Comparison of growth,leaf water relations and gas exchange of Cistus albidus and C. monspeliensis plants irrigated with water of different NaCl salinity levels

Four-month-old potted Cistus albidus and Cistus monspeliensis plants growing in a greenhouse were submitted to saline stress from 9 August to 2 December, using irrigation water containing 0, 70, and 140 mM NaCl. C. monspeliensis plants are more tolerant to saline irrigation water than C. albidus plants, mainly due to their capacity to resist stress with a lower plant biomass and canopy area; furthermore, they showed no leaf necrosis symptoms. Under saline stress conditions the main growth limiting factor in both species was photosynthesis. Both Cistus species responded to saline stress by developing avoidance and tolerance mechanisms. The avoidance mechanisms took place at a morphological and physiological level. Morphologically, the reduction in the canopy area can be considered a mechanisms for regulating water loss via transpiration. Treated C. monspeliensis plants showed a greater capacity to absorb water and were able to conserve it more efficiently than C. albidus plants. Tolerance mechanisms included Na⁺ and Cl^? inclusion and osmotic adjustment. However, the reaction of each species to osmotic adjustment was different, because in C. monspeliensis plants the osmotic adjustment was unable to prevent a decrease in leaf turgor. The curvilinear relationship between P_n and g_l observed in C. monspeliensis plants indicated stomatal limitation of photosynthesis below a leaf conductance of about 160 mmol m^?2 s^?1. In C. albidus plants, a linear relationship between photosynthesis and leaf conductance rather a curvilinear model was significant, indicating limitation of the photosynthetic capacity.