DYNAMICS OF PLANT PIGMENTS ACCUMULATION IN Amelanchier alnifolia LEAVES THROUGHOUT THE GROWING SEASON

ABSTRACT

In the current study, the seasonal dynamics of pigment composition in Amelanchier alnifolia leaves were investigated from April to October using UV-Vis spectroscopy. Chlorophyll content peaked in July–August during the active growing season and decreased in autumn with wilting. Carotenoid concentrations were the highest in May–June, coinciding with increased sunlight exposure and their photoprotective role. Anthocyanins reached a maximum in May, aligning with active plant growth, and decreased by autumn as dormancy approached. Linear regression and Pearson's correlation analysis revealed a significant relationship between the pigments, confirming their joint participation in the regulation of photosynthesis and stress response mechanisms. Obtained results provide insights into the physiological adaptation of A. alnifolia, demonstrating its dynamic pigment composition adjustments to optimize photosynthesis and mitigate environmental stress.

АННОТАЦИЯ

В настоящем исследовании изучена сезонная динамика пигментного состава листьев ирги ольхолистной (Amelanchier alnifolia) с апреля по октябрь месяцы с помощью УФ-Вид-спектроскопии. Содержание хлорофилла достигало максимума в июле-августе в период активной вегетации и снижалось осенью по мере увядания. Концентрация каротиноидов была самой высокой в мае - июне, что соответствовало увеличению воздействия солнечного света и их фотозащитной роли. Содержание антоцианов достигало максимума в мае, что соответствовало периоду активного роста растений, и снижались к осени, с наступлением периода покоя. Линейная регрессия и корреляционный анализ Пирсона выявил значительную взаимосвязь между всеми пигментами, подтверждая их совместное участие в регуляции процесса фотосинтеза и механизмах реагирования на стресс. Полученные результаты дают расширенное представление о физиологической адаптации A. alnifolia, демонстрируя динамическую корректировку состава пигментов для оптимизации фотосинтеза и смягчения негативного воздействия окружающей среды.

Keywords: Amelanchier alnifolia, plant pigments, anthocyanins, carotenoids, chlorophyll.

Ключевые слова: Amelanchier alnifolia, растительные пигменты, антоцианы, каротиноиды, хлорофилл.

Introduction

Plant pigments are key phytocomponents that perform a number of vital functions, including protection from excessive solar radiation and the formation of an adaptive response to changing environmental conditions. This highlights the strategic importance of studying their structure, properties, and seasonal dynamics for a deeper understanding of the physiological processes and mechanisms of plant adaptation depending on environmental factors [22]. Natural pigments are a very important group of molecules, widely distributed in nature and playing a key role in everyday life. Plant leaves are a rich source of natural pigments, including chlorophylls and a variety of carotenoids and anthocyanins [29]. Structural, functional, and physicochemical properties of plant pigments are summarized in Table 1.

Table 1.

Structural, functional, and physicochemical properties of plant pigments

Chlorophyll a and chlorophyll b are the primary pigments in plant cells involved in photosynthesis, where they use the sunlight energy to synthesize carbohydrates from CO₂ and water [11]. They are structurally related to porphyrins and have distinct structural differences. Chlorophyll a contains a methyl (-CH₃) group at position R3, while chlorophyll b has a formyl (-CHO) group at the same position. Both pigments have a porphyrin ring with a magnesium ion at the center and a hydrophobic phytol tail. Chlorophyll a absorbs light primarily in the blue-violet (430 nm) and red (662 nm) regions, appearing blue-green, while chlorophyll b absorbs in the blue (453 nm) and red (642 nm) regions, appearing yellow-green. Chlorophyll b acts as an accessory pigment, broadening the light absorption spectrum for photosynthesis.

Carotenoids, including carotenes and xanthophylls, are polyene compounds with conjugated double bonds. As antioxidants and auxiliary pigments that absorb light, carotenoids are also vital components of plants [15]. They function as accessory pigments, protecting chlorophyll from photooxidation and attracting pollinators. Carotenoids are yellow, orange, or red and absorb light in the 400-500 nm range, with variations reflected in many fruits, flowers, and vegetables, which contribute to their economic value as well [24]. In addition to their primary roles in plants, carotenoids are vital parts of human diets and play important roles in human nutrition and health [31].

Anthocyanins are phenolic, water-soluble pigments with a glycosylated anthocyanidin core, accumulated in cell vacuoles [12] and provide red, purple, or blue coloration of flowers, fruits, and vegetables [1]. They attract pollinators, aid in seed dispersal, and offer UV protection of plants, absorbing light in the 500–600 nm range. The color of anthocyanins is determined by their chemical structure and depends on the nature and position of the substituents in the aromatic patterns of the molecules [6].

