The objective of this study was to evaluate the effects of varying irradiance on the growth andrnphysiology of Coffea arabica. Growth measurements such as stem height (SH), number ofrnbranches (NB), stem girth (SG), internode length (IL) and specific leaf area (SLA) werernmeasured, whereas, net assimilation rate, transpiration rate and water use efficiency werernmeasured for gas exchange measurements. Two Coffea arabica populations (Bale and Sheko)rnwere grown in pots placed in the greenhouse of Addis Ababa University under two irradiancernregimes, high irradiance (HI)/direct sunlight and 20% of shade (20% HI) for six months. Thernresult of this study shows that stem height, number of branches and stem girth was found higherrnfor exposed coffee plants than the shaded in both populations. However, the Bale coffeernpopulation achieved higher growth rate than Sheko coffee population under their respectivernirradiance level with respect to the three parameters. Specific leaf area in shaded coffee plantsrnwas found to be higher than exposed in both populations. But under shade environment, Shekorncoffee plants attained lower value than Bale, where, under high irradiance the reverse isrnachieved. Internode length was also higher in shaded coffee plants of Bale populations, whereas,rnit showed the same growth with exposed ones in Sheko populations. For internode length,rnShaded Bale coffee plants showed higher growth rate than shaded Sheko coffee plants. But underrnhigh irradiance Sheko coffees achieved higher value than Bale. As stated for gas exchangernmeasurements, net assimilation rate (Anet) and stomatal conductance (gs) for exposed coffeernplants was higher in both populations than shaded. But transpiration rate (E) for both coffeernpopulations under direct sunlight was found lower than under shade. Compared to Sheko coffeernplants, Bale coffees showed higher value for daily integrated net assimilation rate andrntranspiration rate under their respective irradiance level. Moreover, as a result of higher netrnassimilation rate and lower transpiration rate in exposed coffee plants, it was expressed thatrnintegrated daily water use efficiency (WUE) was found higher in exposed coffee trees in bothrnpopulations. And comparing the two populations, under high irradiance Bale coffees exhibitedrnhigher daily integrated water use efficiency than Sheko, while both populations showed similarrnvalue under shade. Exposed Bale coffee plants had higher light saturated rates of photosynthesisrnthan the shaded one, which shifted from 200 Î¼ mol Photons m-2 s-1 to 600 Î¼ mol Photons m-2 s-1.rnBut leaves of Sheko coffee trees under high and low irradiance levels were saturated betweenrn800 Î¼ mol Photons m-2 s- and 900 Î¼ mol Photons m-2 s- respectively.rnFurthermore, it is observed from the result that the coffee plants exhibited substantial phenotypicrnplasticity for most of growth and physiological traits in response to varying irradiance level;rnthereby the magnitude of plasticity differed markedly among the populations. In particular,rnplants originating from drier climate (Bale region) showed the highest amount of plasticityrnsuggesting fast adaptation to changing irradiance.rnThese results suggest that this species performs well under high irradiance light though it hasrnbeen grouped under shade adapted plants .It provides the growth and physiological responsernand reveal the difference among the two coffee populations in morpho-physiologicalrncharacteristics. This differential response to light conditions may contribute to its ability tornsucceed in natural and agricultural environments or to endure control strategies. Moreover, thisrnstudy contributes as baseline information for further research on the management and use ofrncoffee production.rnKey words: Coffee, Growth, Net CO2 assimilation, Stomatal conductance, Internal CO2rnconcentration, Irradiance Photosynthetic active radiation