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Article

Agro-Physiological and Morphological Responses of Pearl Millet to Varying Water Regimes in Semi-Arid Conditions of Namibia

by
Ofentse Moseki
1,*,
Grace Kangueehi
1,
Vasco Chiteculo
2,
Matthias Zink
3 and
Maliata Athon Wanga
1
1
Faculty of Health, Natural Resources and Applied Sciences, School of Agriculture and Natural Resource Sciences, Namibia University of Science and Technology, Private Bag 13388, Windhoek 9000, Namibia
2
Southern African Science Service Centre for Climate Change and Adaptive Land Management, SASSCAL-Angola National Node, Rua da Granja, Cidade Alta, Huambo 13301, Angola
3
International Centre for Water Resources and Global Change, ICWRGC, UNESCO Category 2 Centre, 56068 Koblenz, Germany
*
Author to whom correspondence should be addressed.
Agronomy 2025, 15(2), 381; https://doi.org/10.3390/agronomy15020381
Submission received: 6 December 2024 / Revised: 19 December 2024 / Accepted: 23 December 2024 / Published: 31 January 2025
(This article belongs to the Section Water Use and Irrigation)
Browse Figures
Versions Notes

Abstract

:
Pearl millet (Pennisetum glaucum (L.) R. BR.) is a C4 plant adapted to semi-arid climates and is one of the primary staple foods in Sub-Saharan Africa, including in Namibia. The decline in yields associated with water scarcity over the years has been a national concern in the country. An experimental field trial was conducted at the Mannheim Crop Research Station, Namibia, during the 2023 and 2024 cropping seasons to investigate the response of two local pearl millet cultivars (Kangara and Okashana 2) to different water regimes (100%, 75%, and 50% crop evapotranspiration [ETc]) according to morpho-physiological and yield parameters. Pearl millet was planted in a split-plot factorial design with four rows per plot under the three water regimes, and the genotypes were planted in subplots. The results revealed that the water regime had a significant effect on plant height, number of leaves, tillers, chlorophyll content, stomatal conductance, leaf temperature, stem thickness, number of productive tillers, panicle diameter, panicle length, dry panicle weight, biomass, grain weight, and 1000-seed weight of the two pearl millet cultivars (p < 0.001). At 50% ETc, the water regime significantly reduced the growth and yield parameters compared with the 75% ETc and 100% ETc water regimes, highlighting the significance of water in plant development and growth. The findings highlighted that both cultivars responded similarly to water stress. Seventy-five percent of ETc is recommended to be applied in pearl millet systems in semi-arid conditions. This research has significant implications for the planning and producing of pearl millet under water-limited environments under changing climatic conditions.

1. Introduction

In recent decades, water availability has been declining owing to climate change and population growth, particularly in arid and semi-arid regions, posing significant challenges to sustainable crop production [1,2]. These challenges are acute in Sub-Saharan Africa (SSA), where unpredictable rainfall patterns and high temperatures limit agricultural productivity [1]. This is reflected in declining crop yields across the region due to drought stress. Drought stress occurs when available soil water is limited for crop utilization, whereas atmospheric conditions, such as high temperatures, cause continuous water loss by transpiration or evaporation [3]. Drought stress negatively affects key physiological, morphological, biochemical, and molecular processes in crops, including growth phenology, water and nutrient uptake, and photosynthesis [4].
Pearl millet (Pennisetum glaucum (L.) R. BR.), a drought-tolerant crop, is promising for sustaining food production under these challenging conditions because of its low water requirement compared to other cereal crops. Millions of people in Sub-Saharan Africa and Asia consume millet as a staple diet [5]. However, pearl millet is susceptible to water stress at critical stages of development [1,6]. Globally, crop production is frequently constrained by moisture stress during the growing season in dryland environments, leading to significant reductions in economic yields [7]. Despite its resilience to arid environments compared with crops such as maize, rice, and wheat, pearl millet faces the risk of yield loss due to frequent drought and heat stress [7]. For example, in Nigeria, erratic rainfall and poor crop management have reduced yields, leading to crop failure in pearl millet production [8].
The benefits of pearl millet extend beyond its role as a source of food. Its by-products, including fodder, porridge, and snacks, contribute to food security and economic resilience [9,10]. It is cultivated in several countries in Southern Africa, including Botswana, Namibia, South Africa, Zambia, and Zimbabwe [11]. Despite its importance, the physiological and agronomic responses of local pearl millet cultivars under water deficit conditions remain underexplored, particularly in Namibia. Given the sensitivity of crops to water stress, understanding how different water regimes affect their physiological and yield traits is critical for improving productivity under semi-arid conditions.
Studies investigating drought tolerance in pearl millet and the effects of water relations on physiological and biochemical traits under different environments have found that drought stress affects the growth traits of pearl millet [6,12,13,14]. Assessing crop performance across diverse agroecological systems is crucial for optimizing agricultural practices. Ref. [15] demonstrated that agroecological niches and associated cropping systems are essential for realizing the potential of underutilized crops. Ref. [16] highlighted the importance of multidimensional and multiscale assessments in evaluating agroecological transitions, emphasizing adaptability to local conditions and social interactions. Considering the differences brought about by agroecological zones, there is a dearth of information regarding the responses of different local cultivars, such as Okashana 2 and Kangara, to varying irrigation treatments in Namibia, where both genetic and environmental factors significantly influence crop yields. This is critical for ensuring food security under the changing environmental conditions in Namibia.
In light of these gaps, this study sought to evaluate the effects of different water regimes on the physiological and yield parameters of two pearl millet cultivars, Okashana 2 and Kangara, cultivated in Namibia. We determined (i) the effect of different water regimes on the cumulative growth and development of pearl millet in the semi-arid conditions of Namibia and (ii) the impact of the growing season, water regime, and variety on morpho-physiological characteristics. We hypothesized that higher irrigation levels would enhance the agro-morphological and physiological traits of both cultivars. Furthermore, it was anticipated that, owing to their genetic differences, Okashana 2 and Kangara would exhibit distinct responses to different water regimes for pearl millet in Namibia.

