Drought stress induces various morphological, biochemical and physiological responses in plants and affects almost all plant functions including photosynthesis, growth and development. In this study, there were some elements of leaf folding which is a significant avoidance mechanism of plants to drought stress. As described earlier by Saruhan et al. (2009), leaf folding is among the major responses to drought by plants as it reduces the leaf area which results to decreased transpiration. These observations are consistent with those of previous studies in some other plants species facing water stress (Kumar et al., 2017; Wei et al., 2017) including oil palm (Azzeme et al., 2016; Wen et al., 2019). Most of the changes observed in this study were prominent in GH indicating the sensitivity of GH palm seedlings to even milder drought conditions. Stopping plants watering process will lower their water status, putting them under stress and affecting their biochemical processes. As a result, the plant will modify its physiological functions and morphology to adapt to unfavorable environmental conditions. Drought stress prevents plants from absorbing water and nutrients. In addition, when plants are subjected to drought stress, the accumulation of abscisic acid (ABA) increases and plants close their stomata to limit transpiration (Pangaribuan et al., 2024).
In this study, decrease in the chlorophyll concentration was observed early enough after subjecting the oil palm to drought conditions. This results from the fact that sufficient quantity of water is required during chlorophyll production by plant system. Within the chloroplasts of plant cells, there are multiple enzymatic stages involved in the formation of chlorophyll in which water is needed as a reactant or solvent in many of these processes. In other words, the transformation of many precursor molecules into chlorophyll molecules occurs directly in the presence of water. Insufficient water within the cells as caused by drought lowers the amount of chlorophyll produced. In the recent past, Huihui et al. (2020) indicated that of the 15 key proteins involved in chlorophyll synthesis, the activity of 13 enzymes were significantly decreased under induced stress, which led to the significant decrease in chlorophyll a and chlorophyll b contents of Morus alba. In addition, the primary means of transporting nutrients into plants systems is water. Magnesium and nitrogen, two nutrients crucial for the formation of chlorophyll, are taken up by plant roots from the soil and delivered to the leaves via the vascular system. This nutrient delivery may be hampered by lack of water, which might render the body without enough nutrients to produce chlorophyll (Dominguez et al., 2023). Although chlorophyll degradation and pigment photo-oxidation have been speculated to be responsible for lowering plant chlorophyll (Anjum et al., 2011), the interplay of poor nutrient uptake, decrease biosynthesis and collapse in cells turgor are evidently the key factors leading to reduced chlorophyll contents (Khatun et al., 2021).
Results in Fig. 1a indicated that prolonging the drought conditions was associated with concomitant decrease in the chlorophyll contents. Earlier studies by Fathi and Tari (2016) revealed that drought stress causes a decrease in chlorophyll contents, which varies according to the length and extent of the drought exposure. Recent studies by Urmi et al. (2023) indicated that extended drought stress substantially reduced the growth indices of Oryza sativa including photosynthetic pigments. Despite the decrease, plants struggle to maintain appreciable chlorophyll in order to meet up with nutrient demand for the most essential metabolic products required to respond to the stress condition. It has been established that chlorophyll stability was an important mechanism of drought tolerance in Robusta coffee (Chutteang et al., 2023). Different plant species, including oil palm, have been reported to show decrease in chlorophyll concentration under drought stress as was observed in this study. More so, several researches also revealed that the degree and length of drought stress affect the state of chlorophyll degradation. Reports by Cha-um et al. (2013) indicated that, with increasing severity of drought stress, the degradation of Chl a, Chl b, and total chlorophyll in oil palm seedlings showed decreasing trends at soil water content ranging between 6 and 42%.
Although there is disparity in the pattern of decrease in chlorophyll of the palm in this study, the overall drought effect on chlorophyll was notable decrease in its concentration. While CX (Fig. 1h) and YM (Fig. 1h) maintained relatively uniform chlorophyll concentration during the milder drought period, the GH variety suffered more within the period and could only bore more chlorophyll during the severe drought period. This might be attributed to the fact that, the former cultivars were more resistant to water shortage which is mediated through different avoidance mechanisms. The rise in chlorophyll contents of the latter during severe conditions however, was a good manifestation of its adaptive strategies that created a balance between stress and the cellular needs of the plant. In their efforts to determine the effects of drought regiments on Paspalum wettsteinii, Huang et al. (2023) reported that milder drought conditions significantly improved the plant growth indices including photosynthesis as opposed to extreme drought conditions. They concluded that increase in photosynthesis contributes to the adaptation of P. wettsteinii to drought stress even though, responses to drought stress depended on time of exposure.
