Effect of plant age, light-spectra, and winter/spring vernalization alleles on the cold acclimation of barley photosynthetic apparatus

Cold temperature has a Janus-face effect on winter-hardy cereals: cold acclimation increasing their frost tolerance, while it also induces vernalization, resulting in a vegetative-generative transition. The degree of cold hardiness is determined by the CBF-regulon while the vernalization requirement is determined by the VRN1 gene. VRN1 has an epistatic effect on the CBF genes, as plants lose their freezing tolerance when the vernalization requirement is fulfilled. However, sudden frost spells could occur both during the vegetative-generative transition phase as well as when the generative transition has completed. By the approaching of the cold season in temperate climate, red:far-red (R:FR) photon ratio decreases, while blue:red (B:R) photon ratio increases in the sunlight spectrum. In contrast to this, from spring to summer, changes with the opposite direction could be observed. Many publications discuss the influence of light spectra on the development of frost tolerance in cereals. Recently, we reported that when frost-tolerant winter barley genotype ‘Nure’ illuminated by White light (W) with FR enrichment (WFR) increased its frost tolerance. The process was more successful, when WFR light was further enriched with blue light. These effects have been observed even at 15°C, where the cold-induced acclimation processes are mostly inactive. However, this effect is known to be negligible in spring varieties. Diminished freezing tolerance can also result from less efficient cold acclimation of the photosynthetic apparatus, however, the spectral dependence of the process is less investigated. Chlorophyll fluorescence parameters could serve as a physiological marker of photosystem II acclimation to cold as well as an indicator of freezing damages. Therefore, we aimed to investigate how winter/spring VRN1 alleles (vrn-H1/Vrn-H1) influence the light-induced cold acclimation of the photosynthetic apparatus of barley plants under different spectral illumination (W, WFR, WFRB) at 15°C for 10 days after a pre-growing phase in W. The pre- and early post-freezing chlorophyll fluorescence parameters of detached leaves were compared in vegetative-generative transition- and early generative developmental phases between two reciprocal Near Isogenic Lines (Cod.43s: Nure background carrying the Tremois Fr-H1 and Nure Fr-H2 alleles; Cod.30s: Tremois background carrying Nure Fr-H1 allele and Tremois Fr-H2 alleles) and the two parents. Interestingly the Nure vrn-H1 allele improved the quantum yield of photosystem II in the Tremois background after freezing, which was further enhanced by WFR treatment.