As can be seen, plant pigments exhibit diverse structures and properties that are closely linked to their essential functions in plants. Their unique properties underscore their ecological and biochemical significance.

Amelanchier alnifolia (Saskatoon berry) is a deciduous shrub native to North America [30], widely known for its ecological significance and high content of bioactive compounds that allow the plant to adapt to low winter temperatures and mildly alkaline soils [16]. Beyond its nutritional and commercial value, the species exhibits unique physiological traits that make it an interesting subject for studying.

While the phytochemistry and health benefits of A. alnifolia are well-known, ongoing research continues to explore ways to optimize cultivation practices, improve fruit quality, and identify new applications in health and nutrition. This highlights the importance of A. alnifolia as a valuable crop with significant potential for health and the economy [32]. This plant grows well in various environmental conditions, ranging from temperate climates to semi-arid regions, where it is exposed to varying light intensity, temperature, and water availability. The study of pigment content dynamics in this species can contribute to optimizing its cultivation and improving its resilience to environmental stress.

Studying the dynamics of plant pigments throughout the growing season is crucial for understanding various physiological, ecological, and agricultural processes. Monitoring their seasonal changes will provide insights into plants mechanisms of optimizing photosynthetic efficiency under varying environmental conditions, such as changes in light intensity, temperature, and water availability. Studying seasonal pigment changes also provides valuable ecological insights and helps to understand plant adaptation strategies and their role in ecosystem processes, such as carbon cycling and energy flow, that is essential for advancing knowledge in plant physiology, agriculture, and ecology. Therefore, the aim of this study is to investigate the seasonal variations in chlorophylls, carotenoids, and anthocyanins in A. alnifolia leaves from April to October using UV-Vis spectroscopy. Linear regression and Pearson’s correlation analysis were employed to assess the interrelationships between different pigments and their potential roles in plant adaptation to stress conditions.

Materials and methods

Collection and preparation of plant material.

Figure 1 shows the appearance of A. alnifolia leaves by month.

Figure 1. The appearance of leaves of A. alnifolia on the growing season

Since the most active leaf growth was observed in April, these samples were examined for two periods, including 1-2 weeks (sample 1) and 3-4 weeks of April (sample 2).

The plant leaves were harvested from their natural habitat. The samples were washed thoroughly with tap water to remove sand and dust, washed with double distilled water, and stored at room temperature until completely dry. The dried samples were then ground in a stainless-steel mill, 1.0 mm sieved, and stored at 4°C in dark glass vials.

Determination of total carotenoids content.

The content of chlorophylls a and b, as well as the total content of carotenoids, was determined photometrically by the own absorption of pigments in an acetone extract, according to an earlier described technique [34].

A dry grinded sample weighing 0.05 g was mixed with 5 ml of acetone and ground thoroughly in a porcelain mortar placed in an ice bath. Subsequently, 1.0 g of anhydrous sodium sulfate was added, and the mixture was additionally gently stirred. The acetone volume was then adjusted to 10 ml and the resulting solution was centrifuged at 26000 rpm for 10 minutes. The supernatant was carefully collected, and its absorbance was measured with SF-56 spectrophotometer at wavelengths of 662, 645, and 470 nm using a 1.0 cm quartz cuvette. The total carotenoid content was calculated as follows:

C_a (µg/g) = 11.24A₆₆₂ – 2.04A₆₄₅

C_b (µg/g) = 20.13A₆₄₅ – 4.19A₆₆₂

C_t (µg/g) = (1000A₄₇₀ – 1.9C_a – 63.14C_b) / 214

where C_a and C_b denote chlorophyll a and b respectively, and C_t corresponds to total carotenoid content. A₄₇₀, A₆₄₅ and A₆₆₂ are absorption at 470 nm, 645 nm, and at 662 nm, respectively.

Determination of anthocyanins content.

Anthocyanins content was spectrophotometrically measured from acidic extract of dried sample as described below [34].

A dry sample weighing 0.02 g was mixed with 4 mL of a 1% HCl solution containing a few drops of methanol and ground thoroughly in a porcelain mortar. The resulting mixture was stored at 4°C in a refrigerator for 24 hours. Afterward, the solution was centrifuged at 13,000 rpm for 10 minutes, and the absorbance of the supernatant was measured at 530 nm and 657 nm with SF-56 spectrophotometer using a 1.0 cm quartz cuvette against a blank. The anthocyanins content (mg/g DW) was calculated by the following equation:

Anthocyanins (mg/g) = A₅₃₀ – (0.25 · A₆₅₇)

where A₆₄₅ and A₆₆₂ denote the absorption at 530 nm, and at 657 nm, respectively.

