Salinity tolerance and growth response of redroot pigweed (Amaranthus retroflexus L.): a comprehensive evaluation

Plant materials and growth conditions

We used a homogeneous crop of young A. retroflexus growing on the Ararat Plain to collect seeds for our experiment. A. retroflexus seeds were surface-sterilized in 10% sodium hypochlorite for 15 min and then rinsed with distilled water many times. One kilogram of clay loam soil was used to fill pots. First, soil was deemed non-saline till soil electrical conductivity (EC_e) value was not reached <2.0 dS m⁻¹ (Allison et al., 1954). After 15 days, the seedlings were moved to plastic pots (20 L capacity with a height of 26 cm and a top diameter of 37 cm) with one plant per container. An experimental design, known as a complete random block (CRD), was employed, where each treatment represented five replicate pots. After the first leaf had fully matured, the plants were exposed to salt stress induced by various NaCl concentrations. The levels of salt stress were: non-saline, slightly saline, moderately saline, highly saline, and extreme₁ and extreme₂ correspondingly and full information of all treatments mentioned in Fig. 1. To prevent salt shock, NaCl was added slowly until the required amounts were reached. One seedling was placed in each of the five replicate containers that made up each treatment. Following a 25-day period of stable adaptation, measurements were taken every 10 days. For the duration of the experiment, non-perforated plastic containers were used to maintain a steady soil electrical conductivity and stop the leaching of salts.

Morphological indices

Physiological indices

Biochemical parameters

Vegetation indices

Vegetation indices are helpful in contemporary measurement approaches to assess plant health and are crucial for investigating (Steddom et al., 2005). In addition to identifying plant stress and diseases, vegetation indicators are useful for assessing the biophysical and biochemical characteristics of plants (Rumpf et al., 2010).

Spectrometers are one of the most often used tools for figuring out a plant’s spectrum characteristics. These instruments are capable of measuring wavelengths between 300 and 1,300 nm in the field and lab (Mac Arthur, MacLellan & Malthus, 2012). According to Wasonga (University of Helsinki, 2024) one of the most widely used spectrometers is the CI-710s SpectraVue Leaf Spectrometer. It is intended to test a plant’s ability to absorb, transmit, and reflect light across a broad wavelength range simultaneously. New functions added to the most recent SpectraVue Leaf Spectrometer include a touch screen, an integrated MS Windows operating system, a broad wavelength range (360–1,300 nm), and a leaf probe connection. As a result, data collection is streamlined, and the sensors are portable. The leaf spectrometer was equipped with two integrated wideband light sources (CID Bio-Science).

Meanwhile, the following spectral vegetation indices in A. retroflexus leaves were measured in vivo from 9 am to 12 am using a CI-710s Spectra Vue Leaf Spectrometer (CID Bio-Science) with compatible software: Anthocyanin Reflectance Index (ARI); Carotenoid Reflectance Index (CRI); Carter Index (Ctr); Structure Intensive Pigment Index (SIPI); Flavanol Reflectance Index (FRI); Triangular Vegetation Index (TVI); Greenness (G); Lichtenthaler Index (LIC); Gitelson and Merzlyak (GM1 and GM2); Zarco Tejada–Miller Index (ZMI); Normalized Difference Vegetation Index (NDVI); Normalized Phaeophytinization Index (NPQI); Normalized Pigment Chlorophyll Index (NPCI); Photochemical Reflectance Index (PRI); Simple Ratio Pigment Index (SRPI); Water Band Index (WBI); Plant Senescence Reflectance (PSRI); Transformed Chlorophyll Absorption in Reflectance Index (TCARI); Vogelmann Red Edge Index (VREI); Red Edge Normalized Difference Vegetation Index (RENDVI); Chlorophyll Normalized Vegetation Index (CNDVI); Modified Red Edge Simple Ratio (MRESRI); Modified Chlorophyll Absorption Ratio Index (MCARI) and Modified Datt (MDATT) Index. The spectrometer on the leaf was calibrated before the measurements. The dried leaf was then placed into a leaf clip. Spectrometric measurements were performed on five leaves from each plant.

Statistical analysis

The data were analyzed using SPSS-19 and Microsoft Excel 2021. Fisher’s least significant difference (LSD) test was used to find statistically significant differences. Confidence intervals of 95% are shown as error bars in figures.

