Plant Responses to Ultraviolet Radiation Stress

Chlorophyll a fluorescence measurements (particularly variable fluorescence, and non photochemical quenching) have begun to be used as non-invasive probes of the effects of ozone depletion and of UV increases to study the following phenomena. Ozone depletion over the southern and northern hemispheres will probably increase ultraviolet (UV)-B radiation (285–315 nm), which will impact crop plants in different ways. Growth reduction, decreased photosynthesis as well as reduced biomass are commonly observed in UV-sensitive plants when UV-B stress is applied in addition to artificial white light, or in some cases even to solar light. Under unfiltered or extremely high UV-B conditions the molecular targets for UV action in photosynthesis are mainly in the Photosystem II (PS II) reaction center area, indicated by a decrease of the variable chlorophyll fluorescence in seedlings — whereas PS I is less sensitive. Thus, chlorophyll fluorescence, that measures PS II activities, holds promise for further understanding of the UV effects. Under combination of solar UV-B with modest increases of artificial UV-B radiation damaging effects of primary reactions are low or absent-shown by qP analysis in sunflower seedlings, but indirect effects on dark reactions — calculated from qNP calculations—and/or stomatal response may reduce biomass production. Yield reductions have occasionally been reported in soybean and rice cultivars sensitive to enhanced UV-B, but increases in yield have also been observed. Recent studies on several plant species and cultivars, attenuating solar UV-B by special UV-filters, showed relative growth and yield reductions in sensitive cultivars caused by a delay of plant development and flowering under higher solar UV-B radiation. Experiments with trees indicate a lower susceptibility to UV-B due to their robust leaf structure and the accumulation of UV absorbing compounds, which seems to be a general response in many plant species. However, over a period of time damages could also accumulate in these plants. Terrestrial ecosystems, e.g. mountain heath vegetation, show in some cases similar biomass reductions under enhanced UV-B-levels as well as changed composition of secondary compounds. A multitude of morphological, physiological and genetic consequences are expected for single plant species and for ecosystems at conditions of increased UV-B radiation, as expected by ozone depletion scenarios. However, the UV-B effects on primary photosynthesis seem to be low under realistic UV-B enhancements in the field.