International Journal of Horticulture 2015, Vol.5, No.17, 1
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Introduction
Tomato (
Lycopersicon esculentum
Mill) is an important
and widely grown vegetable around the world and
belongs to the family of solanacae. Tomato plants can
be grown under a wide range of climatic conditions,
they are extremely sensitive to hot and wet conditions.
El-Ahmadi (1979) also said that fruit setting in tomato
is interrupted at temperature above 26 ᵒC and 20 ᵒC
day/night and is often completely arrested at
temperature above 38/27 ᵒC day/night. According
to El-mahdy (1990), very recently, Bangladesh
agricultural research institute (BARI) has strengthened
the program for year round tomato variety development
and already succeeded to develop some heat tolerant
hybrids. Difference existed among the cultivars in
their ability to transmit their fruit setting ability under
high temperatures to their immediate hybrid progenies.
Rick (1969) said that tomato is a tropical day neutral
plant and is mainly self pollinated but a certain
percentage of cross pollination also occurs, it is a
warm season crop reasonably tolerant to heat and
drought. So far efforts of many vegetables breeders
from both public and private sector have resulted in
spectacular improvement in yield and quality
characters. As a result of these efforts, hundreds of
new cultivars have been developed since 50years to
meet the diverse needs and varied situation and
climate under which tomato is grown. The
development of a meaningful breeding programme
needs information on the nature of gene action
controlling the economic characters and other
characters of importance. Knowledge of genetic
architecture of the characters under improvement is
essential for adopting appropriate breeding procedures.
Such knowledge leads the plant breeder to develop
new commercial varieties of the crop. Hybrid bred
for heat tolerance might have better performance over
any open pollinated varieties but should be evaluated
under particular situation according to Yordanovs
(1983). Veershety (2004) stressed the information on
variation attributable to genetic difference and also on
the relationship among various quantitative traits is
fundamentally significant in crop improvement
programme. Viredelwala (1987) reported that heat
tolerance is controlled by largely recessive genes and
inherited in a complex fashion with low heritability
which are typical of polygenetic traits. They also
suggested that heat tolerance genes are easily
influence by environment. In another observation
(AVRDC, 1998), pointed out that heat tolerance in
tomato may not be as complex as had been reported
previously by Viredelwala (1987). In the face of
mounting pressure, sustainable advance in tomato
productivity and product is perhaps salutary to realise
nutritional security particularly in areas with high
temperature stress, so as to meet the ever increasing
demand for this vegetable, there is a need for
improvement and to develop superior stable and heat
tolerance varieties and also for development of hybrid
with better yield and quality. One of the methods to
improve yield and quality is heterosis breeding which
was first observed by Hendrick and Booth (1968) for
higher yield. Chaudhary (1965) emphasized the
extensive utilization of heterosis to step up tomato
production. Heterosis manifestation in tomato is in the
form of tolerance to biotic and abiotic stresses
according to Yadav (1989). The efficiency of selection
and development of improved hybrids solely depends
on the general combining ability and specific
combining ability expressed for yield and related
characters. Hence, combining ability which is
important in the development of breeding procedures
is of notable use in crop hybridization either to exploit
heterosis or to combine the favourable fixable genes.
Mahesh (2006), reported that in breeding larger fruited
cultivars for hot conditions, additional problems such
as fruit cracking, black shoulder and rough blossom
ends become major obstacle. Thus it is difficult to fix
all desired traits in a single line. According to Hari
(1997), that tomato is considered as an important
source of vitamin A, C and minerals. Apart from these,
lycopene is valued for its anti-cancer property and acts
as an anti-oxidant and scavenger of free radicals which is
often associated with carcinogenesis as reported by
Bose (2002). Lycopene may also interfere with oxidative
damage to DNA and lipoproteins inhibit the oxidative
LDL (low density lipoprotein) cholesterols. Tomatoes
are being used in sandwish, salads and processed
products like paste, puree, juice and ketchup. The
fruits are mainly consumed either raw or in the
preparation of food. Although tomato production in
Nigeria is constrained by high temperature and
relative humidity limiting it production to the short
harmatan period. The study is conducted to estimate
the magnitude of heterosis in F1 hybrids and general
combining ability on heat tolerant tomato.