Transpiration and drought stress are intimately connected. Drought stress induces a water deficit in soil, causing plants to face a critical challenge in maintaining their physiological functions. As soil water potential decreases, the plant responds by closing its stomata, which reduces the rate of transpiration (Kaiser, 2010). This is among the first adaptive responses triggered by water deficit which is aimed to conserve water. This response is orchestrated by a complex interplay of genes and hormonal signalling pathways. Drought-responsive genes, such as those encoding aquaporins, abscisic acid receptors, and transcription factors, play crucial roles in the regulation of transpiration rate under drought conditions (Wilkinson and Davies, 2002; Yoshida et al., 2019). Although beneficial in limiting the adverse effect of drought on plants, decrease in transpiration is seldom associated with consequences including limiting the plant's ability to take up essential nutrients and minerals from the soil (Vadez et al., 2014), in addition to increased leaf temperatures due to ineffective cooling capacity which can exacerbate heat stress (Chaves et al., 2002). It has been reported that, decrease in transpiration rate in oil palm was mediated by stomatal closure that was accompanied by marked decreases in net CO2 assimilation and limited water use efficiency during a drought condition (Silva et al., 2017).
Photosynthesis is the central metabolic process for carbon assimilation in plants and becomes the main process affected during drought stress. As observed in this study, the rate of photosynthesis was substantially affected with prolonged exposure of oil palm to drought. Under drought conditions, the photosynthetic rate considerably decreases due to the interplay of reduced nutrient uptake, stomatal closure and decreased chlorophyll contents. Reduced CO2 availability resulting from stomatal closure impairs the carbon-fixing reactions of photosynthesis, specifically the Calvin cycle, leading to decreased photosynthetic rates (Flexas et al., 2006). It has been established that, down-regulation of photosynthesis is the primary response to drought stress (Flexas et al., 2018). However, maintained photosynthetic capability under drought is an important feature of drought-resistance in crops and plays an important role in stress tolerance (Chaves et al., 2009; Suzuki et al., 2012). The relationship between photosynthesis and drought stress is characterized by a multifaceted series of responses aimed at conserving water and minimizing damage caused by water scarcity. While these responses are adaptive, they ultimately lead to reduced photosynthetic rates, impacting plant growth and productivity under drought conditions. Previous studies on the effects of drought on oil palm indicated that the plant photosynthetic rate is significantly reduced which led to ultimate reduction in the plant growth and overall yield (Kamil and Omar, 2017; Najihah et al., 2019; Salgado et al., 2022; Pradiko et al., 2023).
Although the role of stomata in decreasing photosynthetic rate has been debated (Campos et al., 2014); the significant reduction in chlorophyll contents observed in this study might be responsible for the decrease in the photosynthetic rate (Fig. 1b; 1e; 1h). Low photosynthesis is generally associated with photosystem II (PSII) inefficiency resulting from inadequate or deficient chlorophyll or chloroplast respectively (Su'udi et al., 2022; Guntur et al., 2023). For an effective metabolism, optimal chlorophyll concentration is essential for maximizing light absorption while too little chlorophyll can limit light capture thus, affecting photosystems (Pfannschmidt, 2003). Photosystem II is sensitive to changes in chlorophyll content, as it requires a balance between chlorophyll and accessory pigments to operate efficiently. Changes in chlorophyll content can modulate the excitation energy distribution between the photosystems, influencing the rate of electron transport and overall photosynthetic efficiency (Müller et al., 2001). In addition, PSII is susceptible to oxidative stress, which can result from imbalances in chlorophyll content. Excessive light absorption by chlorophyll can lead to the formation of ROS that damage PSII components (Allakhverdiev et al., 2003).
