Water stress reduced the growth of E. myrcianthes seedlings and negatively affected photosynthetic metabolism. However, the seedlings recovered after stress suspension, suggesting the physiological plasticity of the species since they showed 100% survival. Applying 2 mmol L−¹ of silicon to the seedlings helped them tolerate the stressful effect of water fluctuation. As a result, silicon contributes to relative water content. Moreover, it decreases transpiration rates, increases osmotic adjustment capacity, and increases water uptake (Rizwan et al. 2015; Zhu et al. 2015).
The relative water content and gas exchange recovered after water fluctuation, regardless of Si application. Furthermore, the characteristics of A, E, Gs, and A/Ci reached higher values than control seedlings when treated with 2 mmol L-¹ of Si. Even though the presence of Si assisted in mitigating the stressor effect during stress, the seedlings recovered regardless of prior Si application for the other traits, such as chlorophyll a fluorescence and antioxidant enzyme activity. However, it is worth noting that both proline and carotenoids remained higher at the end of the evaluations. It suggests that these compounds, related to antioxidant signaling and protection, remained active, which can be understood as a defense mechanism in E. myrcianthes.
We emphasize that the seedlings with Si, regardless of the water regime, showed higher chlorophyll a content, with I2Si seedlings being 19.50% higher than I0Si seedlings. Meanwhile, S2Si seedlings were 3.59% higher than S0Si seedlings. Similar results were reported by Hussain et al. (2021), where Si treatment increased chlorophyll content in soybean plants, which resulted in increased A.
We point out that the increase in carotenoids indicates protection against stress since it protects the photosystems' chlorophyll molecules. Moreover, carotenoids act as light-collecting pigments, thus improving photosynthesis (Pattanaik and Lindberg 2015). Foliar application of Si improves chlorophyll synthesis and photosynthetic capacity in plants subjected to water deficit stress (Savvas and Ntatsi 2015; Verma et al. 2020; Younis et al. 2020).
The chlorophyll index reduction under flooding may be due to the reduced assimilation of nitrogen, which is important in chlorophyll synthesis (Linné et al. 2021). Helaly et al. (2017) reported that Si + flooding treatment increases chlorophyll and nutrient content in Mangifera indica L., which increases the photosynthetic rate.
All gas exchange traits such as A, E, A/Ci, Gs A/Gs and WUE reduced under stress, however, a reduction was less significant in seedlings treated with 2 mmol− 1 silicon. The time to reach zero photosynthesis and to recover the photosynthetic rate varies between species (Silva et al. 2021; Linné et al. 2021; Barbosa et al. 2021). In our study for E. myrcianthes seedlings, a 1st F0 under water deficit occurred in 21 days and a 2nd F0 under flooding in 50 days after the plants remained under stress.
We emphasize that although Ci increased under stress in the S2Si changes, the values were significantly similar to the control. Although these characteristics were recovered after the stress was suspended, A, E and WUE outperformed the controlled seedlings when treated with 2 mmol− 1 silicon, and A/Gs outperformed the control seedlings regardless of the presence of silicon. However, A/Ci only recovered when seedlings were treated with 2 or 4 mmol− 1 Si. Thus, we suggest that the use of 2 mmol L− 1 of Si mitigated the effect of water stress, both by deficit and by flooding, on most gas exchange characteristics.
The behavior of E. myrcianthes seedlings can be attributed in our research to the plasticity of the seedlings and the perception of the stress environment. Phenotypic plasticity can be determined as the advantage that a species has in response to a changing environment, being adaptive when it reaches similar or higher values for a given physiological trait in relation to its initial environment, without abiotic stresses (Becklin et al. 2016).
Usually, higher values of A/Gs and WUE are related to characteristics of plants tolerant to lower water availability in the soil (Galindo et al. 2018; Liao et al. 2022).
The water status of plants directly influences their ability to assimilate carbon, both regulated by Gs and, under conditions of water deficit, plants normally reduce A due to the greater stomatal resistance created to reduce water loss through the transpiration process (Harrison et al. 2020). However, despite the reduction of Gs in plants under water stress, plants treated with Si showed a higher rate of Gs, and this effect was even more visible in the 2nd REC for S2Si seedlings, proving the beneficial effect of Si on gas exchange in seedlings under S2Si.The maintenance of the A rate associated with the values of Gs and E are also characteristics of plants tolerant to lower water availability in the soil, which reflects in the higher A/Gs and WUE (Hatfield et al. 2019), exactly what was observed in the E. myrcianthes seedlings treated with Si in this study.
