Pigment content in leaves of okra seedlings under drought stress.
Photosynthesis light phase response curves of okra leaves under drought stress.
Correlation coefficients. (A) Phenotypic traits. (B) Physiological characters. (C) Phenotypic traits and photosynthetic parameters. (D) Phenotypic traits and chlorophyll fluorescence parameters. The color scale represents the magnitude of correlation: darker colors indicate stronger correlations. Blue indicates negative correlation, red indicates positive correlation, and colors close to white indicate correlations near zero.
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Water deficit stress significantly impacts the yield and quality of okra, an important edible and medicinal vegetable. Understanding the physiological adaptation mechanisms of okra under drought conditions and identifying effective adaptation strategies are critical to its sustainable development. This study examined the phenotypic traits, photosynthetic characteristics, and chlorophyll fluorescence parameters of okra seedlings subjected to continuous natural drought for 0, 5, 7, 15, and 20 days. The results revealed that drought dramatically inhibited the growth and photosynthetic processes of okra seedlings, as evidenced by reductions in chlorophyll a, chlorophyll b, carotenoids, gas exchange parameters, apparent quantum efficiency, light compensation point, light saturation point, and several chlorophyll fluorescence metrics. Conversely, water deficit stress significantly enhances root development, as evidenced by increases in total root length, surface area, root tips, and root biomass. Notably, drought stress also elevates the chlorophyll a/b ratio, intercellular CO2 concentration, dark respiration rate, and maximum net photosynthetic rate. These findings illustrated the complex physiological responses of okra to water deficit stress and highlighted a tradeoff between aboveground growth inhibition and enhanced root system development, thereby facilitating the plant’s adaptation to water deficit.
Okra (Abelmoschus esculentus L.) is a widely cultivated and consumed green vegetable that is predominantly grown in tropical and subtropical regions. This warm-season crop is primarily valued for its tender edible seed pods, which are harvested at a young stage for various culinary applications (Haq et al. 2023). Okra is characterized by its low caloric content as well as significant presence of essential nutrients, such as potassium (K), magnesium (Mg), vitamins A and C, and a variety of antioxidants. These nutritional properties contribute to okra’s reputation for bolstering the immune system and reducing the risk of several chronic diseases, including cardiovascular disease, diabetes, cancer, and stroke (Gemede et al. 2015). Additionally, okra contains a lectin protein that has been shown to inhibit the proliferation of cancer cells in human studies (Boakye-Gyasi et al. 2021). Furthermore, the soluble fiber present in okra aids in the binding and regulation of cholesterol levels within the human body.
Previous research has markedly enhanced our comprehension of the adaptive mechanisms that various okra genotypes use in response to drought. In this context, Ahmed and El-Sayed (2021) analyzed the physiological traits of hybrid okra cultivars subjected to drought stress, while Ayub et al. (2021) focused on the responses of diverse okra cultivars to water scarcity, thus elucidating critical growth stages and physiological mechanisms that underpin drought resilience. Additionally, Mkhabela et al. (2023) provided valuable insights regarding the phenotypic responses of okra under both drought-stressed and nonstressed conditions. More recently, Asante et al. (2024) investigated the synergistic effects of drought and heat stress across 63 okra genotypes and uncovered significant impacts of these stressors on the morphophysiological characteristics of the crop, with drought stress identified as particularly detrimental. Their research highlighted several critical morphophysiological traits—such as the Fv/Fm ratio, chlorophyll content, electrolyte leakage, relative water content, and biomass—as pivotal indicators for selecting stress-tolerant okra germplasm. Collectively, these findings accentuate the pressing necessity for integrated stress management strategies and the development of crop cultivars that demonstrate enhanced resilience to multiple environmental stressors, thereby ensuring sustainable agricultural practices in increasingly variable climates.
