Growth features of Acacia tortilis and Acacia xanthophloea seedlings and their response to cyclic soil drought stress

Seedlings of Acacia tortilis (Forsk) Hyne and Acacia xanthophloea Benth. were raised under controlled glasshouse conditions. Control plants were watered daily while other treatments involved withholding water for 2, 4 and 6-days with 1-day rehydration to container capacity. Compared to A.tortilis, A.xanthophloea seedlings showed higher leaf area, relative growth rates and total dry weight production under adequate water supply conditions. However, with increased water stress, A. xanthophloea seedlings could not alter their pattern of carbon allocation, retaining their root:shoot (r:s) ratio of about 0.5. By comparison, A. tortilis seedlings shifted carbon allocation to the roots, leading to a r:s ratio of 1.5 in water-stressed seedlings, compared to 0.5 in the control plants. The ability of A. tortilis to reallocate carbon to the roots away from the shoots and to actually increase root growth compared to A. xanthophloea was a dehydration postponement strategy that may be important in species survival during drought.