| dc.description.abstract | In an attempt to understand how trees overcome drought stress and to
establish mechanisms of stress tolerance in tropical aridland species, seedlings of
Acacia tortilis (Forsk) Hyne and Acacia xanthophloea Benth. were raised under
controlled glass house conditions in polythene pots of size l Icm in diameter and
I8cm high at The Kenya Forestry Research Institute (KEFRI) Muguga. The controls
were watered daily while the other three treatments involved withholding water for
2, 4 and 6 days respectively with one day re-hydration to container capacity after
every subsequent drought for each species.
Growth measurements showed relatively higher growth rates and total leaf
area in A. xanthophloea compared to A. tortilis under favourable moisture
conditions. A. tortilis responded, to increased water stress by shifting carbon
allocation to the roots leading to root:shoot (r:s) ratio of about l.5 compared to 0.5
for the controls, unlike in A. xanthophloea, where r:s ratio remained at 0.5 in both
repeatedly stressed and control seedlings.
A. xanthophloea seedlings, consistently had lower shoot xylem water
potentials than A. tortilis for all the treatments b.ut exhibited adjustment in cell wall
elasticity when repeatedly stressed. A. tortilis, wi~ repeated stress, showed osmotic
adjustment. Adjustments in cell wall elasticity as observed for A. xanthophloea
resulted in improved membrane integrity. A. tortilis however, showed higher
adjustment. Adjustments in cell wall elasticity as observed for A. xanthophloea
resulted in improved membrane integrity. A. tortilis however, showed higher
membrane integrity at all levels of water stress likely due to inherent drought
tolerance and adjustments during stress.
The study concluded that pre-conditioning could improve drought tolerance
in seedlings and that growth reduction as a result of water stress is likely due to
integration of several drought tolerance mechanisms which are detrimental to
growth. | en |
