Plant growth, stomatal response, pigments and photosynthesis of Althea Officinalis as affected by SO₂ stress

Growth responses of marshmallow (Althea officinalis L.) to SO₂ (0.5,1.0 and 2.0 ppm) stress were studied at pre-flowering, flowering and post-flowering stages. Compared to controls, dry masses of treated leaves, stems and roots were higher at 0.5 ppm SO₂ concentration and significantly decreased with the higher doses at each stage. Number of leaves, leaf area, length of stems and roots and number of flowers and fruits were also greater at 0.5 ppm concentration, but showed a decrease with higher concentrations. Roots showed greater reductions than stem. Size of stomatal aperture, stomatal density and stomatal index declined under higher SO₂ stress, though 0.5 ppm concentration hardly caused any noticeable difference over the control. The net photosynthetic rate of the leaves significantly increased, and the stomatal conductance and intercellular CO₂ concentration decreased at 0.5 ppm SO₂; the reverse was true for 1 ppm and 2 ppm concentrations. The photosynthetic pigments significantly declined under higher SO₂ stress at each stage of plant development, but the lowest SO₂ concentration exhibited stimulatory effect.