Negash, Legesse (Professer)Melke, Abayneh2019-05-062023-11-092019-05-062023-11-092019-01-04http://etd.aau.edu.et/handle/123456789/18189The 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. V 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.enApple PhenologyTropical HighlandsDrought StressDrought Susceptibility IndexBiomassWater Use EfficiencyLeaf Gas ExchangeProlineThesis