Relative Water Content and Gas Exchange of Three Wild Pear Genotypes under Water Stress Conditions

Three pear genotypes (Nazkeh, Lasoreh and Qevilah), grown in 2l containers, were studied under four irrigation regimes in order to evaluate the leaf relative water content (RWC), photosynthesis, leaf gas exchanges and plant dry weight induced by water stress. Control treatment everyday was irrigation. Irrigation was practiced when soil water potential reached at -0.4, -0.8, and -1.2 MPa for the water stress treatments. All traits measured during water stress and two days after re-watering. The leaf relative water content (93.95%), Leaf photosynthesis rate (10.86 µmolm-2s-1), stomatal conductance (0.22 molm-2s-1) and transpiration (11.43 molm-2s-1) were the highest in control treatment. But sub-stomatal CO2 increased in -0.8 and -1.2 MPa treatments and reached 265.7 and 265.2 molmol-1 respectively. Qevilah genotype had the highest RWC (80.66%) under water stress conditions compared to other two genotypes. RWC had a close relationship with photosynthesis (R2= 0.89). Reduction in RWC caused low photosynthesis rate. Leaf photosynthesis was reduced in all genotypes under stressed treatments and was 10.86, 6.72, 0.55 and 0.14 µmolm-2s-1 in control, -0.4, -0.8 and -1.2 MPa treatments, respectively. Photosynthesis and sub-stomatal CO2 concentration were negatively related (R2=0.95). Photosynthesis rates decreased with decrease in stomatal conductance, and high relationship between them (R2=0.93) implied that stomatal limitation to photosynthesis might have been in operation. Also plant dry weight decreased in all genotypes in drought stress conditions, but, there was no significant differences between control, -0.4 and -0.8 MPa treatments in Qevileh genotype. Plant dry weight decreased in -0.4 MPa water stress treatment in Nazkeh genotype. There were no significant differences between measured traits after re-watering in all treatments. The results showed that Qevileh genotype was more tolerant to drought than other genotypes because of a high RWC and no reduction in plant dry weight in -0.4 and -0.8 MPa than control during water stress.