Singlet oxygen, flavonols and photoinhibition in green and senescing silver birch leaves

Abstract

Key message Decreased absorptance and increased singlet oxygen production may cause photoinhibition of both PSII and PSI in birch leaves during autumn senescence; however, photosynthetic electron transfer stays functional until late senescence. Abstract During autumn senescence, deciduous trees degrade chlorophyll and may synthesize flavonols. We measured photosynthetic parameters, epidermal flavonols, singlet oxygen production in vivo and photoinhibition of the photosystems (PSII and PSI) from green and senescing silver birch (Betula pendula) leaves. Chlorophyll a fluorescence and P700 absorbance measurements showed that the amounts of both photosystems decreased throughout autumn senescence, but the remaining PSII units stayed functional until ~ 90% of leaf chlorophyll was degraded. An increase in the chlorophyll a to b ratio, a decrease in > 700 nm absorbance and a blue shift of the PSI fluorescence peak at 77 K suggest that light-harvesting complex I was first degraded during senescence, followed by light-harvesting complex II and finally the photosystems. Senescing leaves produced more singlet oxygen than green leaves, possibly because low light absorption by senescing leaves allows high flux of incident light per photosystem. Senescing leaves also induced less non-photochemical quenching, which may contribute to increased singlet oxygen production. Faster photoinhibition of both photosystems in senescing than in green leaves, under high light, was most probably caused by low absorption of light and rapid singlet oxygen production. However, senescing leaves maintained the capacity to recover from photoinhibition of PSII. Amounts of epidermal flavonols and singlet oxygen correlated neither in green nor in senescing leaves of silver birch. Moreover, Arabidopsis thaliana mutants, incapable of synthesizing flavonols, were not more susceptible to photoinhibition of PSII or PSI than wild type plants; screening of chlorophyll absorption by flavonols was, however, small in A. thaliana. These results suggest that flavonols do not protect against photoinhibition or singlet oxygen production in chloroplasts.

Biophysical and molecular characteristics of senescing leaves of two Norway maple varieties differing in anthocyanin content

“…In the senescing leaves of green maple, PSII activity remained high (F V /F M 0.7-0.8; measured before any high light treatments) until the point where almost no chlorophyll was left (Figure 1C), in line with previous reports (e.g., Keskitalo et al, 2005;Mattila et al, 2021;Moy et al, 2015). On the contrary, despite its slower photoinhibition, a dramatic decrease in PSII activity (measured before any high light treatments) co-occurred with the chlorophyll degradation in the purple maple (Figure 1C).…”

“…In many tree species, chlorophyll a‐ to‐ b ratio decreases during autumn senescence (Lee et al, 2003; Moy et al, 2015; Wolf, 1956), suggesting that a common strategy during autumn senescence is to first degrade photosystem core complexes and only later the chlorophyll b enriched LHCII. However, in some tree species, for example in striped maple ( Acer pensylvanicum ) and in sycamore maple, the chlorophyll a‐ to‐ b ratio increases during senescence (Fulgosi et al, 2012; Lee et al, 2003; Mattila et al, 2021). Our pigment analyses demonstrate that the chlorophyll a/b ratio only slightly decreased during senescence, both in green and purple maple (Figure 2), which is surprising as, particularly during the late senescence, very little of the core complexes were observed, while considerable amount of LHCII was still detected (Figure 5).…”

Changes in Light Energy Utilization in Photosystem II and Reactive Oxygen Species Generation in Potato Leaves by the Pinworm Tuta absoluta

“…An increased NPQ formation decreases 1 O 2 production [74][75][76], and zeaxanthin, a pigment involved in a form of NPQ, may also directly quench 1 O 2 [77]. Thus, the increased NPQ at the whole leaflet level, after 15 min feeding (Figure 3a), might have contributed to the observed similarity in 1 O 2 formation before and after feeding.…”

Ultraviolet screening by slug tissue and tight packing of plastids protect photosynthetic sea slugs from photoinhibition

“…4A, B). The same phenomenon can be seen in senescing birch leaves, as green leaves absorb light at longer wavelengths than senescing ones (Mattila et al 2021). Furthermore, the red edge of the reflectance spectrum of E. timida moves toward shorter wavelengths when the slugs lose plastids during starvation (Fig.…”