Ultraviolet-B irradiation of seeds affects photochemical and reproductive performance of the arid-environment ephemeral dimorphotheca pluvialis

Abstract

A higher polyphenolic content and thicker sclerenchymatous cylinder in the pericarp of ray than of disc seed morphs (diaspores) of Dimorphotheca pluvialis (L.) Moench (Asteraceae) could limit possible damage to the embryo during long-term seed exposure to solar UV-B radiation. This hypothesis was tested by irradiating sun-dried disc and ray diaspores continuously for 6 weeks with four different doses of biologically effective UV radiation, viz 0.0, 0.2, 9.46 and 11.97 kj m −2 8 hr −1 of visible (> 400 nm), UV-A (320–400 nm), ambient and enhanced UV-B (280–320 nm) radiation, respectively. Total effective UV-B doses approximated those received over an 18-week period following seed dispersal at this species' northerly distribution limit (26° 38′ S), at normal ozone levels (ambient UV-B) and anticipated 20% ozone depletion (enhanced UV-B). Irradiation of diaspores with enhanced UV-B improved germination in both seed morphs. However, disc diaspores exhibited a greater fractional increase in germination than ray diaspores. Disc and ray plants grown from diaspores irradiated with enhanced UV-B exhibited decreased photochemical efficiency (reduced variable to maximal fluorescence, F v/ F m), but only disc plants showed decreased potential photosynthetic activity (reduced areas over fluorescence curves, A fd). This was accompanied by increased diaspore production and reduced diaspore mass. Irradiation of diaspores with photoreactivating UV-A produced a contrasting response (increased F v/ F m and A fd, accompanied by decreased diaspore production) in disc plants only. Altered photochemical and reproductive performance, and visibly diminished leaf pigmentation, in disc plants indicated increased sensitivity to photoinhibition, a possible consequence of UV-B induced cellular (membrane and DNA) damage in seeds.