Studies on the Economic Benefits and Extent of Drought Adaptation of Apple (Malus Domestica Borkh.) Genotypes Introduced To Ethiopia
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Date
2019-01-04
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Abstract
The present study was aimed at evaluating the economic benefits and adaptability of apple
(Malus domestica Borkh.) genotypes domesticated, propagated, and cultivated in five
different locations of Ethiopia’s highlands. In addition to in depth studies on the economic
benefits of this useful fruit crop, the present thesis also focused on identifying early maturing,
drought tolerant, as well as diseases and pest tolerant genotypes with desirable fruit yield
and quality characteristics preferred by both the farmers and consumers. To this end, major
apple genotypes introduced to Ethiopia during the last century were evaluated at five distinct
geographical locations and under controlled glasshouse conditions. Eco-geographic
characterization in five selected locations (namely: Holetta, Debrebirhan, Degem, Hidabuabote
and Agena) on eight apple genotypes (Anna, Dorsette golden, Princesa, Granny smith,
Crispin, Gala, Golden delicious and Red delicious) identified specificity in the time of
flowering, fruit setting and maturity, as well as adaptability of genotypes to the environments
with respect to their chilling requirements for flowering and fruit setting. Results from field
study conducted at these locations revealed that cultivar evaluation and selection will depend
on growers’ preferred attributes such as maturity status of the genotypes (early, medium or
late), fruit yield per tree, fruit weight, size and color, the type of rootstock used and
branching habit of the scion. The present thesis found that genotypes Anna, Dorsette golden
and Princesa consistently showed early maturity and high fruit yields at all the tested sites.
Field studies conducted at Debrebirhan addressed physiological response of the eight
genotypes to drought stress, by considering different physiological traits, including plant
water relations (RWC), leaf water potential (ψw), net photosynthesis (Pn), stomatal
conductance (Gs), transpiration rate (E), as well as water use efficiency (WUE). Total
chlorophyll (Chl) content; growth performaces such as root dry mass (RDM), total biomass
(BM), total leaf area (TLA), specific leaf area (SLA) and leaf area ratio (LAR) were
determined and compared among the studied genotypes. Highly strong positive relationships
were obtain between biomass and water use efficiency (r2 = 0.92); and between biomass and
root dry mass (r2 =0.70). Drought susceptibility index identified that Anna, Dorsette golden
and Granny smith were drought tolerant genotypes. Throughout the study period, these
genotypes maintained higher RWC, ψw, WUE, Pn, RDM and low rate of Gs and E, compared
to Golden delicious, Red delicious and Royal gala.
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Conversely, genotypes Golden delicious, Red delicious and Royal gala showed higher rates
of Gs and E, hence their classification as drought susceptible genotypes. The aforementioned
genotypes were also evaluated for drought tolerance in a glasshouse to further characterize
their adaptability for drought prone areas. In addition to repeating measurements on the
physiological parameters considered for the field studies, biochemical determinations on
chlorophylls ‘a’ and ‘b’, proline, soluble sugar, lipid peroxidation expressed as
malondialdehyde (MDA) content, drought induced soluble proteins (dehydrins) and
antioxidant enzyme (AOX) activities, such as superoxide dismutase (SOD), catalase (CAT),
ascorbate peroxidase (APX) and glutathione peroxidase (GPX) were measured in leaves of
apple genotypes maintained in a glasshouse and subjected to induced soil water potential
(Ψmd) of ~ −2.75 MPa). Induced drought stress resulted in reduced RWC, leaf water
potential (LWP), Gs, E and Pn in all the genotypes studied. Under drought stress, apple
genotypes Anna, Dorsette golden and Granny smith maintained higher RWC, higher LWP
and lower SLA, compared to Golden delicious, Red delicious and Royal gala. Clearly, the
glasshouse studies confirmed the field studies that the former group of genotypes (Anna,
Dorsette golden and Granny smith) are considered as drought tolerant, compared to Golden
delicious, Red delicious and Royal gala. The overall results from the glasshouse and field
experiments showed that plant water relations (RWC, LWP) and gas exchange measurements
(Gs, E and Pn) appeared as a greater index of genotype’s tolerance or susceptibility to
drought stress, followed by the elevated activities of antioxidant enzymes.
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Keywords
Apple Phenology, Tropical Highlands, Drought Stress, Drought Susceptibility Index, Biomass, Water Use Efficiency, Leaf Gas Exchange, Proline