The earth moves around the sun in one year. The axis of the earth, however, is not perpendicular to the orbit of the sun. This is the reason why the length of the day—defined as the number of hours that the sun is above the horizon—varies at different locations of the world.
At the equator, the day length or photoperiod is 12 hours per day year-round. At the North and South poles, the sun does not set on June 21 and December 21 respectively and from March 21 to September 21, the days of the equinox, the lengths of the days are 12 hours worldwide. In summer, the longest day length at around 50° latitude North and South is some 16 hours, whereas it is only eight hours six months later. In regions where potatoes are grown in winter because the summers are too hot, the day length during tuber initiation is even shorter than 12 hours and may be between 10 and 11 hours such as in Northern India.
Potato crops grown at longer days, say at 15-hour day lengths rather than at 12-hour day lengths, have more stems. Those stems live longer and have more branches or lateral stems. They also have more, albeit thicker and smaller, leaves. The main impact of the day length, however, is its effect on tuber initiation. Originally potato is a short-day plant coming from the short-day highlands in the Andes with a day length close to 12 hours. Therefore, originally potato plants only formed tubers at short days in autumn. Moreover, current genotypes were selected to form tubers on long days of 16 hours and more. The effect of growing the same variety at longer daylengths on the moment of tuber initiation is considerable. A longer day postpones stolon initiation as well as subsequent tuber initiation and decreases the number of tubers. Once tubers are formed, the plant can allocate part of the daily growth to the tubers. This partitioning to the tubers takes place at a lower rate on longer days, especially at a higher average daily temperature of 20 °C rather than 15 °C. So the tuber growth rate is slowed down by high temperatures and long days.
Owing to delayed tuber initiation and slower tuber growth, more dry matter is invested in the haulms. Haulm growth halts already well before crop maturity so the foliar mass does not increase anymore but diminishes. If the length of the frost-free or heat-free season does not limit such late-maturing crops, it ultimately shows the highest yields.
Top photo: Potatoes grown in short day conditions of 12 hours in equatorial Africa and Asia complete a cycle from planting to maturity in 100 days. The same variety under the long days of 15 hours or longer in temperate climates completes a cycle in 160 days. Some varieties such as Bintje are more sensitive to day length than others, such the more or less day length neutral Désirée.
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