Under flooding, root hypoxia is known to reduce photosynthetic rates, causing a reduction in transpiration and stomatal conductance, and has been related to both stomatal and non-stomatal factors (Barbosa et al. 2021). We believe that the smaller reduction in transpiration of seedlings treated with Si was due to other mechanisms in stress control, since Si acts to reduce E (Zhu et al. 2015; Rizwan et al. 2015), which can be attributed to the higher CRA on the sheets as mentioned.
In flooded Dipteryx alata Vogel. showed low values of A and Gs were observed, possibly indicating stomatal causes as well as a decrease in A/Ci in these plants that may be related to non-stomatal factors (Linné et al. 2021). Similar results were verified in Cedrela fissilis Vell. during flooding (Barbosa et al. 2021). The lower A/Ci values in flooded plants may be due to the damage to gas exchange, such as stomatal closure, that hypoxic conditions cause to the photosynthetic apparatus in plants exposed to flooding for long periods. This can compromise and reduce Rubisco activity (Liu et al. 2014; Junglos et al. 2018).
E. myrcianthes seedlings under stress by flooding showed a Ci value 23.55% lower compared to the seedlings under water deficit, this reduction can be more observed in the seedlings of the S2Si treatment, due to the greater carboxylation of Rubisco. Barbosa et al. (2021) conclude that the lower internal concentration of CO2 in flooded plants is attributed to the lower Gs and storage capacity of this element, due to stress having induced stomatal closure and continuous consumption of previously stored CO2. A reduction in CO2 assimilation rates in response to flooding was also identified in young plants of C. fissilis, under seasonal flooding (Rocha et al. 2018). E. myrcianthes seedlings under water stress (deficit and flooding) showed sensitivity of the photosynthetic apparatus, with regard to the photochemistry of photosynthesis, it is important to highlight that, after periods of water stress, the plants reached recommended values and close to their respective controls, showing once again the physiological plasticity of the species. The dose-concentration of 2 mmol L− 1 of silicon proved to be an alternative in mitigating water stress, avoiding irreparable damage to the photosystems.
Even though the seedlings recovered, in the stress periods, S2Si seedlings showed higher Fv/Fm values, indicating that they did not experience severe stress and could withstand a longer period of exposure to these conditions. The decrease in Fv/Fm may also contribute to the occurrence of reactive oxygen species (ROS), such as superoxide anions and H2O2. They damage chloroplast structures and consequently impair A (Yu et al. 2015) if there is no efficient protective mechanism, represented by increased activity of antioxidant enzymes, as observed in this study.
Slabbert and Krüger (2011) reported that PSII is very sensitive to water stress, probably due to damage to the oxygen-evolving complex or the reaction at the system's integration centers. Therefore, Fv/Fm is considered significant for assessing the quality and integrity of the photosynthetic apparatus (Verma et al. 2020).
The Fv/Fm values in seedlings exposed to flooding were lower than in control seedlings. The authors suggested the occurrence of damage to the reaction centers of photosystem II, which implies the sharp decreases in photosynthesis. However, the damage was considered reversible since there was recovery after stress suspension, which was also observed for E. myrcianthes seedlings.
Partial inactivation in the reaction centers results in reduced light energy harvesting potential, indicated by increased F0 and F0/Fm and decreases in the other photochemical indicators under water stress (Faseela et al. 2019; Khatri and Rathore, 2019). Accordingly, our study showed increased P0, especially for S0Si and S4Si seedlings at both the 1st and 2nd P0. Furthermore, we highlight that after the 1st P0, S0Si seedlings did not decrease the P0 values, indicating a positive effect in maintaining chlorophyll a fluorescence by silicon, given that S2Si seedlings remained below S0Si seedlings.