The extant literature has extensively examined the physiological responses of numerous agricultural crops to drought stress (Sato et al. 2024); however, the precise mechanisms governing the reactions of okra to differential water deficit conditions remain incompletely understood. The present investigation aimed to systematically evaluate the phenotypic characteristics and photosynthetic parameters of okra subjected to varying durations of irrigation cessation. By using a controlled pot experiment, this study sought to elucidate the plant’s photosynthetic response mechanisms in the context of water deficit stress. The elucidation of the photophysiological responses of okra to varying levels of water availability can serve as a valuable reference for researchers and practitioners working in the fields of crop physiology, agronomy, and plant stress biology, potentially informing the development of adaptive strategies and cultivation techniques to mitigate the adverse impacts of drought on okra production.
In this study, pot culture experiments were systematically conducted to investigate the effects of water deficit stress on the growth of okra plants. Xianzhi is the drought-tolerant okra cultivar from the Zhenjiang province of China. The experimental conditions were meticulously controlled. Plants were cultivated for a duration of 30 d under relative humidity of 75% and temperatures maintained at 25 ± 2 °C. Okra seeds were directly sown into pots containing nutrient-enriched soil; each pot had eight seeds to ensure optimal growth conditions. After germination, seedlings were irrigated every 3 d to maintain adequate moisture. Following a 2-week acclimatization period, a water deficit stress treatment was applied to simulate drought conditions. Morphological and photosynthetic parameters were measured at intervals of 0, 5, 7, 15, and 20 d posttreatment to assess plant responses to dehydration. The experiment used a completely randomized design with five treatment groups (A0, A5, A7, A15, and A20), with each replicated six times to ensure statistical reliability.
The growth characteristics of the okra plant were assessed following established methodologies encompassing various parameters, including plant height, number of green leaves per plant (NGL), number of leaves per plant (NL), stem diameter (SD), maximum blade length (MBL), maximum blade width (MBW), length of the main root, aboveground part fresh weight, aboveground dry weight (AGDW), fresh root weight, dry root weight, and root-to-shoot ratio (RSR). Measurements of the leaf area (LA), leaf type index, total root length (RL), root projected area (PA), total surface area (SA), root average diameter, total root volume, number of root tips (T), number of root forks (F), and crossing number (C) were conducted using the WinRHIZO™ system (model LA2400; Regent Instruments, Quebec Canada). The drought damage rate (DR) (%) was calculated using the following formula: DR = (CK − drought treatment) / CK × 100%.
The contents of chlorophyll and carotenoid in whole leaves were determined according to the method of Chazaux et al. (2022).
Photosynthetic parameters were evaluated using a portable photosynthesis system (LI-6800; LI-COR, Lincoln, NE, USA) at 9:00 AM on sunny days. Parameters such as the photosynthetic rate (Pn), transpiration rate (Tr), intercellular CO2 concentration (Ci), and stomatal conductance (Gs) of okra leaves were meticulously measured. The stomatal limitation (LS) was calculated using the formula (Ca − Ci)/Ca, where Ca represents the atmospheric CO2 concentration. Photic phase-response curves (PRCs) were determined following the methodology of Dollish et al. (2022), and light intensities varied from 25 to 1800 μmol·m−2·s−1. Preliminary tests identified the light saturation point for okra seedlings at 700 to 800 μmol·m−2·s−1. Consequently, the leaves were initially acclimated to 750 μmol·m−2·s−1 until Pn stabilized (approximately 5 min); thereafter, automatic measurements were performed using the light response curve program. The evaluated parameters included Pn, apparent quantum efficiency, light compensation point (LCP), light saturation point (LSP), dark respiration rate (DR), and maximum net photosynthetic rate (Pnmax).
Additionally, chlorophyll fluorescence parameters such as initial fluorescence (F0), maximum fluorescence (Fm), steady-state fluorescence under illumination (Fs), maximum fluorescence under illumination (Fm′), and minimum fluorescence under illumination (F0′) were determined using a portable photosynthesis system (LI-6800; LI-COR) at 9:00 PM. Using these data, several indices were calculated, including potential Photosystem II (PSII) activity (Fv/F0), the maximum quantum yield of PSII (Fv/Fm), effective PSII photochemical quantum yield (Fv′/Fm′), actual PSII photochemical quantum yield (ΦPSII), electron transport rate (ETR), photochemical quenching coefficient (qP), and nonphotochemical quenching coefficient (NPQ). Nonregulatory energy dissipation [Y(NO)] was calculated as 1/[NPQ +1 + qP × F0′/Fs × (Fm/F0 − 1)], while regulatory energy dissipation [Y(NPQ)] was calculated as 1 − ΦPSII − 1/[NPQ +1 + qP × F0′/Fs × (Fm/F0 − 1)], as described by Demmig-Adams et al. (1996).