Morphological indices

Plant morphological traits, such as stem diameter and plant height, are important markers for determining the capacity of plants to tolerate salt stress. With increasing NaCl concentrations, plant height and stem diameter dramatically decreased. At extremely saline levels, a significant reduction in plant height was observed. In comparison to non-salinity levels, plant height was significantly decreased at extreme ₂ salinity levels. More specifically, the maximum plant height was recorded at the non-salinity level, whereas a drop in plant height (by 54.8%) was noted at extreme ₂ salinity levels. The trends for 10 daily measurements of plant height are presented in Fig. 2A. There was no discernible variation in average plant height up to moderate salinity levels. A stronger reduction was registered in plants under high to extreme₂ salinity levels, reaching a reduction of 1.4-fold for A. retroflexus. The reduced accessibility of nutrients and impaired cell development could be the cause of the reduction in plant length. Plants cultivated in salty soils first encounter osmotic stress, which restricts the absorption of water, and subsequently ion toxicity, which results in nutrient imbalance. The stem diameter of A. retroflexus was likewise affected by NaCl treatment. Over the course of the trial, the stem diameter data followed an identical trend to that of the plant height data. Up to extreme ₂ salinity level, a declining trend in stem diameter was also noted (the difference being approximately 54.7% when compared to plants cultivated in non-saline soil) (Fig. 2B). Salinity stress generally decreased stem diameter and plant height; the extreme and non-salinity levels of the data were separated by a cluster of results. While the initial growth rates in the early weeks showed little difference, the differences increased throughout the remainder of the experiment.

Physiological indices

Effect of salinity stress on A. retroflexus biomass productivity and water content

A considerable effect of different NaCl concentrations on plant biomass was observed. Increasing the salinity level from the first phase (non-salinity level) to the last phase (extreme₂ salinity level) reduced DW and FW gain in the roots, stems, and leaves of plants. The adverse concentration effects of salinity stress were clearly demonstrated by plants under high salinity levels. FW of roots, stems, and leaves of plants decreased by 86.6%, 93.7%, and 78.5%, respectively, and DW of roots, stems, and leaves decreased by 87.2%, 92.4%, and 78.2% under conditions of extreme₂ salinity level compared to plants grown in non-saline soil. Slightly salinized soil had the highest DW and FW values for A. retroflexus roots, stems, and leaves. Compared to the corresponding control, the statistical analysis showed a significant decrease in plant biomass. Comparative trends for roots (FW and DW), stems (FW and DW), and leaves (FW and DW) are shown in Figs. 2C–2E.

WC values for roots, stems, and leaves generally showed a tendency toward decline. When the value of WC dramatically dropped after the high salinity level, a tipping point was found. Following the high salinity level, the value of root, stem, and leaf WC greatly dropped. In comparison to plants growing in non-saline soil, stem water content (SWC) and leaf water content (LWC) of plants dropped by 22.1% and 2.1%, respectively, under high salinity conditions, while root water content (RWC) increased by 6.47%, respectively. Figures 3 and 4 illustrate the correlation between salinity level and WC factors.

FW_r/FW_s and DW_r/DW_s of A. retroflexus gradually increased with increasing soil salinity. The increase was steeper from moderate to extreme ₂ salinity levels (1.4-fold for FW and 1.2-fold for DW, respectively) compared to the control. DW_r/DW_s and FW_r/FW_s decreased up to a moderate salinity level and then gradually increased. Figures 2F–2H and 3 depict the effect of salinity stress significantly affecting water distribution in plants. While root and shoot weight ratios increased, relative WC and declined, highlighting osmotic stress in response to salinity.

P_n, E and WUE

These findings indicated that the P_n value of A. retroflexus decreased (Fig. 5). This might be because throughout that time the plants developed a partial ability to adjust to the effects of the stress factor. An identical trend was also observed for the E values. In comparison to plants grown in non-saline soil, P_n and E values in plants under extreme ₂ salinity level generally dropped by 51.8% and 82.5%, respectively. Nevertheless, distinct WUE trends were noted throughout that time. More specifically, the plants started to use water more efficiently, and the WUE value increased by 2.7-fold as a result of improved tolerance to salinity stress and the development of more potent defense mechanisms against salt stress. Salinity stress significantly reduced P_n and E across all measurements, indicating impaired gas exchange and reduced plant physiological performance (Fig. 5). While WUE showed an inconsistent trend, its increase in the third measurement suggests a possible compensatory mechanism under extreme salinity. Overall, higher salinity negatively affected plant gas exchange, likely leading to reduced growth and productivity.

