Assessment of Photosynthesis Tolerance to Herbicides, Heat and High Illumination by Fluorescence Imaging

Fluorescence imaging represents a non-invasive tool for revealing and understanding spatial heterogeneity in leaf performance caused by external factors, such as abiotic stress. Sun (Rosa meillandina and Chrysanthemum morifo- lium) and shade (Spathiphyllum wallisii) plants were used to study their tolerance to heat and high illumination. Fluores- cence yield, effective PSII quantum yield and non-photochemical quenching were analysed in leaves attached to plants by fluorescence imaging. The control plants of all species showed homogeneous images of the fluorescence parameters throughout the leaf. The fluorescence yield (F) was 0.1 or less, the effective PSII quantum yield (Y(II)) around 0.75 and non-photochemical quenching (NPQ) less than 0.3. The two sun plants showed higher tolerance to stress conditions. Few variations were observed in F and Y(II) images after stress photoperiods and some leaf regions showed an increase in NPQ, indicating more thermal energy dissipation in these zones than in other leaf regions. The images of the fluorescence parameters were similar to those of control plants after one recovery photoperiod without stress conditions. Shade plant showed lower tolerance and irreversible damage was observed after the first photoperiod, particularly at the base of the leaf and in the areas adjacent to the ribs. The centre and top of the leaf were less damaged, and effective PSII quantum yield remained high because the leaf curved to reduce the incident radiation. Incubation with the herbicides DCMU and paraquat led to differences in the fluorescence parameter images. The effect of DCMU (0.1 mM) was visible after 30 min incubation, beginning at the ribs and adjacent areas of the leaf. The three species studied showed different degree of sensi- tivity to paraquat (0.2 mM), and the effective quantum yield in each species was affected at different incubation times.