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. |