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Chlorophyll is synthesized in the daytime in angiosperms e. Pan et al. Diurnal fluctuations in leaf chlorophyll content can also be caused by circadian oscillations in amounts or activities of other enzymes required in chlorophyll synthesis Papenbrock et al. Our data confirm that fluctuations in chlorophyll contents of tree leaves are small see Reddy et al. Diurnal variation in the rate of chlorophyll degradation during senescence has not been earlier studied. It could be argued that it is beneficial for plants to degrade chlorophyll during the night when the phototoxic intermediates of chlorophyll degradation would be less dangerous.

However, in S. Thus, our data indicate that in spite of possible production of singlet oxygen during daytime, chlorophyll degradation is not confined to the dark period in the studied plants, but instead these species degrade chlorophyll in the light, probably relying on ROS-protection mechanisms like early light induced proteins Andersson et al.

This is consistent with the hypothesis that plants keep the photosynthetic machinery functional until a very late stage of senescence Keskitalo et al. When the chlorophyll content had reached the limit, the ratio decreased in A. Yamazaki et al. In those B.

Also in S. However, the relationship between brightness of the day and flavonol synthesis did not hold for data collected during the early autumn Table 1. Synthesis of flavonols also takes place in the daytime Fig.

Photoprotection in Plants

Night and day temperature, total irradiance sum and maximum daily light intensity max , and flavonol synthesis in rowan and silver birch leaves during selected days during the autumn. It has been previously shown in birch trees that certain flavonols increase in response to UV-B radiation e. Morales et al. Due to the short diffusion distance of singlet oxygen for review, see Mattila et al.

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Chlorophyll degradation may enhance flavonol synthesis because leaves that contain less chlorophyll are more efficiently illuminated throughout the leaf tissue than heavily pigmented leaves. Flavonols have also been shown to increase in response to cold in dwarf birch Stark et al. Here, when the first S.

Furthermore, weather was not particularly cold when flavonols strongly increased in B. However, more data would be needed to confirm the generality of these findings. In addition to their suggested roles in ROS-scavenging, flavonols might also function in protection against pathogens or herbivores Martemyanov et al. However, birch leaves with intense yellow colour have been shown to attract more aphids than greener leaves Sinkkonen et al.

Some flavonols are toxic also to plants e. Kandil et al. It may take over 2 months for a tree to shed all its leaves during autumn senescence, but our data suggest that most of this time, individual leaves retain a relatively high chlorophyll content and the average chlorophyll content decreases only because some leaves lose chlorophyll rapidly. For that reason, a complete picture about tree senescence cannot be achieved by studying senescence as an averaged phenomenon.

The rate of chlorophyll degradation, once it commences, is quite uniform in a tree. Thus, trees keep photosynthetically active leaves alongside with leaves from which nitrogen is rapidly remobilized before abscission. Constant slow decrease in chlorophyll content during the whole autumn observed in S. The present data also suggest that increase in flavonols is common during senescence, but their role still needs to be resolved.

Table S1. The effect of the measurer five persons on the chlorophyll or flavonol Flv levels in the four species, calculated by ANOVA. Figure S1. The data are from the Finnish Meteorological Institute. Figure S2. Phenolic compounds detected with high-performance liquid chromatography HPLC at nm from leaf extracts. A Chromatograms. Retention times RT and classifications of the compounds are given. Figure S3. Chlorophyll content in Sorbus aucuparia leaves during autumn senescence.

At every measurement date, 10 randomized leaflets were collected and chlorophylls were extracted in N , N -dimethylformamide DMF and quantified spectrophotometrically. Mean values and error bars showing SD A ; individual measurements B. Figure S4. Figure S5.

Rates of changes in chlorophyll contents from Sorbus aucuparia A , Acer platanoides B , Betula pendula C and Prunus padus D leaves plotted against rates of changes in flavonol contents. Rates during 2.

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Figure S6. Chlorophyll a to b ratio as a function of total chlorophyll concentration of the sample in the four species, measured during different phases of autumn senescence.

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Figure S7. Comparison of leaf chlorophyll content measured with a destructive method by extracting pigments in N , N -dimethylformamide DMF or with non-destructive measurements from Sorbus aucuparia A , Acer platanoides B , Betula pendula C and Prunus padus D leaves, collected at different dates during autumn. Figure S8. The total amount relative units of all compounds detected with high-performance liquid chromatography HPLC circles or those compounds identified as flavonols by their spectra squares , from extracts of Sorbus aucuparia leaflets A , Acer platanoides leaves B and Betula pendula leaves C , compared to the flavonol index measured from the same leaves with Dualex.

Photoprotection in Plants

Figure S9. Absorbance at nm, reflecting the amount of each individual compound detected with high-performance liquid chromatography HPLC after extraction of phenolics from leaves of Betula pendula A , Acer platanoides B and Sorbus aucuparia C compared to the flavonol index measured from the same leaves with Dualex. The compounds are classified to six groups. Figure S Effect of ambient light A, B and handling C on the measuring accuracy of Dualex.

C Chlorophyll hatched bars and flavonol grey bars contents of P. In agreement with earlier literature Cerovic et al. To validate the non-destructive flavonol measurements from senescing leaves, flavonols were also measured with high-performance liquid chromatography HPLC [see Supporting Information—Fig. In the case of senescing leaves, a good correlation between the optical method and HPLC was obtained in Betula pendula leaves, although the relationship is somewhat curvilinear [see Supporting Information—Fig. S8C ]. The reason for the good correlation in B. Accordingly, large amounts of phenolic acids and non-flavonol flavonoids were observed in S.

S9 ] , but the optical method correlated with both the amount of flavonols and with the total amount of phenolic compounds [see Supporting Information—Fig. A good correlation with the amounts of flavonols in the leaf measured with the optical method and with HPLC is in accordance with previous research Barthod et al. However, flavonols cannot be distinguished from other phenolic compounds with the optical method. In our case, this difficulty has only minor importance because relative differences in the amounts of flavonols in leaves at different stages of senescence were highly correlated with relative differences in the amounts of all phenolic compounds [see Supporting Information—Fig.

We conducted the measurements in field conditions, either at sunrise or at sunset, both defined by the position of half of the Sun above the horizon. Ambient light was found not to affect chlorophyll measurements [see Supporting Information—Fig. S10A ]. Flavonol values showed some variability with ambient light [see Supporting Information—Fig. However, no consistent dependence of the flavonol signal on ambient light was observed.