| dc.description.abstract | Sweet potato is an important food crop in Kenya not only
because it is a good source of carbohydrates, vitamins
and minerals but also because it fits very well in the
existing cropping system in Eastern Province of Kenya
which is mainly semi-arid except for some hill masses
which receive more rainfall. There is however only
limited information on the adaptability and performance
of either the local or improved cultivars over a wide
range of environments. A study on yield stability was
therefore undertaken to identify the cultivars having
specific or general adaptation.
The study was conducted during short rains 1986-1987 and
long rains 1987 in eastern Kenya. Ten varieties, six
of which were exotic introductions, three local land
races and one hybrid selection made at Katumani were
grown at five environments, Katumani, Kitui, Kampi ya
Mawe, Kabete and Kiboko. Of these environments,
Katumani, Kitui, Kampi ya Mawe and Kiboko are in
agroecological zones 4' and 5 receiving limited and
irregular rainfall. Kabete is in agroecological zone 3
and receives adequate rainfall. The experiment was
planted in a randomized complete block design with
four replications. The stability for yield was studied
usLng the methods developed by Finlay and Wilkinson
(1963) and Eberhart and Russell (1966).
Genotype x environment interaction for tuber weight,
vine weight, mean number of tubers per plant, mean vine
length, percent dry weight of vines and tubers were
highly significant. The genotype x season interaction
was also highly significant showing the importance of
testing the genotypes over different seasons. The
genotype x season x environment interaction was highly
significant for tuber weight vine weight, mean number of
tubers per plant, mean vine length and percent dry
weight of tubers and vines.
Using Finlay and Wilkinson's (1963) and Eberhart and
Russell's (1966) stability models it was possible to
categorize the different varieties into different
groups using the stability parameters. Varieties KSP3,
KSP17, KSP23, KSP21 and KSP102 had average stability for
tuber yields with regression coefficients of
approximately 1.0. Varieties KSP15, KSP20 and KSP97 had
above average stability wit regression coefficients
above 1.0 while varieties KSP11 and KSP135 had above
The exotic cultivars were relatively higher yielding
than the local land races and had better tuber yield
adaptation over a wide range of environments compared to
the local varieties. The exotic varieties KSP21, KSP17,
KSP102 and KSP3 showed good general adaptation while
some exotic varieties KSP15 and KSP20 showed specific
adaptation to the better endowed environments since they
had below average stability.
Both the genotypic and phenotypic correlation
coefficients between tuber weight and mean number of
tubers were positively correlated. The correlation
coefficient between vine weight and mean vine length to
fresh tuber yield was either very small or negative
indicating that these two traits only played a
negligible role in fresh tuber yield production. The
direct path values between mean number of tubers per
plant and fresh tuber yield were high confirming the
role of mean number of tubers per plant as a major
determinant of final tuber yield in sweet potato
cultivars. | en |