The fraction of light energy absorbed by PSII was calculated based on the method of Demmig-Adams et al. (1996) by determining the fraction of excitation energy used for photochemistry (P) as qP × Fv′/Fm′ × 100%, the fraction of absorbed light dissipated thermally (D) as (1 − Fv′/Fm′) × 100%, and the fraction of excitation energy not used for photochemistry (Ex) as (1 − qP) × Fv′/Fm′ × 100%.
The morphological characteristics of okra seedlings under drought stress conditions are presented in Table 1. Prolonged drought treatment leads to significant reductions in plant height (PH), SD, NL, NGL, MBL, MBW, LA, aboveground fresh weight, and AGDW. In contrast, certain root system characteristics, including RL, total PA, root SA, root tips (tips), root forks (forks), crossing roots (crossing), dry root weight, and RSR, exhibit significant increases with extended drought treatment. Notably, the fresh root weights in the A7 and A15 treatment groups were significantly higher than that of the control group (CK) (P < 0.05), with other treatment groups showing no significant differences compared with control group (A0), except for a decrease in RV in the A5 group. No significant differences were observed in root average diameter across all treatments. Additionally, the drought damage rates for PH, SD, NL, NGL, MBL, MBW, LA, aboveground fresh weight, and AGDW increased with the duration of drought stress. However, certain drought treatments do not adversely affect PH (D5 and D7), NL (D7), and leaf type index in A5, A7, and A20. Most root morphology parameters exhibit a positive growth trend, with the exception of RV in A5 and A7 (Table 2). These results suggest that moderate drought conditions may facilitate the growth of okra.
The data presented in Fig. 1 indicated a declining trend in the contents of chlorophyll a, chlorophyll b, total chlorophyll, and carotenoids in okra leaves as the duration of drought treatment increased. Conversely, the chlorophyll a/b ratio in okra seedlings exhibited an increasing trend under drought stress. Notably, the A20 treatment showed a significant increase compared with the control group (D0), with an increase of 37.59% (P < 0.05), whereas the differences in the chlorophyll a/b ratios for other drought stress treatments were not statistically significant.
Citation: J. Amer. Soc. Hort. Sci. 150, 3; 10.21273/JASHS05490-25
The data of the photosynthetic parameters of okra leaves, as illustrated in Table 3, revealed a consistent decline in the Pn, Tr, Gs, and stomatal limitation with the prolongation of drought treatment. Conversely, the Ci in okra leaves exhibited an increasing trend as drought stress persisted.
As depicted in Fig. 2, under the same level of photosynthetically active radiation (PAR), the efficiency of light utilization by okra decreased with extended drought treatment. The maximum net photosynthetic rate showed a downward trend. Within the same drought treatment duration, when PAR was ≤100 μmol·m−2·s−1, the Pn increased sharply with the increase in PAR, indicating a linear relationship between the two variables. At this stage, PAR emerged as the sole limiting factor for okra photosynthesis. As PAR continued to increase, Pn remained on an upward trajectory; however, when PAR exceeded 900 μmol·m−2·s−1, Pn leveled off, suggesting that the carbon dioxide concentration became the limiting factor for photosynthesis in okra seedlings. Furthermore, the maximum Pn in the control group was significantly greater than that of the drought-treated plants.
Citation: J. Amer. Soc. Hort. Sci. 150, 3; 10.21273/JASHS05490-25
Table 4 further illustrates that with prolonged drought treatment, the apparent quantum efficiency, DR, and maximum net photosynthetic rate (Pmax) of okra seedlings progressively declined. In contrast, the LCP and LSP showed a gradual increase, indicating that the photosynthetic apparatus of okra seedlings experienced a degree of impairment. This impairment resulted in reduced capacity for weak light utilization and diminished ability for dry matter accumulation.
