Effects of soil and air drought on growth, plant water status and leaf gas exchange in three Mediterranean cedar species:: Cedrus atlantica, C-brevifolia and C-libani

Three- and four-year-old potted, greenhouse-grown cedar seedlings were subjected to two different watering regimes: half received full water supply and the other half was submitted to moderate drought (50% of the full water supply). Height growth was the greatest for C. atlantica and the most-limited for C. brevifolia in the well-watered set. However, in the dry set, height growth was less affected by drought conditions for C. brevifolia than for C. atlantica. Cedrus libani gave intermediate results for both watering regimes. Moderate drought provoked a decrease in osmotic potential at full leaf turgor and a long-lasting osmotic adjustment. When irrigation was withheld completely to induce severe soil drying, gas exchange decreased and then stopped at predawn water potentials of -3.0 MPa for C. brevifolia, between -2.6 and -2.8 MPa for C. libani, and at -2.4 MPa for C. atlantica, irrespective of watering regime. For all species, the dry set showed lower net photosynthesis (A) and stomatal conductance (g(s)) than the plants in the well-watered set. A and g(s) responded to variations in atmospheric water-vapour pressure deficit (VPD). As VPD increased, A and g(s) decreased, and this trend was proportionate to initial values at low VPD, but remained independent of previous watering treatments, plant water status or species. To conclude, C. brevifolia appears to be a species with limited growth potential but strong soil drought tolerance whereas C. atlantica has strong growth potential when an adequate water supply is available but is more sensitive to soil drought. C. libani shows an intermediate behaviour for growth and drought tolerance.