Statistical analysis

The results are expressed as the mean values obtained from three independent measurements, accompanied by their respective standard deviations (SD). Linear regression and Pearson correlation analysis were conducted utilizing the MS Excel 2019 software.

Results and discussion

The results of the determination of the content of chlorophyll a and chlorophyll b in A. alnifolia leaves from April to October are shown in Fig. 2.

Figure 2. The content of chlorophyll a and b in leaves of A.alnifolia

Figure 2 demonstrates the seasonal dynamics of the chlorophyll a and b content in A.alnifolia leaves. As can be seen, the highest content of chlorophyll a is observed in October (38,91 ± 2,16 μg/g DW), while the highest content of chlorophyll b was detected in August (59.44 ± 2,81 μg/g DW). The lowest content of both pigments was found in early April (1,57 ± 0,17 and 2,31 ± 0,22 μg/g DW respectively). In the early months of spring, chlorophyll a and b levels gradually increase as leaf development progresses and photosynthetic activity intensifies. This increase is associated with the plant’s adaptation to rising temperatures and increased daylight, which promote chloroplast formation and the synthesis of photosynthetic pigments [33]. An insignificant decrease in chlorophyll levels in early autumn suggests stress factors such as early frost, drought, or nutrient deficiencies affecting pigment stability. The observed trends in chlorophyll a and b content in A. alnifolia align with known seasonal physiological processes in deciduous plants. The high chlorophyll a content in October may be an adaptive strategy to optimize photosynthesis in changing lighting and temperature conditions. Most likely, this variation is due to the fact that at the end of summer, plants maximize the intensity of photosynthesis, preparing for a decrease in sunlight intensity in autumn. Similar results are known for some plants growing in Russia and Poland. A study carried out for Alnus cordata and Tilia europaea showed that chlorophyll a and b levels were higher at the end of the growing season [18].

The ratio of chlorophylls a and b in leaves of A. alnifolia over the growing period is shown in Fig. 3.

Figure 3. The chlorophyll a/b ratio in leaves of A.alnifolia

Figure 3 illustrates seasonal variations of the chlorophyll a/b ratio in the leaves of A. alnifolia. As can be seen, the chlorophyll a/b ratio in the A. alnifolia leaves ranged from 0.56 (July) to 0.84 (May). In early spring, the chlorophyll a/b ratio starts at 0.68 in the first decade of April (Sample 1), increases to 0.81 at the end of April, and reaches a peak value of 0.84 in May. The observed trend suggests active chlorophyll synthesis, driven by increasing light availability and the onset of the growing season. This result aligns with known reports that young leaves prioritize photosynthetic efficiency by accumulating higher proportions of this pigment [20].

During the summer, the ratio decreases to 0.62 in June and to 0.56 in July. This decrease may indicate increased environmental stress, such as high light intensity, temperature fluctuations, or drought conditions, which accelerate chlorophyll degradation [27]. Additionally, chlorophyll b levels might increase relative to chlorophyll a as a photoprotective response, facilitating the dissipation of excess energy to prevent damage to the photosynthetic apparatus.

From late summer to autumn, the chlorophyll a/b ratio shows a moderate recovery, reaching 0.60 in August, 0.65 in September, and stabilizing at 0.67 in October. This partial rebound may be attributed to decreasing temperatures and reduced sunlight intensity, which alleviate photodamage and promote chlorophyll retention. Similar adaptations have been previously reported in deciduous species as they prepare for cold weather while maintaining photosynthetic function under changing light conditions [28].

Nevertheless, the chlorophyll b content in leaves rarely exceeds the chlorophyll a content during the entire growing season under normal conditions. Chlorophyll a is typically more abundant because it plays a primary role in photosynthesis, directly participating in the light reactions by capturing light energy and transferring electrons. Chlorophyll b, on the other hand, acts as an accessory pigment, broadening the range of light wavelengths absorbed and transferring energy to chlorophyll a. However, plants growing in shaded environments often have higher chlorophyll b content relative to chlorophyll a. This adaptation increases light-absorption efficiency under low-light conditions [26].

The total anthocyanin content in the leaves of A.alnifolia is shown in Fig. 4.

