Abstract

The drought adaptation of two annual legumes, Medicago minima (L.) Bartal andOnobrychis aequidentata (Sibth and Sm) D’Urv., was studied at the H₂O and CO₂efficient use level over the drought period investigating whether, besides the already known drought adaptation mechanism, annual legumes are dealing with drought by controlling stomatal and mesophyll conductance. O. aequidentata maintained higher leaf water potential and leaf hydraulic conductance than M. minima and consequently exhibited better water balance. The data revealed that these two legumes exhibit a completely different photosynthetic behavior. The photosynthetic rate in M. minima is mainly depended on stomatal conductance while in O. aequidentata seems to be depended both on stomatal and mesophyll conductance. O. aequidentata seems to be more efficient having significantly lower values of the ratio internal CO₂ concentration to stomatal conductance at the same value of leaf water potential compared to M. minima. This verifies the higher values of mesophyll conductance and consequently the higher values of carboxylation efficiency. Each one of the two species has different traits to cope with the adverse environmental conditions. The differentiation of drought adaptation mechanisms seems to be a major key for co-existence of several plants of the same life form in Mediterranean grasslands.

Key words: Photosynthesis, transpiration, stomatal conductance, mesophyll conductance, carboxylation efficiency, Medicago minima, Onobrychis aequidentata.