Biochemical parameters

Uptake nutrient content, K⁺/Na⁺ ratio, REL

Increasing salinity stress caused distinct ion accumulation in the roots, stems, and leaves in diverse ways (Fig. 6). The findings indicate that, in comparison to control plants, a drop in the total K⁺ concentration was seen in the roots and leaves of the plants (K⁺ content decreased in roots and leaves 3.3 and 1.3 times, respectively). The opposite pattern was observed in the stems of plants; with increasing salinity level, there was an increase in total K⁺ content compared to the control plants (K⁺ content increased 1.0 times). Salinity enhanced Na⁺ content in roots 6.6 times, stems 158.1 times and leaves 33.1 times, but decreased in the contents of K⁺ and K⁺/Na⁺ ratio in roots (3.3 and 21.8 times, respectively), stems (K⁺/Na⁺ ratio 157.3 times) and leaves (1.3 and 45.0 times, respectively) in response to salinity. Although the increase in Ca²⁺ content did not occur in such a dramatic manner (the content in roots and stems increased by 2.9 and 2.4 times, respectively, and decreased by 1.4 times in leaves), the content of Cl⁻ in the roots, stems, and leaves was also significantly increased by 12.0-, 22.3 times and 24.6 times, respectively, along with the rise in salinity level. Salinity stress leads to a significant increase in Na⁺ and Cl⁻ accumulation, disrupting ionic homeostasis in plant tissues (Fig. 6). The sharp decline in K⁺ and K⁺/Na⁺ ratio highlights severe potassium depletion, which may impair essential cellular functions. The mixed response of Ca²⁺ suggests tissue-specific regulation, possibly playing a role in stress adaptation. Overall, high salinity induces ionic imbalance, potentially reducing plant growth and metabolic efficiency. Figure 6 depicting the correlation between ion content in plant tissues and salinity level.Salt stress also caused a considerable increase in REL value. While the rise was proportionate, there was a noticeable sharp shift in the REL value after the level of salinity reached a moderate one (Fig. 2K). When comparing non-saline soils to those with extreme₂ salinity, the value of REL rose by 4.5 times.

Assessment of STI

The value of STI of plants showed the highest value at non-saline level. The variation was 89.7% when compared to plants cultivated in non-saline soil. As demonstrated in Fig. 2L, the STI value drastically dropped as salinity levels increased.

Evaluation of vegetation indices

Based on carotenoids and chlorophylls, the PRI and PSRI are commonly used to evaluate changes in the physiological status of plants. Five leaves from each plant were measured at different heights to calculate the PRI (Fig. 7L). The PRI normalizes the reflectance at 531 nm (R531) by combining it with a reference wavelength (R570) that is resistant to transient variations in the light energy conversion efficiency.

Throughout the vegetative period, the PSRI underwent substantial changes. Initially, the plants experienced less stress. It was found that when the PSRI falls below 0, it indicates the beginning of leaf senescence (Fig. 7K). Hence, the PSRI serves as a quantitative indicator of leaf senescence, while the photosynthetic efficiency is characterized by the photosynthesis rate (PRI). PSRI was found to be sensitive to carotenoids and the concentration ratio of chlorophyll.

Plants grown in saline and non-saline soils showed substantial differences in CRI2. Salinity stress can cause carotenoids to rise or fall according to the CRI2 data (Fig. 7AB). SIPI (Fig. 7I) of salt-affected plants (0.467) was significantly lower than that of the control plants (0.541) under NaCl treatment. Significant variations between salt-affected and control plants were identified under NaCl treatment, as demonstrated by the PRI (Fig. 7L), which followed the same trend as SIPI.

The NDVI is an index that is frequently and extensively used. NDVI is calculated as the ratio difference between reflectance in the red and near infrared bands. The NDVI (Fig. 7O) as greenness and disease indices of salt affected (0.456) experimental plants were significantly lower than the NDVI of non-affected (0.546) plants under NaCl treatment.

