POTATOHANDBOOK Contributions to potato yield Relative contributions to crop yield 50 100 40 80 30 60 20 40 below the threshold value. Only on the basis of long-term temperature data is it possible to determine for each place on earth when a potato crop can be grown between planting and harvest. In regions with a prohibitively cold winter period, farmers have a frost-free period and in regions with extremely hot summers, the high-heat period is excluded as being unsuitable for growing potatoes. 10 20 0 Subsistence farming Rainfed crop Irrigated crop Environment the crop is grown in Propagation material Environment Management 0 Subsistence farming Rainfed crop Irrigated crop Environment the crop is grown in Propagation material Environment Management The influence of environmental factors such as temperature and rainfall predominate in subsistence farming. change outside the field during the growing season. Once planted, the crop is subjected to them: temperature, solar radiation, day length and the gases in the atmosphere. There are various sources of weather information and climate data (long-term weather) with an increasing degree of aggregation. These are meteorological stations placed in the field and weather reports from local, regional and national weather services. Finally, global data is available based on satellite observations. The closer to the potato field the observation takes place, the more relevant the weather data are for the potato grower to make decisions. More general, long-term climatic data are at the basis of risk analysis and the exploration of potato yield and quality. Temperature as the most important development factor In addition to the photo period, the temperature is the most important factor that determines the development of the potato crop. Higher temperatures lead to earlier crop emergence after planting and faster initial development of the leaf. If the average day and night temperatures are below 5°C, it is too cold for crop growth. In a temperate climate, growers wait with planting in the spring until it gets warmer. At the end of the season in autumn, they harvest in good time for fear of low temperatures, which ultimately endanger the crop. The frost-free seasons in continental temperate climates, such as those in the eastern United States and Canada and northeastern China and in Hokkaido in Japan, are shorter than those of the maritime climates. North-western Europe also benefits from a warm ocean current, the Gulf Stream. With average day and night temperatures above 28°C, the crop stops growing. This means that for the subtropical lowland winter crop, arable farmers are looking for cool niches with relatively low temperatures to grow their potato crops. In a Mediterranean climate, farmers plant their crops as soon as the risk of frost has disappeared in the second half of the winter. They harvest their potatoes before prohibitively high temperatures occur in the early summer. A second season is possible in autumn, as soon as the summer temperatures moving towards autumn have dropped Temperature sum has more meanings Crop development, especially the early stages of emergence and early leaf growth, is inter-related and can most easily be explained by the temperature sum. Also, the physiological age of tubers is often expressed as the temperature sum, from a certain event such as tuber initiation, haulm killing and storage. A temperature sum usually has more significance than the number of calendar days, because physiological processes depend on the temperature. The germination speed of the tubers, for example, is faster at a higher temperature. The temperature sum (Tsum) takes this into account. The temperature sum calculates the average daily temperature (Taverage) that is derived from the daily maximum temperature (Tmax) and the minimum temperature (Tmin). Taverage=(Tmax+Tmin)/2. Cumulative temperature of the Tsum at the time T of observation is the sum of the average daily temperature (Taverage) with the increase in time from the emergence of the crop, day 1 (or any other temperature-related event like germination after the harvest). Tsum = Taverage day1 + Taverage, day2 + Taverage, day3 + ...... The Tsum is expressed in degree days, dd or °Cd. For example, if the storehouse is cold in winter, it will take longer for a tuber to germinate than when it is warmer. Suppose it takes 800°Cd for a tuber to germinate from when the storehouse is filled. This is achieved after 200 days when the temperature in the storehouse and the tubers is 4°C and after 80 days at a temperature of 10°C. ● Potato World 2019 • number 4 17 Crop yield (t/ha) Crop yield (t/ha) Pagina 16
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