over the course of the time many other reasons have been added.
10% increase in production
Rootstocks are back on the agenda again, first and foremost to boost production. This, for example, is an important reason to seriously consider using a rootstock for roses grown on a substrate. In this case production can rise by some ten per cent. The grafting of tomatoes offers better resistance to different diseases and thus greater security over production. In addition, these grafted plants yield more.
But rootstocks offer many more possibilities. Production at a lower temperature, improved fruit quality, and improved resistance to heat and drought stress. Nevertheless, they are still used very little; the emphasis remains – also in research – on improving yield. But this is changing: The last few years have seen plenty of developments. In an international context, research is being carried out into the mechanisms whereby the rootstock influences the processes that take place above ground and how the genetic mechanisms function.
Greater tolerance to cold
Greater tolerance by fruit vegetables to lower temperatures would be very attractive. It would also allow further energy savings to be made and increase the crop’s resilience. Now, for example, a tomato crop that has encountered a temperature that is far too low (under 12ºC) for some time hardly recovers.
The roots, however, appear to play an important role in tolerance to lower temperatures. Photosynthesis continues as usual when it's cold, but the development of the leaves deteriorates. This is due to the poor absorption of water and nutrients and a disturbed hormone balance. So it makes sense to select species with a root system that functions better under cold conditions. For this it is often necessary to return to the wild varieties. Grafting tomatoes onto Solanum habrochaites is a way to achieve better growth under cold conditions. In cucumbers the fig-leaf gourd (Cucurbita ficifolia) is a good example.
Larger root system
The types of rootstock that are best suited to achieving tolerance to low temperatures appear to have a number of features in common. Quite simply, when it becomes cold they produce a larger root system than rootstocks that are more sensitive. Also, research on the fig leaf gourd shows that uptake of nutrients is better. And, the uptake of phosphate by tomatoes drops considerably at low temperatures but this is not a problem in wild varieties, according to trials.
If a crop of fruit vegetables has had far too much cold the leaves wilt. This clearly points to a water absorption or transport problem. This problem is much less apparent with cold tolerant rootstocks, due primarily to the more extensive root system and the longer roots. In addition, they are more capable of suppressing the formation of free radicals (which affect the cell membranes) during cold stress. Finally, the hormone production is also different. Cold tolerant roots, for example, produce more cytokinins. These hormones stimulate the root meristem and ensure that sufficient assimilates are drawn to the roots.
On the other hand, theoretically, the rootstock should also play a role in the crop’s resistance to heat stress. The formation of the hormone ethylene is partly causing the lagging root growth when it is hot, as well as damage by free radicals. There are undoubtedly differences in ethylene production and free radical formation between the different rootstocks, but research findings so far offer little perspective. However, it has been possible to increase eggplant production by 10% under hot conditions by grafting onto a heat tolerant rootstock.
Fruit quality can also be influenced by the choice of rootstock. This includes appearance, firmness, taste and healthy ingredients such as vitamins. In general, the external fruit quality (size, colour, shape) improves with greater vigour as a result of the chosen rootstock, so you don't have to select specifically for this.
Firmness is a property that is more complicated. This depends on the shape of the cells, cell turgor, composition of the cell wall and its chemical properties. All these features are influenced by the hormone systems, water and nutrient uptake. Trials have shown that grafting has a positive influence on the firmness of watermelons and cucumbers while courgettes grown on rootstocks were actually softer.
The taste of a fruit is a combination of acids, sugars, volatile substances and other components. Based on international research it is hard to say to what extent this is dependent on the rootstock. There is a more positive picture for the healthy substances, such as vitamins and lycopene. It appears that in watermelon, cucumber and tomato the levels are indeed sometimes determined by the chosen rootstock. But before a grower can start to steer the crop based on this a lot more research is needed.
The conclusion is that rootstocks certainly offer fruit vegetables perspective in the search for more energy-efficient or cold tolerant production. The impact of rootstocks on fruit quality is still a largely unexplored area.
The ultimate rootstock doesn’t exist
If you inoculate a graft with the best above-ground characteristics onto a rootstock with the best under-the-ground characteristics you’ll have the best plant. Right? Unfortunately it’s not that simple.
It all cases it’s about the combination. Characteristics of the graft and the rootstock can reinforce each other but they can also work against each other. In the worst case they can even be incompatible. (see page 46).
Usually trial and error leads to the best combination. For example, some rose rootstocks produce lots of cytokinin. If you graft onto this a variety that has difficulty developing, it has a positive effect. The graft forms more flowers. But if you graft on a variety that easily develops then you’ll have too many flowers. Sweet peppers that have a problem with setting would very much welcome a good rootstock that encourages setting.
There is renewed interest in rootstocks when growing on substrate, first and foremost to increase production. However, rootstocks offer more possibilities. There certainly provide opportunity to improve cold tolerance and thereby save energy.
Text: Ep Heuvelink (Wageningen University), Dietmar Schwarz (IGZ Grossbeeren) and Tijs Kierkels. Images: Henk Bouwman and Eric van Houten.