PhD

The present study entitled “Genetic diversity analysis for nutrient efficiency
and identification of nutrient responsive genes in cassava (Manihot esculenta
Crantz)” was carried out in the Department of Plant breeding and Genetics, College
of Agriculture, Vellayani and Division of Crop Improvement, ICAR-CTCRI,
Sreekariyam, during 2019-2022. The study was undertaken to screen and
characterize N and K-efficient genotypes in cassava and identify the traits
contributing to nutrient efficiency and study their gene expression.
Thirty genotypes of cassava comprising of landraces, released varieties and
pre-release accessions from ICAR-CTCRI and KAU were taken for the study. Field
evaluation was done to identify low input nutrient efficient lines where analysis of
variance revealed significant difference among the genotypes for all the characters
studied except the girth of tuber. High GCV, PCV, heritability and genetic advance
were observed for traits like tuber yield, N and K efficiency, plant height, number
of leaves, number of branches, stem N and K content. Tuber yield was positively
correlated to N and K efficiency, plant height, leaf and stem weight, stem girth,
mean weight of tubers, number of nodes, girth and length of tubers, while negatively
correlated to tuber starch and dry matter content. Path analysis showed that tuber
length gave highest positive direct effect on yield followed by number of tubers,
number of leaves, tuber starch content, plant height and nitrogen efficiency.
Diversity studies grouped the genotypes into eleven clusters and the characters that
gave the maximum contribution to divergence were tuber dry matter content, N and
K efficiency.
Root studies on thirty genotypes revealed that root traits like number of nodal
roots, basal roots, adventitious roots, storage roots and root length displayed
significant positive correlations with N and K efficiency.
Evaluation of five highly nutrient efficient genotypes at the three levels of
fertilizers (0%, 25% and 50% of the recommended dose of fertilizers) showed
overall significance for traits like plant height, the number of leaves retained, stem
weight per plant, girth of the stem, tuber yield, length of tuber, stem K content,
tuber K content, total N and K uptake, N and K efficiency. The highest yielder was
KBH 18 followed by 8S501-2. Maximum N efficiency was observed for KBH18
followed by 15S409, while maximum K efficiency was recorded for 8S501-2
followed by 15S409. Results showed that from 0 to 25 % there is an escalation in
tuber yield as well as N and K efficiency, while the values at 25 and 50% are on
par. All the selected four genotypes showed significant superiority over the K
efficient check variety – Sree Pavithra – in terms of tuber yield, N efficiency and K
efficiency.
The study identified nine nutrient responsive genes in cassava which includes
NRT1, NRT3, NLP1, GPT2, AMT1, TAR2 for nitrogen and KUP3, KUP4, KUP8 for
potassium. Expression studies done using these genes in contrasting genotypes for
N and K efficiency raised under field conditions showed significant upregulations
and downregulations in their expression for efficient and less efficient genotypes.
Allele mining for allelic variations in contrasting genotypes (15S409 and Export
kappa) for two genes (AMT2 and NTR3) didn’t show much significant variation.
Although SNP differences were observed for NRT3 gene, further functional studies
are required to confirm this result.
The study identified three main nutrient efficient genotypes viz., KBH18,
which was the highest yielder, highest in N efficiency and third highest in K
efficiency, 8S501-2, which was second highest in yield, highest in K efficiency,
third highest in N efficiency and also exhibited early bulking and 15S409, which
was the third highest yielder, second highest in N efficiency & K efficiency. The
superior genotypes identified in the present study can be subjected to field trials for
confirming their superiority and release as a variety.