Morphological and physiological responses of canola (Brassica napus) siliquas and seeds to UVB and CO₂ under controlled environment conditions

Combined effects of UVB radiation and CO₂ concentration on plant reproductive parts have received little attention. We studied morphological and physiological responses of siliquas and seeds of canola (Brassica napus L. cv. 46A65) to UVB and CO₂ under four controlled experimental conditions: UVB radiation (4.2 kJ m^-2 d^-1) with ambient level of CO₂ (370 μmol mol^-1) (control); UVB radiation (4.2 kJ m^-2 d^-1) with elevated level of CO₂ (740 μmol mol^-1); no UVB radiation (0 kJ m^-2 d^-1) with ambient level of CO₂ (370 μmol mol^-1); and no UVB radiation (0 kJ m^-2 d^-1) with elevated level of CO₂ (740 μmol mol^-1). UVB radiation affected the outer appearance of siliquas, such as colour, as well as their anatomical structures. At both CO₂ levels, the UVB radiation of 4.2 kJ m^-2 d^-1 reduced the size of seeds, which had different surface patterns than those from no UVB radiation. At both CO₂ levels, 4.2 kJ m^-2 d^-1 of UVB decreased net CO₂ assimilation (A_N) and water use efficiency (WUE), but had no effect on transpiration (E). Elevated CO₂ increased A_N and WUE, but decreased E, under both UVB conditions. At both CO₂ levels, the UVB radiation of 4.2 kJ m^-2 d^-1 decreased chlorophyll fluorescence, total chlorophyll (Chl), Chl a and Chl b, but had no effect on the ratio of Chl a/b and the concentration of UV-screening pigments. Elevated CO₂ increased total Chl and the concentration of UV-screening pigments under 4.2 kJ m^-2 d^-1 of UVB radiation. Neither UVB nor CO₂ affected wax content of siliqua surface. Many significant relationships were found between the above-mentioned parameters. This study revealed that UVB radiation exerts an adverse effect on canola siliquas and seeds, and some of the detrimental effects of UVB on these reproductive parts can partially be mitigated by CO₂.