Hydroxycinnamic acids in sunflower leaves as UV protecting secondary metabolites responsive to different abiotic stress conditions
Phytochemical analysis of Helianthus annuus L. showed absence of flavonoids, indicating a strategy of UV resistance in sunflower leaves different compared to most other angiosperms that accumulate flavonoids for UV-A screening (UAS) and hydroxycinnamic acids (HCAs) for UV-B screening (UBS). The identification of foliar UAS pigments in sunflower, conducted by combined determination of epidermal UAS via chlorophyll fluorescence, HPLC and mass spectrometry analysis, showed caffeoylquinic acids (CQA), a group of HCAs, as the only candidates for UAS. The autofluorescence of CQAs showed predominant accumulation in the epidermis suggesting the CQAs to be optimally located for a screening function. Consistently, UAS was correlated to the foliar CQA contents with a high coefficient of determination. Analysis of screening efficacy provided strong evidence for the CQAs to be effective UAS compounds. In the majority of angiosperms, flavonoid biosynthesis is responsive to abiotic stress conditions such as UV-B, high PPFD, low temperature or nitrogen depletion. Whether the UAS mechanism of sunflowers responds analogously was tested in shifting experiments. The epidermal UAS and foliar CQA content in sunflowers with initially low UAS increased simultaneously after exposure to UV-B, high PPFD and low temperature. Nitrogen depletion alone had no effect. The qRT-PCR showed the mRNA levels of two of three analysed key genes (HaHQT2 and HaHQT3) toincrease to a transient maximum prior to the CQA accumulation whereas relative gene expression of HaHQT1 appeared to play a subordinate role. In summary, this work promotes the understanding of the UAS mechanism in sunflower and provides first evidence for an immediate response of the UAS pigment biosynthesis by analysed abiotic stress conditions.