2. Materials and Methods

2.1. Study Area

Namibia is a semi-arid country in Sub-Saharan Africa (SSA) [17,18]. This study was conducted at Mannheim Research Station experimental field sites in the Oshikoto Region, Namibia (Lat. −19.168611 and Lon. 17.763056). Both crop production and livestock farming are practiced in this area. Namibia is characterized by highly variable rainfall and temperature in space and time (Figure 1), recurrent droughts, and water scarcity. According to Namibia Water Corporation Ltd. (NamWater) [19], the annual rainfall varies between 550 mm and 600 mm in the north and 250 mm and 300 mm in the south. Nearly 80% of the country is supplied by underground water, mainly because it is naturally protected from evaporation; hence, it depends on this source. Evapotranspiration rates vary from 2600 mm per annum in the northeast to 3700 mm per annum in the south. Higher evapotranspiration rates were observed between October and December. During the September to December 2023 cropping season, the monthly maximum temperature ranged between 38 °C and 36 °C; the average temperature ranged between 28 °C and 25 °C, and minimum monthly temperatures ranged between 18 °C and 12 °C, and the corresponding rainfall during that period was in November and December with 7.2 mm and 33.6 mm, respectively. Conversely, during the January to April 2024 cropping season, the maximum monthly temperature ranged between 36 °C and 32 °C; the average monthly temperature ranged between 25 °C and 22 °C, and minimum monthly temperatures ranged between 18 °C and 14 °C, while the rainfall received during that period ranged from 63.3 mm to 18.8 mm.

2.2. Methodology

2.2.1. Experimental Design

First, the land was prepared by clearing debris, followed by the placement of the irrigation system. The experiment was then laid out in a blocked split-plot design with three irrigation treatments as the main plot factor and crop cultivars (Kangara and Okashana 2) as the subplot factor, with four replicates. Before sowing, soil samples were taken to make a composite sample that was analyzed in the lab for fertility and classification according to a previously described method [20]. Soil samples were collected from five locations in each plot to create a composite. Polyethylene drip laterals (20 mm inside diameter) were installed before planting in every plot, with emitters (rated at 1 L/h discharge) spaced every 0.2 m on the laterals. Each plot was 3 m by 3 m, and a buffer zone with 1.0 m and 2.0 m spacing was provided between the plots and blocks (Figure 2). After irrigation, observations of emitter wetting patterns showed complete closure between adjacent emitters on the same lateral side and more than 85% between rows, which ensured uniform wetting within the plots. Each plot consisted of four rows spaced at 0.75 m; only the two middle rows were used for monitoring, while the other two were used as borders. Several seeds were planted in each hole at a spacing of 0.15 m, and thinning was performed after emergence to achieve a population of 30 plants, with one plant in each stand. The first trial season was conducted from September to December 2023, and the second from January to April 2024.