The fluorescence parameters of okra seedlings under drought stress are presented in Table 5. As the duration of drought treatment increased, the following key photosynthetic metrics exhibited a declining trend: maximum photochemical quantum yield of PSII (Fv/Fm); potential photochemical efficiency of PSII (Fv/F0); actual photochemical efficiency of PSII (PhiPS2); and ETR. Conversely, the qP showed an increasing trend with prolonged drought exposure. Additionally, the NPQ values under drought conditions were significantly higher than those of the control group (P < 0.05).
As shown in Table 6, with the increase in the drought treatment duration, both P and D exhibited an upward trend, while Ex showed a downward trend. In the control group (A0), approximately 50% of the energy absorbed by the okra leaves is used for thermal dissipation, approximately 24% is allocated to nonphotochemical reactions, and the remainder is used for photochemical reactions. This indicated that the utilization efficiency of absorbed light energy in okra seedlings was relatively low, with a predominant reliance on thermal dissipation to prevent photoinhibition. In contrast, under water deficit stress, the fraction of excitation energy used for photochemistry (P) and fraction of absorbed light dissipated thermally (D) in okra leaves gradually decreased while the Ex increased. This trend suggests that the photochemical reaction centers of PSII are impaired, leading to a diminished protective regulatory effect. Consequently, drought stress damages the PSII system in okra seedlings, reducing the efficiency of primary light energy conversion and compromising the integrity of the potential active centers of PSII, ultimately inhibiting the primary reactions of photosynthesis.
Figure 3A illustrates significant positive correlations among various morphological traits of okra, specifically PH, DS, NL, NGL, MBL, MBW, and LA. In contrast, RL, RV, SA, average diameter, PA, T, F, C, and RSR exhibit significant negative correlations under drought conditions. Figure 3B highlights positive relationships among five key physiological parameters, NPQ, Ex, Ci, LCP, and chlorophyll a/b ratio, thus demonstrating significant negative correlations with other variables. Additionally, chlorophyll a, chlorophyll b, and carotenoids are highly positively correlated. Pnmax, Fv/Fm, and DR also show positive correlations. Moreover, Pn, Gs, Ls, Fv/Fm, Fv/F0, PhiPS2, ETR, and qP have significant positive correlations with PH, SD, NL, NGL, MBL, MBW, LA, aboveground fresh weight, and AGDW, whereas Ci and NPQ exhibit negative correlations with these traits. Finally, Pn, Gs, and Ls demonstrate negative correlations with RL, PA, SA, F, T, C, fresh root weight, dry root weight, and RSR, while Ci and NPQ reveal positive correlations with these parameters (Fig. 3C, 3D).
Citation: J. Amer. Soc. Hort. Sci. 150, 3; 10.21273/JASHS05490-25
Under drought stress, okra seedlings exhibit significant reductions in plant height, stem diameter, leaf number, and leaf area compared with well-watered conditions, indicating impaired growth (Asante et al. 2024; Swindell et al. 2007). To mitigate water loss, plants often reduce transpiration by decreasing leaf area while increasing leaf thickness, although this may compromise gas exchange (Castro-Díez et al. 2000; Jin et al. 2008). Conversely, drought triggers adaptive root morphological changes, such as increased primary root elongation, total root length, and fine root proliferation, enhancing water and nutrient uptake (Anjum et al. 2011; Grzesiak 2009; Wasaya et al. 2018). Studies of Pistacia lentiscus L. and oats demonstrate that moderate drought promotes fine root development, whereas severe stress suppresses it (Cortina et al. 2008; Wang et al. 2020). These adjustments, including greater root surface area and reduced average diameter, improve metabolic efficiency and anchorage (Fernández et al. 2002; Munson et al. 2018). For instance, Sun et al. (2024) observed reduced root diameter, increased surface area, and decreased volume in reed-like plants under drought, with peak root length and biomass under moderate stress. Similarly, Liu et al. (2023) reported increased specific surface area and root length but decreased diameter and volume in Haloxylon ammodendron seedlings. While drought reduces aboveground biomass because of moisture sensitivity, it elevates root biomass, facilitating deeper soil exploration (Hincha and Thalhammer 2012; Wang et al. 2018). Such plasticity optimizes resource acquisition in water-limited environments.
