Variation in drought tolerance and morphological plasticity among two provenances of Acacia senegal (Senegalia senegal) seedling in North Eastern Nigeria

• Main Author: Jibo, Abdullahi Umar
• Format: Dissertation
• Language: English
• Published: University of Aberdeen 2015
• Subjects: Acacia senegal

One of the most serious environmental stress factors limiting plant growth and productivity is drought. To survive this adversity, plants are equipped with various means of altering their metabolism, morphology and developmental processes. Acacia senegal (L) Willd is a multipurpose native tree species that occurs throughout semi-arid Africa. Increasingly poor yield of the species in the semi-arid region of northern Nigeria may be related to shifts in climate. A series of experiments was conducted to compare morphological and physiological responses in A. senegal from the arid northern region of Nguru and, in contrast, the semi-arid more southern region of Gujba. The relationship between climatic variables and growth patterns was investigated for A. senegal in two contrasting areas in NE Nigeria. This study analyses annual tree growth pattern as way of exploring climate change and examines its dendrochronological (the study of tree rings) potential. Cross dating (the procedure of matching ring patterns among trees and wood fragments in a given area) of annual ring width growth was successful within and among selected A. senegal trees, which indicates that this species forms annual rings. The mean annual radial growth rates were 3.6 mm year-1 in Nguru and 8.13 mm year-1 in Gujba. Ring-width index showed annual patterns similar to seasonal precipitation in both provenances. A reciprocal transplant experiment was performed to investigate responses of A. senegal seedlings to changes in available soil moisture within a drought gradient in northern Nigeria. Differences occurred between provenances across all treatments: Nguru provenances had higher root/shoot ratios and tap root length than seedlings from Gujba provenance. Responses tended to be greatest in the more northerly, arid Nguru provenance. This suggests genetically controlled, adaptive plasticity between populations. A. senegal seedlings were subjected to three watering regimes in the greenhouse: well-watered, mild- and severe-water-deficit conditions. Biomass allocation to roots, stomatal conductance and chlorophyll fluorescence were higher in the drought treatments than in the control. Biomass allocation to root and stomatal conductance was highest in the Nguru population. This suggests a stronger drought tolerance in the Nguru population then the Gujba population. In an experiment using a rhizotron technique, to understand the role of root distribution and root activity at varying soil depths and how this may affect the expression of morphological and physiological traits developed during drought, A. senegal seedlings were subjected to two watering regimes in the greenhouse: well-watered and severe-water-deficit conditions. Measurements of soil water content, shoot elongation and stomatal conductance were carried out. Root distribution and number of mean lateral roots between the provenances showed a significant difference. The variation in morphological traits along an environmental gradient is reflected in the magnitude of the phenotypic plasticity response. To test if morphological diversity was related to genetic diversity, microsatellite markers were used to estimate and compare genetic variation among two provenances (Nguru and Gujba) in NE Nigeria. Contrary to our expectations, genetic diversity showed no polymorphism between Nguru and Gujba accessions of A. senegal. Evidence from these investigations indicated a higher tolerance to water deficits in A. senegal seedlings from the Nguru arid provenance compared with those from Gujba semi-arid provenance. Any genetic difference between these provenances has implications for management of this important species in a changing climate.