The increased F0/Fm under stress is a commonly reported response in the literature. However, for S4Si E. myrcianthes seedlings in the 2nd P0, F0/Fm values were higher than the other treatments, indicating that values returned to normal during recovery. In C. langsdorffii flooded plants, higher F0/Fm values occurred during the flooding periods (Cremon et al. 2020). The values were higher than the reference values (0.14 and 0.20), suggesting that the increase in this ratio indicates stress (Rohácek 2002). Our results are similar to those observed for other species considered stress-tolerant and with the potential for recovery after stress suspensions, such as C. odorata (14 days underwater deficit and 12 days under flooding) (Silva et al. 2021)d langsdorffii (42 days under flooding) (Rosa et al. 2018).
Furthermore, the reduction in Fv/F0 values in E. myrcianthes seedlings followed the results found here with other species under water stress, considering that the reference values (between 4 and 6) reflect the maintenance of good functionality in photosystem II reaction centers (Rohácek 2002; Herrera 2013). After the 1st P0, S0Si seedlings failed to recover Fv/F0 values above 4, suggesting damage to these seedlings' photosynthetic apparatus. On the other hand, S2Si seedlings in both the 1st and 2nd REC showed values above 4, indicating the Si potential to mitigate stress in seedlings.
Ashraf (2012) mentions that using chlorophyll fluorescence as a flooding stress indicator is due to the fact that it is a physiological factor that determines the primary processes related to photosynthesis, such as excitation energy transfer, light absorption, and photochemistry, reactions occurring in the PSII (Silva et al. 2021).
Silicon effects on the antioxidant enzyme activity of E. myrcianthes subjected to water stress varied depending on the organ evaluated and the enzyme type. However, we emphasize that regardless of the silicon doses, all seedlings recovered their enzyme activity values after the stresses' suspension, suggesting the damage's reversible effect.
Silicon application to stressed plants can increase the activities of antioxidant enzymes and regulate defense systems (Savvas and Ntatsi 2015; Verma et al. 2020) and the detoxification of ROS, consequently alleviating oxidative damage in plants during stress (Kim et al. 2017; Verma et al. 2020). It is worth noting that the root showed 16% higher SOD levels when compared to the leaf. Furthermore, for the stress periods, we can observe the flooding effect on the SOD enzyme increase since during the 2nd P0, the enzyme activity was 35.06% higher in the root.
Larré et al. (2016) showed the Si efficiency in mitigating water stress effects by activating enzyme activity that varies with the plant organ. Furthermore, they showed that the balance between an increase in the production of reactive oxygen species (ROS) and the ability to respond to stress by rapidly activating the antioxidant defense system relates to tolerance or adaptation to this limiting condition.
The increased antioxidant enzyme activity (SOD and POX) in D. alata seedlings under water stress contributed to the recovery of activities in the photochemical apparatus (Linné et al. 2021). In addition, we observed an increase in proline content in stressed seedlings of E. myrcianthes, regardless of the presence of Si. Proline plays an important role in osmoregulation and stress mitigation in plants (Funck et al. 2012).
Seedlings from S2Si showed better development with higher RL and DQI. However, we point out that RL decreased in the 2nd REC. This effect is possibly due to the damage caused by flooding in the root and stem development, which reduced the yield (Bouman et al. 2007). Similarly, the growth and biomass of S. officinarum plants were lower after the plants were subjected to stress. However, Si application decreased the pronounced water stress effect (Verma et al. 2020).
C. Odorata seedlings recovered biochemical, physiological, and growth characteristics and the DQI after water fluctuations, which demonstrates the physiological plasticity of the species under adverse conditions (Silva et al. 2021), similar to what we observed with E. myrcianthes. However, we observed that in most of the traits studied, Si2 mitigated stress and favored recovery.
Plants tolerant or adapted to stress can acclimate even when their roots are under limited water or oxygen availability. They can maintain stomatal conductance and net photosynthesis at stable levels, enabling survival (Linné et al. 2021; Silva et al. 2021). Therefore, we suggest the adaptability of E. myrcianthes seedlings regardless of Si supplementation since most of the traits evaluated in this study recover regardless of silicon use. However, this adaptability was greater in seedlings treated with Si since, during the stress and recovery periods, seedlings treated with 2 mmol L−¹ of Si showed higher values in gas exchange, indicating that besides the physiological plasticity of the species, Si can promote greater tolerance.