Chlorophyll plays a pivotal role in photosynthesis by capturing sunlight and converting it into biological energy and food. Its structural and functional integrity are highly dependent on the availability of water. Under drought conditions, a decline in the chlorophyll and carotenoid contents in okra leaves aligns with the findings of Kocheva et al. (2005), Peiró et al. (2020), and Plazas et al. (2019). This phenomenon may be attributed to several factors, including chloroplast degradation, photoxidation, inhibited synthesis, and altered activity of chlorophyll enzymes (Ahmad et al. 2018). Drought reduces leaf moisture content, which subsequently decreases the protease activity associated with photosynthetic pigments, thereby diminishing the rate of photosynthesis and adversely affecting the plant’s photosynthetic characteristics (Liao and Wang 2014).
This study indicates that drought stress significantly diminishes the photosynthetic rate (Pn), Tr, and stomatal conductance (Gs) in okra leaves, thus corroborating the findings reported by Zhang et al. (2024). Drought stress can be classified into stomatal and nonstomatal factors, which frequently occur simultaneously (Zhang et al. 2010). With increasing drought severity, Pn declines while the Ci incresases, and the stomatal limitation falls, suggesting that nonstomatal factors primarily limit photosynthesis in okra under drought. Contributing factors may include reduced chlorophyll content and diminished electron transport rates in Photosystem I and PSII because of light inhibition. Stomatal closure serves as a protective measure under drought, preserving leaf water content but inhibiting photosynthesis (Ohashi et al. 2006). In mild drought conditions, the drop in photosynthesis is largely caused by stomatal diffusion limitations, which also disrupt mesophyll metabolism (Kosmala et al. 2012). Conversely, severe drought directly hampers photosynthetic activity by limiting carbon dioxide uptake, significantly reducing ribulose-1,5-bisphosphate carboxylase/oxygenase activity (Rinalducci et al. 2008) and leading to increased photorespiration. This further reduces water flow into the leaves, decreasing leaf water potential and hydraulic and stomatal conductance (Sade et al. 2015).
The concurrent decline in AQY and Pmax is a key characteristic of light inhibition in plants (Shi et al. 2022). Our research indicated that under drought stress, the gradual reduction in AQY, Pmax, and DR reflected a decreased ability of okra to harness light energy as well as a significant decline in the conversion efficiency of light to chemical energy. Li and Rao (2024) further established that variations in Fv/Fm are critical indicators of light inhibition severity in plants. The observed reductions in AQY, Pmax, and Fv/Fm under heightened drought conditions suggest that okra experiences considerable light inhibition. Additionally, increases in LCP and LSP demonstrate okra’s adaptive responses to light-inhibited environments. Water deficit stress limits the ability of chloroplasts to use CO2, leading to reduced energy consumption and a shift in the balance of energy and electron transfer toward CO2, which results in elevated reactive oxygen species levels (Yordanov et al. 2000). These reactive oxygen species can cause membrane lipid peroxidation, thus damaging membrane integrity and impairing photoprotective mechanisms, ultimately decreasing light utilization efficiency and raising LCP. An increased LSP indicates that drought conditions require okra to achieve maximum photosynthetic rates at higher light intensities to maintain metabolic functions. The increase in LCP signifies that okra must perform photosynthesis under intense light to accumulate organic materials as a strategy against drought stress. Overall, these findings underscore the adaptive modifications of okra’s photosynthetic processes in response to drought, aligning with the findingsd of previous studies by Yu et al. (2015) and Jat et al. (2024).
