Morphological and Physiological Responses of Senegalia senegal (L.) Britton Provenances to Drought, Salinity, and Fertility

Increasing drought and salinity tolerances in economically important trees adapted to dry land areas is key challenge for maintaining the socioeconomic welfare of dry land areas. Strategies to improve drought and salt tolerance must examine the tree physiological mechanisms that link to the trees survival and growth. This study examined physiological adaptive traits allowing Senegalia senegal to grow better in both saline and dry lands. We conducted two greenhouse experiments and one field study to characterize growth, photosynthetic capacity, water use efficiency (WUE) and gum arabic yield potential among different Senegalia senegal provenances. In the first experiment, we tested early growth and photosynthetic response of seed sources to a cyclic drought treatment. The second greenhouse study examined seed source response to drought, salinity and fertility conditions. Gum yield assessment, growth and WUE of mature trees were determined from a field trial.

In general, results showed a high intra genetic variability of Senegalia senegal on gum yield, biomass accumulation and growth. Ngane provenance presented superior growth characteristics as both mature trees and seedlings and exhibited a more conservative water use strategy under drought. Both greenhouse studies revealed similar photosynthetic capacity among Senegalia senegal genotypes when conditions are not limiting. However, when factors such as salinity, fertility and severe drought are involved, different physiological and morphological responses appear and at times this was dependent on seed source. But, at moderate drought stress (chapter 2), no drought by provenance interaction was found. Results of chapter 3 revealed that Ngane has larger stomata with low density in comparison with Diamenar and Kidira provenances. With the exception of Ng21B1, all seed sources displayed similar adaptations to salt stress in term of biomass accumulation. Fertilizer increased total biomass of all seed sources from 63% to 213% for Ng21B1 and K17B19, respectively. However, salinity reduced the fertilizer effect on biomass increment. Leaf gas exchanges were affected by salinity and fertilizer within various responses among seed sources. Results of chapter 4 revealed that gum yield was found to be positively correlated with tree height, crown width, stem volume index and crown area index. Ngane and Diamenar appeared the best provenances in term of annual gum yield per hectare. Diamenar had a higher survival rate than Ngane. This finding reveals the need to consider the tradeoff between tree survival rate and individual tree gum yield in Senegalia senegal stands. In addition to being more sensitive to salinity, Ngane also seems to be more susceptible to low soil pH in terms of survival, but this result needs to be tested further.

This study suggests that improvement of gum arabic production can be possible through genetic selection. But, for the best adapted genotypes, research should explore new genetic combination and investigate physiology and genetic diversity. Moreover, the findings on the relationship between soil pH and tree survival rate suggests a need for care in selecting appropriate sites for Senegalia senegal stands. Therefore, silviculture practices as well as genetic selection appear critical in improving gum arabic production.