Roots - Tubers
The tropical origin root crops are cultivated in different climatic zones of the sub-tropics because of the fact that it needs shorter growth periods like potato and sweet potato and capable for tuberization in longer day lengths and lower temperatures. Daylength is also important but it varies between 6 to 12 hours. The short day conditions are highly favourable for economic yield of vegetables and tuber crops. The different varieties are suitable for different day length and temperature. However, adaptability in different micro-climatological regions needs suitable varieties for different environmental conditions and capacity for growing in different ecosystems. Thus physiological characters play an important role in adaptation. The day length plays an important role in tuberization of root crops. To secure good yield for Potato a day length of 12-13 hours is needed and night temperature 200C increase tuberization. With short day length and temperatures sweet potato can be cultivated in sub-tropics round the year.
Soil water is necessary for germination and establishment of crops and this applies to root crops where vegetative propagation in used. The Yams also has a high correlation between rainfall, development of vine and final yield. The sweet potato needs 50-100 cm rainfall per year and yield is reduced even when drought occurs for a month or so but water logging is also not desirable. Tapioca however withstands low rainfall of 30-50 cm. Potato needs 60-75cm but it must be evenly distributed every week during growth and development period for better yield.
It has been shown by research workers that the photosynthesis was increased with increase of CO2 concentration and photosynthesis was closely related to the potassium content of the leaves and maximum photosynthesis occurs when 4% or more potassium are available. Similarly with high leaf area photosynthesis is more.
The yield of root crops grown during summer season in sub-tropical condition is generally lower due to long days and high temperature and also the high night temperature and as a result there is slow tuber initiation and growth while high humidity increases pests and diseases. But during winter all the conditions are favourable for higher yield.
It is well known that nitrogen increases dry matter by increasing leaf area and thus there is a relationship between nitrogen content and leaf area. But in root crops high nitrogen increases top growth and leaf area, while net assimilation decreases. Root yield depends mostly on potassium. High potassium does not change leaf area index but increases net assimilation rate and thus high dry mater.
In short, the favourable conditions for higher yield of tropical root crops in the sub-tropical conditions are favourable nutrient levels, light condition, photosynthetic activity, adaptability and soil conditions for endorsement of tuberous roots.