POTATOHANDBOOK P is for Plant The recently published Potato Handbook, written by Professor Anton Haverkort, covers in around six hundred pages all conceivable aspects that relate to the potato. Never before has a unique and complete reference book about the nutritious tuber been published, intended for everyone in the world who works with the potato in any way whatsoever and/or wants to know more about it. In order to establish a clear line in the format of the extensive manual, Haverkort has used the formula S ociety x P lant = P ropagation x E nvironment x C ultivation as a guideline. This brings together all the factors related to the potato. To give a first impression of all the information that the Potato Handbook has to offer, in a few consecutive editions of Potato World magazine you will find a very brief summary of each chapter in the column ‘Potato Handbook’. Last time Chapter 1 was discussed, this time we will publish two short and topical excerpts from chapter 2. In the second chapter of the Potato Handbook, the author does not look at the potato from the perspective of society, but focuses on the potato plant itself. The plant’s growth resulting from its biological properties are discussed here. This chapter discusses how the plant multiplies in different ways and how abiotic and biotic factors influence the growth and quality of the tubers. For potato growers it is important to know that, in essence, the plant is not multiplied by seed, but vegetatively in the same way as cuttings. Ultimately, from a botanical point of view, the tuber is a thickened piece of stem. Diseases pass easily from generation to generation, which requires a cleansing and a seed potato programme. For potatoes, the underground parts are the most important, since these parts of the stem form stolons and tubers that grow and mature after initiation. They are then ready to cook and eat. Potato handbook Crop of the future Anton J. Haverkort Efficient use of water and drought tolerance Two relevant factors that influence the growth of the potato plant and that are discussed in the Plant chapter are the efficient use of water and drought tolerance. For growers in countries with low water availability, such as around the Mediterranean Sea, these are two factors that should always be taken into account. However, given the weather last summer, many northern European potato growers will meanwhile also be very interested in this aspect. Haverkort writes the following about this issue in the nearly one-hundred page Plant chapter: ‘The availability of water during an entire cultivation cycle hardly influences the water use efficiency of the potato crop itself under field conditions. The crop adapts its perspiration to the availability of water by adjusting its amount of evaporating foliage. When there is a lack of water, the crop drops its leaves to reduce evaporation. In pot tests, the water utilisation efficiency can increase by withholding water, because the stomata partially close. However, this is at the expense of the growth of the tuber. In practical field conditions, no gain can be expected in terms of increased tuber yield by increasing the yield per available amount of water for the crop. The crop adjusts the length of the growing cycle to the situation of the available moisture. A crop has another way of tapping into more water, namely through deeper rooting. Genotypes differ slightly in root mass and root depth, whereby 10 cm of deeper rooting provides 10 mm of extra water availability. Varieties that escape an early drought due to their earli ness benefit only once from this process. Late-maturing, deeper-rooting varie ties benefit from this process after every dry period when the soil water level recovers after rainfall. Three mechanisms to escape from drought The definition of drought tolerance is the relative yield under drought conditions compared to fully-watered conditions due to rain and/or irrigation. The potato crop has three mechanisms to optimise tuber production. These are: to escape from an increasing lack of water through early tuber initiation and early maturity of the crop, to have late maturation and the survival of a number of dry periods through leaf fall with the formation of new leaves when it rains again, and to have deeper rooting to tap into more water. With rain patterns such as wet winters and dry summers and with average, not-too-high Mediterranean widths, an early-maturing crop develops the tubers early and fills them early when there is still water in the soil from the winter and spring rains. By the time that the rains stop, the crop has matured and has reached the optimum harvest index of 0.75. The plants have then distributed 75% of all the produced dry matter to the tuber and, at an earlier 16 Potato World 2019 • number 2 Pagina 15

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