Differential Response of the Photosynthetic Machinery to Fluctuating Light in Mature and Young Leaves of Dendrobium officinale.

Ying-Jie Yang,Wei Huang,Hu Sun,Ren-Qiang Mei,Qi Shi

doi:10.3389/fpls.2021.829783

Abstract

A key component of photosynthetic electron transport chain, photosystem I (PSI), is susceptible to the fluctuating light (FL) in angiosperms. Cyclic electron flow (CEF) around PSI and water-water cycle (WWC) are both used by the epiphytic orchid Dendrobium officinale to protect PSI under FL. This study examined whether the ontogenetic stage of leaf has an impact on the photoprotective mechanisms dealing with FL. Thus, chlorophyll fluorescence and P700 signals under FL were measured in D. officinale young and mature leaves. Upon transition from dark to actinic light, a rapid re-oxidation of P700 was observed in mature leaves but disappeared in young leaves, indicating that WWC existed in mature leaves but was lacking in young leaves. After shifting from low to high light, PSI over-reduction was clearly missing in mature leaves. By comparison, young leaves showed a transient PSI over-reduction within the first 30 s, which was accompanied with highly activation of CEF. Therefore, the effect of FL on PSI redox state depends on the leaf ontogenetic stage. In mature leaves, WWC is employed to avoid PSI over-reduction. In young leaves, CEF around PSI is enhanced to compensate for the lack of WWC and thus to prevent an uncontrolled PSI over-reduction induced by FL.

Highlights

A typical light condition for plants in nature is the fluctuations of light intensity owing to cloud, wind, and shading from upper leaves and plants (Pearcy, 1990)
Many previous studies have indicated that this rapid re-oxidation of P700 in angiosperms is caused by the fast outflow of electrons from photosystem I (PSI) to O2 mediated by the water-water cycle (WWC) activity (Shirao et al, 2013; Huang et al, 2019b, 2021; Sun et al, 2020; Yang et al, 2020)
Since an important role of Cyclic electron flow (CEF) activation under fluctuating light (FL) is to alleviate PSI overreduction, we examined the relationship between electron transport rate through PSI (ETRI)–electron transport rate through PSII (ETRII) and Y(NA), and found that the ETRI–ETRII value was strongly correlated to Y(NA) (Figure 5B)

Summary

Introduction

A typical light condition for plants in nature is the fluctuations of light intensity owing to cloud, wind, and shading from upper leaves and plants (Pearcy, 1990). When light intensity transiently shifts from low to high, photosystem II (PSII) electron flow rapidly increases but CO2 assimilation rate increased slowly (Gerotto et al, 2016; Acevedo-Siaca et al, 2020; De Souza et al, 2020; Grieco et al, 2020; Kimura et al, 2020; Yamori et al, 2020), leading to the imbalance between light and dark reactions (Yamori et al, 2016; Slattery et al, 2018). Photosynthetic Regulation Under Fluctuating Light from PSII to photosystem I (PSI) cannot be immediately transported to NADP+ because the consumption of nicotinamide adenine dinucleotide phosphate (NADPH) is restricted, resulting in the accumulation of reducing power in PSI as demonstrated by PSI over-reduction (Yamamoto et al, 2016; Wada et al, 2018). Fluctuating light (FL) can give rise to a risk of PSI photoinhibition in photosynthetic organisms (Suorsa et al, 2012; Kono et al, 2014; Yamamoto and Shikanai, 2019; Storti et al, 2020). Plants should protect PSI from damage when exposed to natural FL conditions (Tikkanen et al, 2012; Allahverdiyeva et al, 2015; Ferroni et al, 2020)

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