Figure 4. Total anthocyanins content in leaves of A.alnifolia

Figure 4 shows the seasonal variation of anthocyanin content in A. alnifolia leaves also reflects the dynamic physiological adaptations of the plant to environmental conditions throughout the growing season. The anthocyanin content in the leaves of A.alnifolia ranged from 0.20 mg/g in October to the highest value of 1.15 mg/g in May. Observed dynamics revealed that in spring, anthocyanins accumulate in young leaves. In early April, anthocyanin content is relatively low, with values of 0.33 mg/g DW in the first sample and 0.63 mg/g DW in the second. This initial increase suggests that anthocyanins serve a protective role as the photosynthetic system forms in young leaves. Young leaves often experience high levels of light stress due to insufficient development of chlorophyll and other photoprotective pigments, which makes anthocyanins vital for protecting leaf tissues from excessive exposure to light and reactive oxygen species (ROS) [13]. The anthocyanin content reaches its peak in May (1.15 mg/g DW), coinciding with increasing sunlight intensity and temperature. At this stage, anthocyanins act as sunscreen pigments, absorbing excess light in high-radiation environments and preventing photoinhibition of chloroplasts [5].

During the summer months, the anthocyanin content stabilizes at 0.56 mg/g, 0.55 mg/g, and 0.56 mg/g DW in June, July, and August, respectively. This relatively stable content suggests that anthocyanins continue to act as photoprotective and antioxidant agents, especially in response to prolonged exposure to intense sunlight. In addition, summer is often characterized by elevated temperatures and periods of drought stress, both of which can lead to increased ROS production in plant cells. Anthocyanins have been previously shown to scavenge these reactive molecules, thereby reducing oxidative damage of leaf tissues [4].

A significant decrease in anthocyanin content is observed in September (0.40 mg/g DW) and further in October (0.20 mg/g DW). This decrease may be due to the onset of wilting, during which plants prioritize the absorption of nutrients from the leaves, preparing for winter. Since anthocyanins are synthesized at an energetic cost, their production is downregulated as corresponding chlorophyll degradation becomes the dominant metabolic process [19]. The decrease in anthocyanins in October is also due to the fact that plants shed their leaves in autumn to conserve water and nutrients. The synthesis of anthocyanins is reduced because they do not play any role in the process of leaf wilt. The plant focuses on absorbing nutrients from the leaves before they fall off, which further reduces anthocyanin levels [2].

Carotenoids are essential pigments in plants, playing a critical role in photosynthesis, photoprotection, and stress responses. These pigments serve as accessory molecules, protect chlorophyll from photooxidative damage, and contribute to the coloration of leaves, fruits, and flowers.  The total carotenoid content determined in leaves of A. alnifolia is given in Fig.5.

Figure 5. Total carotenoids content in leaves of A.alnifolia

In April, carotenoid levels are relatively low, with values of 0.28 µg/g DW in the first sample and 0.40 µg/g DW in the second one. This low concentration is due to the early stage of leaf development when pigment biosynthesis is still in progress. At this stage, young leaves rely on carotenoids to protect developing chloroplasts from excessive light and oxidative stress [9]. As the leaves expand and photosynthetic capacity increases, carotenoid content rises sharply, reaching a peak of 3.91 µg/g DW in May. This significant increase suggests that carotenoids play a crucial role in light absorption and energy transfer within the photosynthetic apparatus, supporting the high demands of actively growing plant tissues. In addition to their role in photosynthesis, carotenoids function as antioxidants, preventing damage caused by ROS generated under intense sunlight [3].

In summer, carotenoid levels gradually decrease. In June, the content drops to 2.49 µg/g DW, followed by a further decrease to 0.98 µg/g DW in July. This decline corresponds with the stabilization of the photosynthetic system as the leaves reach full maturity. At this stage, carotenoids continue to support photoprotection by dissipating excess energy through the xanthophyll cycle, which prevents photoinhibition under strong summer sunlight [25]. However, a slight increase in carotenoid content is observed in August, reaching 1.51 µg/g DW. This temporary rise may be attributed to environmental stress factors such as high temperatures, drought, or increased light exposure. The increased carotenoid content during this period suggests enhanced photoprotective mechanisms that help maintain photosynthetic efficiency under suboptimal conditions.

In late summer and early autumn, carotenoid content begins to decline again. In September, levels drop to 0.93 µg/g DW, and in October, they reach 0.51 µg/g DW. This progressive reduction is characteristic of leaf senescence, a process in which plants degrade pigments and reallocate nutrients to storage organs in preparation for winter dormancy. Unlike chlorophyll, which breaks down rapidly, carotenoids degrade more slowly, often contributing to the yellow and orange coloration observed in autumn leaves [21].

