Studies on Some Drought Resistance Characteristics of Wild Coffee Populations

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Addis Ababa University


Volumetric soil water content, soil water potential (SWP), leaf water potential (LW?) and chlorophyll fluorescence were measured to evaluate physiological responses and adaptability to water deflcits in the wild arabica coffee populations along climatic gradients in the southeastern (SE), South and Southwestern (SU] of Ethiopia. The sites selected were Harena (Bale) forest ji-om the SE, Bonga and Shekoforestsji-om the South and Yayuforestji-om the SWregions. Each of the four sites was sub divided in to three sub sites that represent different coffie accessions (sub populations or land races). Therefore, a total of 12 coffee accessions or sub populations were studied in situ. The soil moisture at two distances ji'Oln the individual coffee trees, wet and dly soil moisture contrasts as well as diurnal variations were measured and analyzed. At the same time, soil moisture deeper than the slilface layer was also measured. The measurement of soil water potential took place at three depths during the wet season but at one depth in the dry season. In the case of leaf water potential, both predawn and midday as well as diurnal changes were recorded in the d,y season but only midday values were recorded in the wet season. Diurnal chlorophyll fluorescence yield and light responses were measured for light adapted leaves. Comparisons were made between leaves of three age groups (older, medium aged and younger), wet and dry seasons as well as shade and sun exposed coffee trees. The wet season in Bale differs ji-om that of the three sites (Bonga, Sheko and Yayu). It was dry in Bale during the two periods of measilrements. Therefore, wet-dlY comparison does not include Bale. Almost similar soil moisture content and soil water potential values were found in Bonga, Sheko and Yayu during the wet season. But considerable variation was found in most of the subsites during the dly season and maximum and minimum values were recorded in Bonga 2 and Sheko 3 respectively. When the soil moisture values of all the sites were compared based on the mean values of their sub sites, Bonga and Yayu had the highest and lowest soi/moisture content ill the d,y season, respectively. Minimulll and maximum range of wet and d,y seasons' soil 1 moisture content was also found in Bonga and Yayu respectively. Bonga had the highest predawn leaf water potential (LWP) followed by Yayu. The highest and the lowest Midday LWP values were recorded in Bonga and Sheko respectively. The highest and the lowest diurnal ranges of LWP were recorded in Yayu and Bale respectively. Based on their LWP values, Yayu and Sheko coffee populations seem to be drought avoiders (by higher re-saturation capacity) whereas, Bale coffees seem to be drought tolerant. Most probably, in Bonga (the wettest site), the coffee populations were not too stressed to display some drought resistance mechanisms. Significant difference was found beflVeen the wet and d,y seasons' midday LWP values. Chlorophyll fluorescence yield of coffee leaves of different age groups did not show significant difference (P=0.05). But Chlorophyll fluorescence yields of most of the sites and sub sites were significantly different (P=0.05). Higher value of fluorescence yield was found in the d,y season than the wet season. Shade coffees had higher values of fluorescence yield than sun exposed coffees. The fluorescence yield was linearly and weakly correlated to both soil moisture and LWP but the correlation to leaf water potential was relatively higher. A linear, somewhat strong correlation was found between soil moisture and leaf water potential. The strongest correlation was found beflVeell soil moisture content and soil water potential. The grand mean soil moisture of all the coffee forests for the wet and d,y seasons were 47.91% and 17%, respectively. Concerning the sub sites, the highest level of stress wasfollnd in Sheko 3and the lowest in Bonga 2, whereas among the sites, the highest level of stress was found in Sheko and Yayu and the lowest ill BOllga for the d,y season.