Red pigments in autumn leaves of Norway maple do not offer significant photoprotection but coincide with stress symptoms

Chlorophyll and topographic patterns demonstrate stress conditions drive the brightness of autumn leaf colour

Autumn leaf colour brightness is an important cultural ecosystem service. As its spatial patterns and ecophysiological mechanisms remain unclear, we analysed relationships among autumn leaf colour brightness, late summer chlorophyll content, and topographic position in both canopy-based micro-scale analysis and site-based macro-scale analysis. Multispectral drone observations were made in three Fagus crenata forests at elevations of 300, 600, and 900 m in Niigata Prefecture, Japan. In a drone-acquired digital surface model of canopy area distribution, we analysed the canopy-averaged autumn leaf colour brightness, four vegetation indices, topographic position index, and elevation. The macro-scale pattern showed brighter yellow leaves at lower elevations, suggesting the effect of temperature through productivity or evaporative stress. The micro-scale pattern showed brighter yellow leaves in canopies that had a low late summer chlorophyll content, or that grew on ridges. The negative correlation between autumn leaf colour and chlorophyll content suggests that low-chlorophyll trees experience stresses (e.g. irradiance, desiccation, senescence) that induce higher carotenoid content to defend against such stresses. Increased autumn leaf coloration on ridges is consistent with a stress gradient. Although further research is needed to reveal the underlying physiological and ecological mechanisms, autumn leaf colour brightness has different meanings at different scales and thus has potential not only as a cultural ecosystem service but also in forest management through early detection of stress.

Transcriptome sequencing and anthocyanin metabolite analysis involved in leaf red color formation of Cinnamomum camphora

Cinnamomum camphora, a key multifunctional tree species, primarily serves in landscaping. Leaf color is crucial for its ornamental appeal, undergoing a transformation to red that enhances the ornamental value of C. camphora. However, the molecular mechanisms underlying this transformation remain largely unexplored. In this study, green leaf (GL), color turning red leaf (RL) and whole red leaf (WRL) were obtained to measure pigment contents, while GL and RL were analyzed for transcriptomic alterations. A decline in chlorophyll content and a rise in anthocyanins were observed during the transition from green to red leaves. Using LC MS/MS, 11 types of anthocyanins showed significant accumulative differences, with cyanidin-3,5-O-diglucoside exhibiting the greatest disparity. Comparative RNA-seq identified 22,948 genes against reference genes, revealing 544 novel genes. Of these, 3,222 genes were up-regulated and 7,391 genes were down-regulated when the FPKM mean value > 1 in at least one group. The ribosome was identified as the most abundant KEGG term, with a substantial number of down-regulated differentially expressed genes (DEGs). The results indicated a downward trend in protein content, with GL exhibiting the highest protein concentration. 22, 4, and 29 DEGs were associated with chlorophyll biosynthesis, chlorophyll degradation, and anthocyanin biosynthesis, respectively. Most DEGs related to chlorophyll biosynthesis were down-regulated. SGR and SGRL, which are associated with chlorophyll degradation, exhibited opposite differential expression, resulting in a significant decrease in chlorophyll content in RL. The significantly up-regulated genes ANS and UFGT are advantageous for anthocyanin biosynthesis, contributing to the red coloration observed. Additionally, differential expression was noted in 40 R2R3-MYBs. Two MYB90 (Ccam01G003512 and Ccam01G003515) homologs of AtMYB113 were also identified showed high levels of up-regulation in RL. These findings suggest a strong correlation between pigment metabolism and transcriptome data, elucidating the mechanism that leads to the red coloration of leaves in C. camphora.