2.2.2. Treatments Determination

The crop evapotranspiration rate was computed using the CROPWAT 8.0 model [21], which uses meteorological, soil, and crop factors as inputs for the study area. Long-term monthly climate averages for the Oshikoto region were obtained from the National Aeronautics and Space Administration (NASA) for 1991–2022 (https://power.larc.nasa.gov/data-access-viewer/, accessed on 15 August 2023) [22]. The data acquired were the mean monthly minimum and maximum temperatures (°C), relative humidity (%), solar radiation/sunshine hours, rainfall (mm), and wind speed (m/s). Crop data were obtained from existing databases and literature, such as the FAO database [23,24]. These data were input into the CROPWAT 8.0 model to determine the potential evapotranspiration rate (ETo) of the area. The actual crop evapotranspiration (ETc) was derived by multiplying the potential evapotranspiration (ETo) by the crop factor (Kc), based on both growth stages and soil water measurements. A total of 75% and 50% of the ETc from CROPWAT 8.0 computation were used to determine the crop response to different percentages of water supplied in addition to the control (100 ETc) (Table 1).

2.2.3. Data Collection

Agronomic trait data were collected using pearl millet descriptor methods described by the International Board for Plant Genetic Resources (IBPGR) and the International Crops Research Institute for the Semi-Arid Tropics (ICRISAT) [25].

Morphological Traits Data

Quantitative data were acquired through biometric parameter measurements, including plant height (PH), stem thickness, number of leaves, and tillers. These data were collected at physiological maturity from five plants in each plot selected randomly. The plant height (PH) was measured from the base of the plant to the tip of the panicle. The number of physiologically active leaves and tillers was counted and recorded accordingly.

Stomatal Conductance Measurements

Stomatal conductance was measured using the Decagon Leaf Porometer (SC-1; Decagon Devices Inc., Pullman, WA, USA). Readings were taken from the abaxial surfaces of fully expanded and exposed leaves. In each plot, three plants were randomly selected to take conductance measurements, which were later averaged to get the conductance for each treatment. These readings were taken just before physiological maturity at 83 days after planting.

Chlorophyll Content Measurements

Using a digital SPAD-502 Plus chlorophyll meter (Konica Minolta Sensing, Inc. Tokyo, Japan), the SPAD values were determined [26]. The chlorophyll meter is a spectral instrument; it measures the difference between the transmittance of a red (650 nm) and an infrared (940 nm) light through the leaf, generating a 3-digit SPAD value, which was used to take chlorophyll content readings from the three uppermost fully expanded leaves from each plot. Three SPAD values per leaf, including one value around the midpoint of the leaf blade and two values 3 cm apart from the midpoint, were averaged to give the mean SPAD value of the leaf as outlined by a previous study [27]. Readings were taken just before physiological maturity at 83 days after planting.

Leaf Temperature Measurements

Leaf temperature measurements were taken using a noncontact laser infrared digital thermometer (EC-Technology, N10IT001–5, Woodridge, IL, USA) between 1200 and 1300 h when the plants were exposed to concentrated sun radiation. Three plants were selected from each plot for measurements. Upper-top leaves fully exposed to the sun’s radiation were targeted. These readings were taken just before physiological maturity at 83 days after planting.

Yield and Yield Attributes Measurements

At physiological maturity, several data were collected from the main stem. Stem thickness (ST) and panicle diameter (PD) (cm) were measured using a Vernier caliper. Panicle length (PL) in centimeters was measured from the lower panicle branch to the tip of the panicle. Aboveground biomass (B) and panicles were harvested and air-dried for two weeks before weighing. The dry panicle weight (DPW) in grams was measured and recorded after drying before hand-threshing them. Grain weight (GW) was measured in grams by weighing the grains per panicle, which was recorded after threshing. The grains were then weighed using a balance scale, and thousand-grain weight (1000-GW) seeds were counted using an electronic seed counter (Pfeuffer Contador GmbH, Kitzingen, Germany).

