Some Aspects of Water Relations of two Mahogany Species

ABSTRACT

The general distribution, growth form and economic importance of Kha%a senega!sir (Bear) A. Juss. and Khaya ivorensis A. Chev. are described. Some aspects of the water relations of seedlings of these two species, the former a savanna species and the latter a forest species were studied with the view of ascertaining whether moisture plays an important role in determining the pattern of their distribution. &rowth of seedlings under four soil watering regimes namely - -0.3 (A), -0,4 (B), -0.8 (C), and -4.5 (D) bars; and under culture solution and culture solution to which polyethylene glycol was added to give the following osmotic potentials (bars): -0.3 (A), -2.8 (B), -5.3 (C), aid -10.3 (D) was studied. The experiment was done in the g r e e n h o u s e . G-rowth of K. S e n e g a lensis was more sensitive to moderate moisture stress and less sensitive to high moisture stress. K, ivorensis on the other hand showed less sensitivity to moderate stress but high sensitivity to severe stress. However, when soil moisture stress was -0.3 bars, growth of K. ivorensis was very poor. This was attributed to a reduction in root permeability due to poor aeration as a result of more permanent near saturation of the soil. Studies of diurnal patterns of plant water status were carried out by examining leaf relative water oontent, (R.W.C.) leaf water potential (L.W.P.) and stem diameter variation, in relation to soil moisture stress. R.W.C. was overall higher in seedlings of K. senegalensis than in those of K. ivorensis. L.W.P. on the other hand was lower for IC. senegalensis than for K. ivorensis seedlings. Stem shrinkage decreased with decrease of soil moisture content from 100 to about 50%, field capacity. The decrease in K. senegalensis was greater than in K. ivorensis-. However at 27% field capacity, shrinkage in K. senegalensis was consistently reduced more than in K. ivorensis. This may indicate better water conservation by the former species. Transpiration was also studied in relation to the soil moisture treatments, both in the greenhouse aid in the research room, the latter being a semi-controlled environment where temperature, relative humidity and light intensity were precisely known. The transpiration of seedlings growing in osmotic solution was also studied in the greenhouse, employing stresses of -0.3 and -10.3 bars. Transpiration generally decreased with moisture stress in the root medium. In the research room transpiration of K. senegalensis was higher than that of IC. ivorensis under all soil treatments. In the greenhouse, however, similar higher transpiration rates were recorded for K. senegalensis seedlings than for K. ivorensis seedlings when stress was from -0.3 to -0.8 bars, but at severer stresses (-4.5 snd -10.3 bars) transpiration of K. senegalensis was reduced more than that of K. ivorensis.