Natural variation in photosynthetic electron transport of wheat flag leaves in response to dark-induced senescence

Early leaf senescence affects photosynthetic efficiency and limits growth during the late production stage of winter wheat (Triticum aestivum). Natural variation in photosystem response to senescence represents a valuable resource for improving the aging traits of flag leaves. To explore the natural variation of different phases of photosynthetic electron transport in modern wheat cultivars during senescence, we exposed the flag leaves of 32 wheat cultivars to dark conditions to induce senescence process, and simultaneously measured prompt fluorescence and modulated 820 nm reflection. The results showed that the chlorophyll content, activity of PSII donor side, PSI and electron transfer between PSII and PSI were all decreased during dark-induced senescence, but they showed different sensitivity to dark-induced senescence. Furthermore, natural variation in photosynthetic parameters among the 32 wheat cultivars were also observed and showed by variation coefficient of the different parameters. We observed that PSII and PSI activity showed less sensitivity to dark-induced senescence than electron transfer between them, while PSII and PSI activity exhibit greater natural variation than electron transport between PSII and PSI. It suggests that Cytb6f might degrade faster and have less variation than PSII and PSI during dark-induced senescence.

Overexpression of FERONIA receptor kinase MdMRLK2 regulates lignin accumulation and enhances water use efficiency in apple under long-term water deficit condition

Water use efficiency (WUE) is crucial for apple tree fitness and survival, especially in response to climatic changes. The receptor-like kinase FERONIA is reportedly an essential regulator of plant stress responses, but its role in regulating WUE under water deficit conditions is unclear. Here, we found that overexpressing the apple FERONIA receptor kinase gene, MdMRLK2, enhanced apple WUE under long-term water deficit conditions. Under drought treatment, 35S::MdMRLK2 apple plants exhibited higher photosynthetic capacity and antioxidant enzyme activities than wild-type (WT) plants. 35S::MdMRLK2 apple plants also showed increased biomass accumulation, root activity, and water potential compared to WT plants. Moreover, MdMRLK2 physically interacts with and phosphorylates cinnamoyl-CoA reductase 1, MdCCR1, an enzyme essential for lignin synthesis, at position Ser260. This interaction likely contributed to increased vessel density, vascular cylinder area, and lignin content in 35S::MdMRLK2 apple plants under drought conditions. Therefore, our findings reveal a novel function of MdMRLK2 in regulating apple WUE under water deficit conditions.

Flavonols do not affect aphid load in green or senescing birch leaves but coincide with a decrease in Photosystem II functionality

Instead of red anthocyanins, birches synthesise colourless (to human eye), UV-absorbing flavonols during autumn senescence. To test if flavonols protect against insects, and if leaves with high or low amounts of flavonols differ in their photosynthetic functions, aphid-free and aphid-infested green and senescing birch leaves were collected from outdoor-grown trees and analysed. Photosynthetic parameters were greatly affected by the leaf chlorophyll content (i.e. the phase of senescence). Photochemical quenching and the amount of functional Photosystem I decreased linearly with chlorophyll content, while FV/FM (Photosystem II functionality) decreased strongly only at the end of senescence. Non-photochemical quenching of excitation energy (NPQ) increased towards the end of senescence. However, no significant differences in the total flavonol amounts, nor in individual flavonol species, were found between aphid-free and aphid-infested leaves, suggesting that flavonols play no role in defence against aphid herbivory. Interestingly, both green and senescing leaves with a high flavonol content showed low FV/FM values. High flavonol content slowed down PSII photoinhibition and improved recovery, but only in green leaves. Previously, we proposed that anthocyanins provide an additional sink for photosynthates at the nitrogen resorption phase during autumn senescence, and the present data may suggest that flavonol synthesis plays a similar role.

Both external and internal factors induce heterogeneity in senescing leaves of deciduous trees

Autumn senescence is characterised by spatial and temporal heterogeneity. We show that senescing birch (Betula spp.) leaves had lower PSII activity (probed by the F V /F M chlorophyll a fluorescence parameter) in late autumn than in early autumn. We confirmed that PSII repair slows down with decreasing temperature, while rates of photodamage and recovery, measured under laboratory conditions at 20°C, were similar in these leaves. We propose that low temperatures during late autumn hinder repair and lead to accumulation of non-functional PSII units in senescing leaves. Fluorescence imaging of birch revealed that chlorophyll preferentially disappeared from inter-veinal leaf areas. These areas showed no recovery capacity and low non-photochemical quenching while green veinal areas of senescing leaves resembled green leaves. However, green and yellow leaf areas showed similar values of photochemical quenching. Analyses of thylakoids isolated from maple (Acer platanoides ) leaves showed that red, senescing leaves contained high amounts of carotenoids and α-tocopherol, and our calculations suggest that α-tocopherol was synthesised during autumn. Thylakoids isolated from red maple leaves produced little singlet oxygen, probably due to the high antioxidant content. However, the rate of PSII photodamage did not decrease. The data show that the heterogeneity of senescing leaves must be taken into account to fully understand autumn senescence.

