Abstract
To set-up afforestation and reforestation projects in arid regions southern Tunisia, several indigenous and exotic forest species were used among them are Acacia spp. However, the success of these projects remains highly sceptical because of the intensified aridity during the last decade. To overcome this issue, the selection of genotypes resistant to severe drought is crucial as first step. For this reason, the aim of the present study is to compare tolerance capacity to severe drought between two Acacia species (A. cyclops and A. salicina) and evaluate efficacy of their biochemical responses at leaf and root levels. Combined physiological and biochemical approaches were adopted. Two-years-old plants of two Acacia species (A. cyclops and A. salicina), frequently used in forestation projects in arid regions southern Tunisia, were subjected to severe water stress by withholding watering during 60 days. At regular intervals, water relations and net photosynthetic rate (P-n) were measured. In addition, the biochemical response was characterized by quantifying one sugar alcohol (arabitol) and three cyclitols (myo-inositol, pinitol and quercitol) in leaves and roots. Our results revealed that A. cyclops was more tolerant to severe drought than A. salicina. The turgor of its leaf tissues and its Pn were less affected. The superiority of A. cyclops to tolerate severe water stress might be attributed to greater efficiency of its biochemical defense mechanisms compared to A. salicina. Comparison of biochemical profiles between species exhibited some differences depending on the organs and the species. For development and survival under severe drought conditions, A. cyclops accomplished efficient osmoregualation and osmoprotection mechanisms by massive accumulation of specific polyols distinctly in leaves and roots. Indeed, compared to A. salicina, A. cyclops accumulated higher amount of arabitol, myo-inositol and quercitol in roots, but pinitol in leaves. So, contents of these polyols might be used as promising criteria for the selection of drought-tolerant Acacia species.