RE SEARCH Searching for root properties for more robust potato varieties underground (root) and above-ground (sprout) mass is then approximately 0.5:1. Only under stress do the differences become visible. In periods of drought, a potato plant produces relatively more roots and less haulm, but some varieties produce significantly more than others. Some varieties respond with up to 200 percent increase in root-sprout ratio. Others get stuck at only 15 percent’, Venema outlines. This increase in drought stress turned out, not entirely surprising, to be also strongly dependent on the greenhouse temperature. On the basis of the differences found, he selected nine genotypes, which he will study in more detail this year. ‘There are varieties with longer roots, thicker roots or that, in fact, produce more roots. Longer, thin roots are more likely to find water, but it is often a “cheap” root system; it consists of fewer cell layers and is less able to penetrate heavily compacted soils’, the researcher gives as an example. Furthermore, plants also react to abiotic stress with adjustments in the foliage. The researcher would also like to find that out, because this can be related to changes in the soil. It is a complex story, according to Venema: ‘The plant’s adaptation to abiotic stress factors is controlled by the hormone balance, which involves multiple genes. The challenge for us is to identify the so-called master switch genes; key genes that control these kinds of processes.’ Yield component What can he say in general terms about the way in which plants adapt their roots to different forms of stress? In a period of drought, a plant reacts with more root mass and therefore relatively fewer nutrients go to foliage and tuber. ‘Less leaf surface means less evaporation, but also less photosynthesis and subsequently yield. We want to know more about that relationship, because breeders have no use for a robust plant that does not produce any yield. The researcher was pleasantly surprised when he heard about the average yields that were achieved in the dry and hot year 2018. ‘Around 25 to 30 percent less than normal was harvested on plots that were not POTATO PLANT MAKES SEVERAL TYPES OF ROOT How do the roots of a potato plant develop? The plant first forms a basal root, the main root system, which grows directly into the depths and provides anchoring and moisture absorption. Its initiation can already be seen as dots on the light germs of potatoes. On the stems of the plant, the socalled adventitious roots are formed, which are thus formed higher up in the ridge. In the absence of oxygen due to flooding at the bottom of the ridge, the plant starts to compensate and its development is stimulated The primordia can already be seen as dots on the light germs of potatoes. more strongly. And then there are still the stolon roots, which arise at the joining of the stem and the stolon. The roots that the stolon makes itself are called stolon side roots. The precise functions of the different roots are not exactly clear, but our guess is that the roots on the stolons play an important role in calcium absorption, among other things. Stolon roots can also go deep. This has a positive effect on the performance of the plant. From the outset, the basal roots provide the haulm of the plant with moisture. When the basal roots are injected with coloured water in the lab, this can be directly observed in the sprouts. But the other roots can also partly take over the function of the basal root and contribute to the moisture supply of the haulm. The moment the potato crop starts filling the tubers – bulking – root growth decreases sharply and during the ripening process the plant can even partially ‘eat’ its own roots. Because of these changes, Venema focuses on the first phase of development in his research. In addition to root mass, he also wants to map out differences in the structure and composition of the roots. irrigated. That shows that a potato is a sturdy product.’ Very little is known about how a potato plant deals with excess salt in terms of root development. Even with thale cress (Arabidopsis thaliana), the model plant that has been turned inside out by plant researchers all over the world, the mechanisms are not yet precisely visible. ‘The question is to what extent the roots play a role in salt tolerance. There are examples of potato plants that make longer roots in search for fresh water. But there are also strong indications that the plant uses salt in a different way. For example by storing it in the stem, in the cell fluid of the vacuole, so that it doesn’t damage the important processes (evaporation and photosynthesis, ed.) in the leaves of the plant.’ There isn’t much you can do about too much water. Damage mainly occurs later in the season, when the tubers are under water for longer than 24 hours. Yet there are indeed differences between varieties earlier in the cultivation process. ‘There are plants in which the root system rots away almost completely, while others remain intact. Some varieties can handle it better than others, and also recover better’, says Venema. The response to the supply of nitrogen is somewhat similar to that with drought. Plants react with deeper rooting if there’s not enough nitrogen around the roots. ‘The roots have receptors, with which they can identify the nitrogen-rich Potato World 2019 • number 4 7 Pagina 6
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