Exposed anthocyanic leaves of Prunus cerasifera are special shade leaves with high resistance to blue light but low resistance to red light against photoinhibition of photosynthesis

BACKGROUND AND AIMS

The photoprotective role of foliar anthocyanins has long been ambiguous: exacerbating, being indifferent to or ameliorating the photoinhibition of photosynthesis. The photoinhibitory light spectrum and failure to separate photo-resistance from repair, as well as the different methods used to quantify the photo-susceptibility of the photosystems, could lead to such a discrepancy.

METHODS

We selected two congeneric deciduous shrubs, Prunus cerasifera with anthocyanic leaves and Prunus triloba with green leaves, grown under identical growth conditions in an open field. The photo-susceptibilities of photosystem II (PSII) and photosystem I (PSI) to red light and blue light, in the presence of lincomycin (to block the repair), of exposed leaves were quantified by a non-intrusive P700+ signal from PSI. Leaf absorption, pigments, gas exchange and Chl a fluorescence were also measured.

KEY RESULTS

The content of anthocyanins in red leaves (P. cerasifera) was >13 times greater than that in green leaves (P. triloba). With no difference in maximum quantum efficiency of PSII photochemistry (Fv/Fm) and apparent CO2 quantum yield (AQY) in red light, anthocyanic leaves (P. cerasifera) showed some shade-acclimated suites, including lower Chl a/b ratio, lower photosynthesis rate, lower stomatal conductance and lower PSII/PSI ratio (on an arbitrary scale), compared with green leaves (P. triloba). In the absence of repair of PSII, anthocyanic leaves (P. cerasifera) showed a rate coefficient of PSII photoinactivation (ki) that was 1.8 times higher than that of green leaves (P. triloba) under red light, but significantly lower (-18 %) under blue light. PSI of both types of leaves was not photoinactivated under blue or red light.

CONCLUSIONS

In the absence of repair, anthocyanic leaves exhibited an exacerbation of PSII photoinactivation under red light and a mitigation under blue light, which can partially reconcile the existing controversy in terms of the photoprotection by anthocyanins. Overall, the results demonstrate that appropriate methodology applied to test the photoprotection hypothesis of anthocyanins is critical.

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

Disassembly and degradation of the photosynthetic protein complexes during autumn senescence, a vital step to ensure efficient nutrient relocalization for winter storage, is poorly understood. Concomitantly with the degradation, anthocyanins are often synthesized. However, as to why leaves accumulate red pigments, no consensus exists. One possibility is that anthocyanins protect senescing leaves from excess light. In this study, we investigated the pigment composition, photosynthetic performance, radical production, and degradation of the photosynthetic protein complexes in Norway maple (Acer platanoides) and in its highly pigmented, purple-colored variety (Faassen's black) during autumn senescence, to dissect the possible roles of anthocyanins in photoprotection. Our findings show that senescing Faassen's black was indeed more resistant to Photosystem II (PSII) photoinhibition, presumably due to its high anthocyanin content, than the green maple. However, senescing Faassen's black exhibited low photosynthetic performance, probably due to a poor capacity to repair PSII. Furthermore, an analysis of photosynthetic protein complexes demonstrated that in both maple varieties, the supercomplexes consisting of PSII and its antenna were disassembled first, followed by the degradation of the PSII core, Photosystem I, Cytochrome b6 f, and ATP synthase. Strikingly, the degradation process appeared to proceed faster in Faassen's black, possibly explaining its poor PSII repair capacity. The results suggest that tolerance against PSII photoinhibition may not necessarily translate to a better fitness. Finally, thylakoids isolated from senescing and non-senescing leaves of both maple varieties accumulated very little carbon-centered radicals, suggesting that thylakoids may not be a major source of reactive oxygen species in senescing leaves.