Significant differences in WBI (Fig. 7B) were exposed between control (1.034) and salt-affected (1.008) plants grown under NaCl treatment. The results of this investigation support the theory that salt stress causes WBI to decline. The WBI of salt-affected plants was significantly lower than that of non-affected plants because of reduced LWC. Pathogen inhibitors, such as flavanols and anthocyanins, were found in greater quantities shortly thereafter. The ARI1 and ARI2 of salt-affected plants maintained the same trend (Figs. 7AE and 7AF). Water scarcity is a result of decreased photosynthetic efficiency, as indicated by an increase in FRI (Fig. 7Y). Therefore, the FRI is inversely proportional to the WBI. Accordingly, it is reasonable to believe that the plants were under stress and forced to activate their defense mechanism, given that they lost a large amount of water and dramatically increased their flavanol content. It is well known that pigments such as xanthophyll, carotenoid, and chlorophyll are highly dependent on photosynthetic area and light. The consequence of this is a decrease in vegetation pigments. Therefore, the state of the host plant can be reliably determined using pigments and their decreasing or increasing indices.

Changes of plant and growth development characteristics of A. retroflexus

Our research findings suggested that A. retroflexus plants were more affected by higher salt treatments. This study examined the morphological, physiological, and biochemical changes in plant growth and development of the A. retroflexus plant genotype under different salt concentrations (non-saline, slightly saline, moderately saline, highly saline, and extreme₁ and extreme₂). As shown in Figs. 2A–2B, plant height and stem diameter were not significantly changed at slightly saline levels when compared to the control. However, these morphological traits decreased as salinity levels increased from moderately salinized to extreme₂ in A. retroflexus plants. Plant development and growth were affected by higher concentrations of NaCl, according to some preliminary research (Grieve & Suarez, 1997; Wouyou et al., 2017). Some reports also support our findings that higher NaCl concentrations decreased plant height and stem diameter in some plant species (Zaman et al., 2020; Borsai et al., 2020; Pulvento, Houssemeddine Sellami & Lavini, 2022). As an indicator of the plants’ ability to survive under different abiotic stressors, plants’] FW and DW, biomass, and plant height indicate the plants’ life-sustaining processes. The current study found that plants subjected to varying levels of salt stress (non-saline, slightly saline, moderately saline, highly saline, and extreme₁ and extreme₂) significantly reduced the biomass of roots, stems, and leaves (Figs. 2C–2E). Despite this, the biomass of A. retroflexus most drastically decreased at salinity levels ranging from high to extreme₂. According to the results, A. retroflexus can survive salinity levels of slightly salinized to moderately salinized without sacrificing their FW and DW of root, stem, and leaves, but these indicators were significantly stunted when exposed to salt levels of highly salinized to extreme₂, compared to the control condition. Various previous study also supports this finding which suggested that increasing the salinity stress decreased the plant biomass (Butcher et al., 2016; Wouyou et al., 2017; Feine et al., 2019; Sarker & Oba, 2020; Singh et al., 2022b; Singh et al., 2022a; Szymańska et al., 2022; Pulvento, Houssemeddine Sellami & Lavini, 2022). Decline in plant biomass is also correlated with the water status in plants. A detrimental impact on plant-water interactions results from the buildup of ions in plant cells caused by fast ion absorption. Under conditions of salinity stress, plants experience a fall in turgor pressure and a decrease in WC within their cells because of an osmotic gradient caused by the soil’s high salt concentration (Ondrasek et al., 2022). Our experiment finding suggested that Figs. 2F–2H, and 3 show the results of total water content in the roots, stems, and leaves of A. retroflexus plants affected at different levels of salinity stress. Salinity stress responsible for two types of stresses ionic and osmotic stress which reduced the photosynthesis rate resulting final carbon product of photosynthesis accumulation reduces and this process is responsible for buildup of biomass in plants (Wouyou et al., 2017; Zeeshan et al., 2020; Tovar et al., 2020; Ondrasek et al., 2022; Liu et al., 2022a; Pulvento, Houssemeddine Sellami & Lavini, 2022; Balasubramaniam et al., 2023; Luyckx, Lutts & Quinet, 2023; Liu, An & Lai, 2024). Thus, the FW and DW of A. retroflexus plants decreased. In response to severe salt stress, plant FW dropped dramatically, and this drop was expected as salinity increased (Figs. 2F–2H, 3–4). Estrada et al. (2021) discovered that the plant biomass matter content of Amaranthus was unaffected at 100 mM but higher salinity level decreased the total water content and biomass.

