Anatomical and functional responses of Calendula officinalis L. to SO2 stress as observed at different stages of plant development

Growth responses of field-grown marigold (Calendula officinalis L.) plants to SO2 (0.5, 1.0 and 2.0 ppm) stress were studied at pre-flowering, flowering and post-flowering stages. Low concentrations of SO2 did not cause any noticeable difference in shoot length, flowers and fruits per plant, and in seeds per fruit, but enhanced leaf number, leaf area and root length. With higher concentrations, however, all these parameters declined. Compared with controls, dry mass of leaves and roots was higher with 0.5 ppm SO2 but significantly lower with higher doses at each stage. With a high SO2 dose stomatal pore size increased on adaxial epidermis but decreased on abaxial epidermis. The net photosynthetic rate significantly increased, whereas stomatal conductance and intercellular CO2 concentration decreased with 0.5 ppm SO2 treatment, the reverse being the case with higher concentrations. The photosynthetic pigments declined significantly under high SO2 stress at each stage of plant development, although 0.5 ppm concentration had a stimulatory effect. SO2 stress delayed the development of interfascicular cambium and altered the proportion of the component tissues in the plant axis. The areas of cortex and vasculature in stems and roots decreased, while pith area increased in the treated plants. The length of fibres and vessel elements increased in the stem and decreased in the root, compared with the control. Vessel density and diameter declined in large vascular bundles and increased in small vascular bundles of the stem. In roots, both the parameters gained under heavy SO2 stress.