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

Differential Sensitivity of Photosynthetic Electron Transport to Dark-Induced Senescence in Wheat Flag Leaves

BACKGROUND

In winter wheat (Triticum aestivum), delayed senescence of the flag leaf is linked to the duration of photosynthesis and grain yield. In different wheat cultivars, various components of the photosynthetic apparatus may display differences during senescence. Furthermore, previous studies related to senescence mostly used a limited number of cultivars, making it difficult to investigate the patterns and reasons for different appearance of damage to electron transport among various cultivars.To tackle these challenges, flag leaves of 32 wheat cultivars were subjected to darkness in vitro to simulate the senescence process. The cultivars were divided into three groups by k-means clustering, based on the rate of decline in their leaf chlorophyll content. Subsequently, we simultaneously measured prompt chlorophyll a fluorescence, delayed chlorophyll a fluorescence, and modulated 820-nm light reflection to examine the alterations in photosynthetic electron transport within the three groups of wheat cultivars during dark-induced senescence.

RESULTS

The results showed that the photosystem II (PSII) donor side, grouping of PSII units, PSII reaction center, PSII acceptor side, and photosystem I (PSI) were all damaged during dark-induced senescence, while the sensitivity of photosynthetic electron transport to senescence gradually increased from the upstream to downstream electron carriers on the PSII acceptor side. The extent of the observed decrease in activity of the different components of the photosynthetic electron transport chain during senescence, was consistent with the chlorophyll degradation rate of the wheat cultivars, while the priority of inhibition for different photosynthetic electron transport processes in each cultivar group was different. The results from the three separate signals align well with each other.

CONCLUSIONS

The sensitivity of different part of photosynthetic electron transport to senescence were varied depended on their chlorophyll degradation rate. The differences in the response of different processes of photosynthetic electron transport to chlorophyll degradation rates might be an important factor influencing the differences in photoinhibition among wheat cultivars, especially in senescence process.

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.

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.