Oil palm exhibits dynamic changes in water conductance in response to drought. As observed in this study, under mild stress (Fig. 1a; 1d; 1g), there was drastic decrease in the conductance occasioned by abrupt exposure to drought. At moderate drought, there was an adjustment in hydraulic properties to maintain water transport efficiency which explained why it improved from the previous. However, it has been reported that severe drought can lead to hydraulic limitations, affecting water uptake and transport within oil palm (Pérez-Ramos et al., 2019). This might be a plausible reason why, the conductance decreased subsequently with prolonged drought stress. In the same vain, the pattern of water conductance observed in this study might demonstrate the underlying activities governing the physio-morphological response of oil palm to drought stress. Structural modifications including changes in leaf morphology or alterations in leaf thickness affect water conductance and transpiration rates (Mia et al., 2020). According to Rahman et al. (2018), oil palm roots demonstrate plasticity in response to drought and can adjust both in terms of biomass allocation and root architecture, potentially reaching deeper soil layers to access water sources. This might explain the reason why there was initial decrease in water conductance at milder stress conditions. Upon extending their roots to water source in deeper soil, the conductance relatively increased despite the worsening water shortage. The works of Jiang et al. (2019)d rez-Ramos et al. (2019) have made similar observations, thus supporting the present findings.
Studies have shown that during initial stages of drought stress, when soil moisture becomes limiting and plants begin to sense water scarcity, there can be a rapid closure of stomata which reduces the influx of atmospheric CO2 into the leaves, causing a build-up of CO2 within the leaves' intercellular spaces (Flexas et al., 2013). This supports the current findings in which increase in intracellular CO2 was observed during the initial mild drought season. The reduced CO2 uptake due to stomatal closure might have led to the accumulation of CO2 within the plant. This increase in intercellular CO2 concentration is often referred to as stomatal limitation and is a characteristic response to water stress (Efeoğlu et al., 2009). Recent studies by Wu et al. (2023) reported similar findings in maize seedlings which further support the findings in this study. As drought stress persists and becomes more severe, other physiological and biochemical responses occur, which can further affect intracellular CO2 levels. These responses including metabolic adjustments, changes in leaf anatomy, and potential shifts in the carbon balance cause subsequent drastic decline in intracellular CO2 (Flexas et al., 2013). This also corroborates our findings because during severe drought, decrease in CO2 concentration was obvious. Studies by Chai et al. (2020) reported different physiological responses to drought stress by Cocos nucifera and Elaeis guineensis including decreased CO2 under severe drought conditions. Therefore, the response of oil palm to drought stress with respect to intracellular CO2 concentrations can vary depending on the specific plant species, the severity and duration of the drought.
Cumulative effects of physiological changes due to drought on the cultivars
In this study, the various factors examined—chlorophyll concentration, transpiration rate, photosynthesis, conductance to water, and intracellular CO2 concentration—are interrelated and together contribute to a comprehensive understanding of how oil palm responds to and tolerate drought stress. These factors collectively establish a grand-effect on the plant's performance and survival under drought stress.
Chlorophyll concentration and intracellular CO2 concentration are the major determinants of photosynthetic rate. As observed in this study during prolonged drought, decrease in CO2 which was assumed to result from reduced stomatal conductance led to a decrease in CO2 availability for photosynthesis. This decrease, coupled with limited water availability, affected the rate of photosynthesis (Fig. 1a). On the other hand, the reduced photosynthetic rate correlated with decrease in chlorophyll concentration (Fig. 1b), which could best be described as a result of energy-demanding chlorophyll synthesis which becomes impaired under drought conditions. However, under mild drought conditions, a relatively higher CO2 was the cause for better photosynthetic as well as chlorophyll concentration despite the low conductance to water. This is further strengthened by the fact that, transpiration rate was minimal during this drought period suggesting a mechanism of good water use efficiency. The response has greatly helped the plant to overcome drought adversity from the onset indicating a good response to drought by the oil palm. In a similar study using Datura stramonium, Javaid et al. (2023) indicated that higher CO2 concentration mitigated the adverse effects of drought and most of the physiological parameters were sustained with increasing drought duration; therefore support the present findings.
Although dehydration reduces photosynthesis, drought-tolerant plants often exhibit a balance between photosynthesis and transpiration. They can maintain some degree of photosynthesis even with reduced transpiration to ensure a continued supply of energy and carbon compounds. The ability to regulate conductance to water while maintaining essential physiological processes such as nutrient transport and structural support contributes significantly to a plant's overall tolerance to drought stress. Studies have shown that dehydration impairs electron chain and induce the production of high ROS (Rejeb et al., 2014) which may affect normal biological process and increase plant internal injury index. Results from this study indicated that, despite the increase in water scarcity during the moderate drought season, substantial photosynthetic rate was maintained amid lower transpiration rate. However, at severe drought conditions, the photosynthetic rate significantly decreased which also correlated with decrease in almost all the physiological parameters examined.