The observed seasonal changes in carotenoid content in A. alnifolia leaves highlight their critical role in leaf development, photosynthesis, and stress tolerance. The peak observed in May aligns with the period of maximum photosynthetic activity, while the subsequent decline indicates the stabilization of the photosynthetic system. The temporary increase in August suggests a protective response to environmental stress, whereas the gradual reduction in autumn corresponds with the natural process of leaf senescence. These findings are consistent with previous studies on the seasonal dynamics of carotenoids in deciduous plants, emphasizing their multifunctional role in plant physiology. Environmental factors such as light intensity and nutrient availability also affect carotenoid biosynthesis [17] [23].

In this work, linear regression and Pearson correlation analysis were also performed to establish the relationships between the content of anthocyanins, carotenoids, chlorophylls a and b, and their ratios. Figure 6 illustrates linear regression and Pearson correlation coefficients for these pigments in A. alnifolia leaves, reflecting insights into their relationships throughout the growing season.

Figure 6. Linear regression (R²) (A) and Pearson’s correlation coefficients (r) (B) for the relationship between plant pigments in leaves of A. alnifolia

Linear regression coefficients indicate the strength of the dependency between pigments. In Figure 6A, chlorophyll a (Chl a) and chlorophyll b (Chl b) show a significant positive correlation (0.93), suggesting synchronized accumulation and degradation patterns. This is consistent with studies showing that Chl a and Chl b function together in light-harvesting complexes (LHCs) and are regulated by similar biosynthetic pathways [8]. The relationship between the chlorophyll a/b ratio (Chl a/b) and other pigments is relatively weak, with coefficients of 0.11 for Chl a and 0.31 for Chl b. These observations may indicate that fluctuations in the ratio are influenced by external environmental factors such as light intensity rather than direct pigment synthesis interactions. Carotenoids exhibit a weak correlation with chlorophyll b (0.27) and a higher correlation with anthocyanins (0.69). The strong association between carotenoids and anthocyanins suggests their cooperative role in photoprotection, particularly during stress conditions. Anthocyanins are known to shield chlorophyll from excessive light exposure, while carotenoids participate in the dissipation of excess energy [10].

The high positive correlation between Chl a and Chl b (0.96, p < 0.05) confirms their coordinated function within the photosynthetic apparatus. Similarly, a moderate correlation is observed between the chlorophyll a/b ratio and Chl b (0.56), indicating that shifts in pigment composition affect the balance between LHC and reaction centers. A particularly strong positive correlation is observed between carotenoids and anthocyanins (0.83, p < 0.05), reinforcing their complementary roles in stress response mechanisms. A similar relationship was previously discovered by Havaux et al., emphasizing that both types of pigments contribute to photoprotection under oxidative stress [15].

Carotenoids demonstrate a moderate correlation with Chl b (0.36) and Chl a (0.52), reinforcing their integral role in light harvesting and photoprotection. However, their negative correlation with the chlorophyll a/b ratio (-0.28) suggests that an increase in carotenoid content is associated with modifications in the LHC, which is a common adaptation to high-light environments. Interestingly, anthocyanins exhibit a negative correlation with the chlorophyll a/b ratio (-0.52), suggesting that higher anthocyanin accumulation coincides with a reduction in the relative proportion of Chl a to Chl b. This pattern aligns with findings that anthocyanins accumulate under stress conditions such as excessive light or temperature fluctuations, leading to corresponding changes in LHC [7].

Conclusion

The present study aimed to analyze the seasonal variations in anthocyanins, carotenoids, and chlorophylls within the leaves of A. alnifolia throughout its growth cycle. The results revealed substantial changes in pigment composition over time, indicating the plant’s ability to adapt to fluctuating environmental conditions. Chlorophylls play a key role in sustaining photosynthetic efficiency during the peak growing period, while carotenoids and anthocyanins primarily function as protective pigments, particularly in early leaf development and under environmental stress. The observed correlation between carotenoids and anthocyanins highlights their synergistic role in preserving the integrity of the photosynthetic system. These findings contribute to a deeper understanding of pigment metabolism in A. alnifolia and similar species, with potential applications in agriculture, forestry, and stress physiology research. Future investigations should focus on uncovering the molecular pathways that regulate pigment production and degradation under diverse climatic conditions to further elucidate plant adaptation mechanisms.

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