Chlorophyll fluorescence is a key indicator of photochemical processes in PSII essential for understanding photosynthesis in plants. Water deficit stress decreases the electron transport rate of PSII, while NPQ only increases significantly after a substantial reduction in leaf water content (Zivcak et al. 2013). The decline in Fv/Fm and Fv/F0 parameters can evaluate a plant’s drought resistance. This study demonstrates a notable decrease in both metrics with heightened drought stress, corroborating the findings of Ghotbi-Ravandi et al. (2016) in cultivated barley and wild types. Okra seedlings show marked PSII dysfunction under drought, with declining resistance as drought duration increases. Additionally, significant reductions in PSII actual photochemical efficiency, indicated by ΦPSII and ETR, align with the observations of Mihaljevi et al. (2021), who linked drought stress to reduced PSII reaction center openness. The resulting accumulation of unutilized light energy causes damage to the photosynthetic apparatus and photoinhibition. The qP serves to illustrate light energy allocation, which is critical under drought stress. Enhanced photorespiration may temporarily protect the photosynthetic system, but severe drought conditions often result in ETR dropping to or below control levels, further diminishing photorespiration efficiency. The NPQ coefficient, indicative of nonradiative energy dissipation, increases with prolonged drought treatment, reflecting enhanced thermal dissipation capacity, consistent with the findings of wheat under drought stress by Xue et al. (2025).
The absorption of light energy by plant leaves is dissipated through three fractions, P (the fraction of excitation energy used for photochemistry), D (the fraction of absorbed light dissipated thermally), and Ex, with each influenced by the plant’s growth environment (Adams et al. 2001). This study revealed that okra seedlings experienced a decline in P under drought conditions, which was likely associated with diminished photosynthesis and fluorescence parameters, such as AQY, Pmax, and Fv/Fm. In contrast, the increased Ex served as a protective mechanism for PSII against excessive reduction and damage to the electron transport chain, thereby preserving the photosynthetic apparatus from photoinhibition (Shi et al. 2022). These results highlight the adaptability of okra to drought stress. Moreover, drought significantly modified the processes by which okra absorbs and distributes light energy; the reductions in P and D indicated a water conservation strategy that suppressed photosynthesis, while the enhanced Ex signified improved light energy utilization efficiency to shield the photosynthetic system during drought (Zhu et al. 2024). Additionally, evidence reported by Chishti et al. (2025) demonstrated that water deficit stress adversely affected photochemical efficiency and photosynthetic performance, resulting in photoinhibition and heightened thermal dissipation to manage excess excitation energy.
Water deficit stress impedes the growth of okra by reducing chlorophyll and carotenoid contents, thereby inhibiting photosynthesis. In response to drought stress, okra enhances root development, increases photoinhibition, and optimizes parameters related to energy dissipation during light absorption. Conversely, moderate water deficit has been shown to promote the growth of okra. This study offers valuable insights for developing novel drought resistance strategies for okra and serves as a critical reference for future research and practical applications.
Pigment content in leaves of okra seedlings under drought stress.
Photosynthesis light phase response curves of okra leaves under drought stress.
Correlation coefficients. (A) Phenotypic traits. (B) Physiological characters. (C) Phenotypic traits and photosynthetic parameters. (D) Phenotypic traits and chlorophyll fluorescence parameters. The color scale represents the magnitude of correlation: darker colors indicate stronger correlations. Blue indicates negative correlation, red indicates positive correlation, and colors close to white indicate correlations near zero.
Contributor Notes
This study was supported by the program for Natural Science Research in Guizhou Education Department (QJJ-[2023]-024), the Sixth Batch of Guizhou Province High-level Innovative Talent Training Program (GCC[2022]009), Innovative training program for college students in Guizhou Province (2024109760072), and the program of Excellent Innovation Talents in Guizhou Province (GCC[2023]071).
All data analyzed during this study are included in this published article.
J.W. and M.M. contributed equally to this work.
M.M. is the corresponding author. E-mail: 13639052573@163.com.
Pigment content in leaves of okra seedlings under drought stress.
Photosynthesis light phase response curves of okra leaves under drought stress.
Correlation coefficients. (A) Phenotypic traits. (B) Physiological characters. (C) Phenotypic traits and photosynthetic parameters. (D) Phenotypic traits and chlorophyll fluorescence parameters. The color scale represents the magnitude of correlation: darker colors indicate stronger correlations. Blue indicates negative correlation, red indicates positive correlation, and colors close to white indicate correlations near zero.
