PhD

Due to global warming the risk of environmental stresses, especially of prolonged
drought, are expected to increase in the coming years. Cassava (Manihot esculenta Crantz.) is
a typical tropical plant with its distribution confined almost entirely to the tropical zone. The
crop is sensitive to soil water deficit during the first three months after planting, and water
stress during the later stages of crop growth causes reduced yield and quality of tubers. In
case supplemental irrigation is not possible, some nutrients are found to play important roles
in imparting drought resistance to the crop. With this background, the present investigation
entitled “Crop-weather-nutrient relation in cassava under drought stress” was undertaken.
Two separate experiments were conducted in the Agronomy farm of College of
Horticulture Vellanikkara (10o 31’ N latitude and 76o 13’E longitude) in Thrissur District of
Kerala. In experiment I, conducted in 2015-16, the response of four cassava varieties of
varying durations viz..Vellayani Hraswa and Sree Vijaya of short duration and M4 and Sree
Athulya of long duration, planted in May, October and December to variations in
environmental conditions. In experiment II, the effect of foliar application of potassium,
silicon and calcium along with major nutrients on short duration variety Vellayani Hraswa
grown under drought stress was investigated in two years, viz., 2015-16 and 2016-17. The
experiments were laid out in randomized block design, with net plot size of 5.4 m x 5.4 m
and three replications. The crops were raised as per the package of practice recommendation
of Kerala Agricultural University (KAU, 2011).
In experiment I, the crops were grown under rainfed condition and were thus subjected to
varying temperature, precipitation and solar radiation during the period of study. May planted
crops received a total rainfall of 2450.37 mm, whereas October and December planted crops
received rainfall amount of 2013.4 mm and 1781.7mm respectively.
Considering the general growth habit, May and October planted crops recorded
considerable greater growth compared to the December planted one. Progressive increase in
plant height was observed during all the three planting seasons, with May planted varieties
outperforming the other two plantings. Among the varieties, Sree Athulya a normal-duration
variety and Sree Vijaya a short-duration variety were significantly taller. Leaf retention and
number of leaves fallen were less for Sree Athulya planted in December planting season
compared to other seasons. Among normal-duration varieties, M4 was seen to produce greater
number of leaves than Sree Athulya throughout the growing period, whereas short-duration
varieties Sree Vijaya and Vellayani Hraswa recorded more number of leaves only towards
later stages. Values of physiological parameters were higher in May and October planted
crops with high stomatal conductance and photosynthetic activity. Sree Athulya variety
recorded higher crop growth rate for all the plantings. Cyanogenic glucoside content and
bitterness of varieties were found to progressively increase in tubers from May to October to
December plantings. For all seasons, higher yield, tuber length and girth, number of tubers
and mean yield per plant were in May. Normal-duration variety Sree Athulya recorded
significantly higher tuber length, tuber girth, number of tubers per plant and mean tuber
weight per plant, which added to significantly greater yield per plant by the variety. Among
the varieties, Sree Athulya recorded significantly higher yields followed by M4 and Sree
Vijaya for all seasons, while Vellayani Hraswa recorded significantly lower yield.
Sree Athulya recorded higher B:C ratio of 3.67 and 2.37 when planted in May and
December plantings respectively. However, short duration variety Sree Vijaya was more
profitable when planted in October.
Correlation between yield and weather parameters suggested that yield influencing
weather parameters at early crop establishment period were maximum temperature, minimum
temperature, rainfall and relative humidity. Reduced maximum and minimum temperature,
high rainfall and high relative humidity were seen to be desirable.
The results placed Sree Vijaya and M4 as the most promising varieties for moisture
stressed conditions followed by Sree Athulya. Even though Sree Athulya was drought
tolerant and produced highest yield, qualitatively it could be regarded as only the second
ordered variety since it cannot be used for edible purpose.
In experiment II, field experiments were conducted with 10 treatments. The treatments
consisted of the recommended package of practices (POP) for nutrients, POP along with KCl,
CaCl2 and silicic acid separately and in combinations. Controls included POP with water
spray and water spray alone. The total precipitation during 2015-16 was recorded to be
514.1mm. The 2016-17 cropping season experienced extreme dry condition with monthly
rainfall of only 136.3 mm during the entire cropping period.
Pooled analysis of data showed that combined application of nutrients as per package of
practices along with KCl + CaCl2 + silicic acid (T8) had significant positive impact on plant
height, leaf retention, root length, stomatal conductance, photosynthetic activity and relative
leaf water content. Significantly higher HCN content in tubers was recorded in 2016-17
compared to 2015-16.
The above treatment combination also resulted in significantly higher tuber girth, number
of tubers, mean tuber weight and yield per hectare in 2015-16. Generally, yield of crop was
comparatively low during 2016-17 cropping period due to insufficient rain. However, POP+
KCl + CaCl2 along with POP + KCl + silicic acid and POP + KCl + silicic acid + CaCl2
recorded higher fresh tuber yield. Tuber girth was highest in the treatment POP + KCl +
silicic acid + CaCl2.
Combination of all three nutrients along with recommended doses of N, P and K resulted
in significant increase in the total uptake of primary and secondary nutrients compared to
control. However nutrient uptake was limited in 2016-17.
In general, foliar application of chemical treatments along with recommended package of
practices were found to have a significant positive influence on cassava even under low
rainfall conditions.