Circadian rhythms are associated with variation in photosystem II function and photoprotective mechanisms

Abstract

The circadian clock regulates many aspects of leaf gas supply and biochemical demand for CO , and is hypothesized to improve plant performance. Yet the extent to which the clock may regulate the efficiency of photosystem II (PSII) and photoprotective mechanisms such as heat dissipation is less explored. Based on measurements of chlorophyll a fluorescence, we estimated the maximum efficiency of PSII in light (Fv'/Fm') and heat dissipation by nonphotochemical quenching (NPQ). We further dissected total NPQ into its main components, qE (pH-dependent quenching), qT (state-transition quenching), and qI (quenching related to photoinhibition), in clock mutant genotypes of Arabidopsis thaliana, the cognate wild-type genotypes, and a panel of recombinant inbred lines expressing quantitative variation in clock period. Compared with mutants with altered clock function, we observed that wild-type genotypes with clock period lengths of approximately 24 hr had both higher levels of Fv'/Fm', indicative of improved PSII function, and reduced NPQ, suggestive of lower stress on PSII light harvesting complexes. In the recombinant inbred lines, genetic variances were significant for Fv'/Fm' and all 3 components of NPQ, with qE explaining the greatest proportion of NPQ. Bivariate tests of association and structural equation models of hierarchical trait relationships showed that quantitative clock variation was empirically associated with Fv'/Fm' and NPQ, with qE mediating the relationship with gas exchange. The results demonstrate significant segregating variation for all photoprotective components, and suggest the adaptive significance of the clock may partly derive from its regulation of the light reactions of photosynthesis and of photoprotective mechanisms.

Thermal imaging as a noninvasive technique for analyzing circadian rhythms in plants

“…In plants, the circadian system regulates a diverse range of activities from growth and organ movements to photosynthesis and gene expression, and is a key part of the day‐length gauging mechanism for photoperiodic development (reviewed in Inoue et al ., ). The circadian system in plants also regulates the exchange of carbon dioxide and water vapour exchange between leaves and the atmosphere by controlling stomatal opening and photosynthesis (Salome et al ., ; Haydon et al ., ; Yarkhunova et al ., ). Although not yet fully understood, mechanisms for circadian control of stomatal aperture have been shown to involve both photoperiod and hormone pathways (Legnaioli et al ., ; Kinoshita et al ., ; Hassidim et al ., ).…”

New connections between circadian rhythms, photosynthesis, and environmental adaptation

“…The study of Yarkhunova et al () has interesting implications. For example, the genetic diversity generated during crop breeding will cause variation in circadian period that could alter photosynthetic performance (Yarkhunova et al, ).…”

“…The authors also took a recombinant inbred line (RIL) population of Arabidopsis, produced from a cross between the Wassilewskija and Landsberg erecta accessions. Using this population, Yarkhunova et al () identified that circadian periods closer to 24 hr were associated with greater operating efficiency of PSII, whereas longer periods within the population associated with decreased PSII operating efficiency. Conversely, NPQ was positively correlated with circadian periods in the range 24.5–27 hr.…”

“…Using this population, Yarkhunova et al (2018) identified that circadian periods closer to 24 hr were associated with greater operating efficiency of PSII, whereas longer periods within the population associated with decreased PSII operating efficiency. Conversely, NPQ was positively correlated with circadian periods in the range 24.5-27 hr.…”

“…In this issue of Plant , Cell , and Environment , Yarkhunova et al () investigated the influence of circadian regulation upon various aspects of photosynthetic light harvesting. Using Arabidopsis thaliana , they found that mutations that alter the circadian period decrease the operating efficiency of Photosystem II and increase the non‐photochemical quenching (NPQ) of chlorophyll fluorescence.…”