Alteration in photosynthesis characteristics of A. retroflexus under salinity stress

Plants depend on photosynthesis, which produces both organic matter and energy that help in continuing plant growth and development. Inhibition of photosynthesis is the primary mechanism of salinity stress, which is responsible for the reduction in the formation of biomass in plants. One of the most essential photosynthetic pigments in plants is chlorophyll, which aids light absorption, transmission, and transformation. Tolerance levels in plants can be evaluated by analyzing their chlorophyll content (Wang et al., 2022). In this study, under salt stress, CCI decreased significantly more in the highly salinized to extreme ₂ NaCl treatment compared to the slightly salinized to moderately salinized and control or non-saline treatments. When plants are exposed to salinity stress, their photosynthetic traits can serve as good indicators to analyses their salinity stress adoptability capacity (An et al., 2021). Plant gas exchange characteristics are known to be negatively affected by salinity stress which directly affected plant photosynthetic capacity (Naz, Akram & Ashraf, 2016; Hnilickova et al., 2021). This study found that A. retroflexus P_n, E, and WUE were reduced when exposed to salt stress. There was a greater reduction in P_n, E, and WUE under highly salinized to extreme₂ stress conditions compared to slightly to moderately salinized and non-saline levels. In the current study (Figs. 2J, 5) the levels of A. retroflexus P_n, E, and WUE were found to be significantly lower under situations of water deficiency or osmotic imbalance developed due to salinity stress (Hawrylak & Wolska-Mitaszko, 2007; Wouyou et al., 2017; Feine et al., 2019; Zeeshan et al., 2020; Liu et al., 2022a; Pulvento, Houssemeddine Sellami & Lavini, 2022; Luyckx, Lutts & Quinet, 2023; Liu, An & Lai, 2024). For a long time throughout the day, the stomata remain closed because of the water stress or osmotic stress caused by salinity stress. The results indicated that the inhibitory effect on A. retroflexus photosynthesis was higher with highly salinized to extreme₂ salt stress compared to slightly to moderately salinized level. Due to stomatal closure under higher saline stress reduces E value and intercellular carbon dioxide concentration due to this in our experiment the P_n of A. retroflexus was too much lower under highly salinized to extreme₂ saline conditions compared slightly to moderate salinized level and control conditions (Gandonou et al., 2018; Tarin et al., 2020). Thus, stomatal factors mostly caused the reduction in photosynthesis of A. retroflexus under increased saline stress (highly salinized to extreme₂). Another key component of the restriction of photosynthesis caused by salt stress is the reduction in chlorophyll content (Ondrasek et al., 2022).

Biochemical activity response of A. retroflexus against salt stress

During salinity stress biochemical changes occurs which induced osmotic and ionic stresses that cased ionic and osmotic imbalance into plants system but among these two ionic stresses is more lethal and altered the biological functions like membrane damage, inhibition of photosynthesis process, imbalance of ionic and osmotic conditions of plants (Singh et al., 2022b; Singh et al., 2022a; Singh et al., 2023; Singh et al., 2024). Plasma membranes are the primary ion-specific target of salt, which causes membrane damage and increases REL (Fig. 2K) (Souri, Hatamian & Tesfamariam, 2019). One of the most essential criteria for identifying salt-tolerant capacity of plants is to monitoring amount of electrolyte leakage from plasma membranes (Ali & Ashraf, 2011). In the current study, as the salt concentration increased, the REL value increased similarly across all treatments (Fig. 2K). Similar results were also observed in lettuce (Lactuca sativa L. cv. Orion), New Zealand spinach (Tetragonia tetragonoides (Pall) Kuntze) and common purslane (Portulaca oleracea L. cv. Green Purslane) (Hniličková et al., 2019). Ionic stress induced the nutrient-shortage or toxicity can hinder plant growth in deficient root zone nutrients. Low nutrient availability due to competition with main ions (Na⁺ and Cl⁻) makes mineral acquisition of nutrients challenging under salt-stress scenarios. This interaction mainly reduces Ca²⁺ and K⁺ like essential ions which participated in various biological function in plants (Fig. 6) (Munns & Termaat, 1986). The link between salt stress and nitrogen (N), phosphorus (P), and K⁺ is complicated. N is a vital mineral in plant cells and increased Cl⁻ absorption in saline environments can reduce shoot N uptake due to Cl⁻/NO₃⁻ antagonism (Munns & Termaat, 1986). Phosphorous is a key element which play important role in photosynthesis, transportation/storage, energy transfer in ETS system and less absorption of phosphorous is affected by salt stress. In soil with high levels of Cl⁻ and SO₄²⁻, phosphorous absorption may decrease due to high ionic strength and limited solubility of Ca ± P minerals. Na⁺ and K⁺ compete for root uptake sites resulting lowering K⁺ and Ca²⁺ (Fig. 6). Na⁺-induced K⁺ uptake reduction in plants is competitive, regardless of Na⁺, Cl⁻, or SO₄²⁻ dominance (Ali et al., 2013). Salinity creates nutritional imbalances and lowers the yield of crops, according to extensive research on Manihot esculenta and Zea mays (Ali & Ashraf, 2011; Ali et al., 2013; Shafiq et al., 2019; Razzaq et al., 2020; Ashraf et al., 2022). Root, stem and leaves Na⁺ and K⁺ concentrations and K⁺/Na⁺ ratios play key markers that determine the salt tolerance capacity of plants. When compared to controls root, stem and leaves Na⁺ and K⁺ levels, as well as their K⁺/Na⁺ ratios, were significantly different under different levels (non-saline, slightly saline, moderately saline, highly saline and extreme₁ and extreme₂) of salinity stress. A lower K⁺/Na⁺ ratio was observed for treatments with highly salinized to extreme₂ salt level, which explained the tolerance capacity of plants under these salinity levels. Previous many experiments also show that in different crop and Amaranthus plants including increasing the salinity levels decreasing the K⁺/Na⁺ ratio (Zeeshan et al., 2020; Estrada et al., 2021; Chourasia et al., 2021b; Liu et al., 2022b; Luyckx, Lutts & Quinet, 2023).

Effect of salt stress on the tolerance activity and vegetation traits of A. retroflexus

A. retroflexus exhibits a wide range of morpho-physiological and biochemical reactions when exposed to salt and shows good tolerance at the initial stages of salinity. This study showed that all biochemical and morpho-physiological factors were linked to STI (Fig. 2L). As salinity stress increased, the STI value decreased (Fig. 2L). Certain STI parameters, such as SST (Fig. 2M) increased as salinity levels increased, while SLSI (Fig. 2N) and SWSI (Fig. 2O) decreased.

Plants absorb radiation slowly in the red band due to chlorophyll pigments, while responding quickly to near-infrared (NIR) wavelengths due to their strong radiation reflection. Xue & Su (2017) suggested that indices based on distinct spectral bands and the corresponding plant response might be used to assess the vegetation conditions. Various vegetation indices, such as the ARI2, CRI2, SIPI, FRI, NDVI, and G, are used to track plant growth and health (Radočaj et al., 2023). The SAVI characterizes green biomass (Blackburn, 1998). The PRI and PSRI are used to assess plant physiological status, with PRI normalizing reflectance at 531 nm and PSRI assessing light energy conversion efficiency. PSRI changes during vegetative periods, indicating leaf senescence. This quantitative indicator is sensitive to carotenoids and chlorophyll concentration ratio, and is a quantitative measure of photosynthetic efficiency, as characterized by the photosynthesis rate (PRI). Salinity stress affects plant growth, affecting carotenoids.

For leaf carotenoids, the SIPI is used. Salt-affected plants showed lower SIPI than control plants under the NaCl treatment. The NDVI, a vegetation index, is also used in studies on global environmental and climatic change (Bhandari, Kumar & Singh, 2012; Gandhi et al., 2015). The NDVI as greenness and disease indices of salt-affected plants were significantly lower than non-affected plants under NaCl treatment. These findings highlight the importance of understanding plant growth and adaptation to salinity stress. The current study found significant differences in WBI between control and salt-affected plants grown under NaCl treatment. This study supports the theory that salt stress causes WBI to decline, with salt-affected plants having lower WBI due to reduced LWC. Pathogen inhibitors, such as flavanols and anthocyanins, were found in greater quantities. Pigments such as xanthophyll, carotenoid, and chlorophyll are highly dependent on photosynthetic area and light, resulting in a decrease in vegetation pigments.