2.3. Data Analysis

The effects of the cropping season, water regime, and variety on morpho-physiological traits, including plant height, number of leaves, number of tillers, leaf temperature, chlorophyll content, stomatal conductance, and yield attributes of pearl millet, were analyzed using a two-way factorial with analysis of variance (ANOVA) in R version 4.4.0 [28] (Equation (1)). Model assumptions were tested by evaluating the normality of error distributions for each dependent variable using the Shapiro–Wilk test (p < 0.05) and assessing the homoscedasticity of variances using the Levene test (p < 0.05). When a significant F-test was detected, a post-hoc analysis using Tukey’s Honestly Significant Difference (Tukey’s HSD) at a 5% significance level was performed to identify the groups that showed significant differences. Akaike information criteria (AIC) were used to compare potential models, with the model with the lowest AIC score considered the best fit for describing the relationships between season, water regime, variety, and their interactions with the morpho-physiological and yield traits studied [28,29].
Yijk = μ + αi + βj + (αβ)ij + εijk
where Yijk is the dependent variable; μ is the overall mean of the dependent variable; αi is the effect of the i-th level of the main effect A; βj is the effect of the j-th level of the main effect B; (αβ)ij is the interaction effect of A and B, and εijk is the residual error [30].

3. Results

3.1. Effects of Water Regime, Variety, Season, and Their Interactions on the Morpho-Physiological Traits of Pearl Millet Varieties

The results analyzing the water regime had a statistically significant effect (p < 0.001) on plant height, number of leaves, number of tillers, chlorophyll content, stomatal conductance, and leaf temperature (Table 2). The variety had a statistically significant effect on plant height (p < 0.05). The season had a statistically significant effect (p < 0.001) on plant height, number of leaves, tillers, and chlorophyll content. Furthermore, there was a statistically significant interaction between the water regime and season on plant height (p < 0.001) and number of tillers (p < 0.001). However, there was no statistically significant difference in the interaction of the water regime by season on the other measured plant parameters. There was a clear positive response between the water regime, season, and the dependent variables, while with variety, there was no distinct difference for most of the measured parameters.

Interaction of Water Regime by Season on Pearl Millet Morpho-Physiological Characteristics

Figure 3a,b shows the interaction effect of water regime and season on pearl millet growth and development. The seasonal effect model had the lowest AIC compared to other model terms. Graph (a) shows the relationship between plant length and the water regime across the two seasons. Statistically significant differences in plant length were observed between the different water regimes in September–December 2023 and January–April 2024. Plants under the 50% ETc treatment, September–December 2023, showed the shortest length, and from January to April 2024, plants showed similar height to those in September–December 2023. It can be seen from the graphs that the 100% ETc treatment resulted in longer plants, and September–December 2023 plants tended to be longer than January–April 2024 plants in each water regime. Under the 75% ETc treatment in September–December 2023, plants were slightly shorter than under the 100% ETc treatment, similar to the treatment with 100% ETc in January–April 2024. Under the 100% ETc treatment, plant lengths were statistically different from those under 50% ETc in September–December 2023. Plants have the highest mean length under 100% ETc in September–December 2023 compared to January–April 2024. These results show the influence of water availability on plant height. Higher water availability resulted in taller plants.
Graph (b) shows the relationship between the number of tillers (presented on a logarithmic scale) and different water regimes across the two seasons. Under 50% ETc in September–December 2023, plants showed a moderate number of tillers, whereas in January–April 2024, plants had the lowest number. Higher water availability (100% and 75% ETc) resulted in a more significant number of tillers, as there were no statistically significant differences between the two water regimes in either season. Under 75% ETc in September–December 2023, plants also showed a higher number of tillers than in January–April 2024 plants under the same treatment. September–December 2023 plants had the highest number of tillers, whereas in January–April 2024, plants had fewer tillers under the same treatment of 100% ETc. There was a statistically significant difference between the seasons in this water regime, as indicated by the different letters. However, September–December 2023 generally produced more tillers than January–April 2024 across all water regimes.

3.2. Effects of Water Regime, Variety, Season, and Their Interactions on the Yield and Yield Attributes of Pearl Millet

Table 3 shows that there was a statistically significant difference in the effect of the water regime on stem thickness (F (2) = 16.215, p < 0.001), and there was no statistically significant interaction between the water regime and season on stem thickness. There was a statistically significant interaction between the water regime and season (F (2) = 12.228, p < 0.001) on the number of productive tillers. There was a statistically significant difference in water regime (F (2) = 20.273, p < 0.001) and a statistically significant difference in season (F (1) = 35.018, p < 0.001) on panicle diameter, and there was no statistically significant interaction effect of water regime by season on panicle diameter; however, there was no statistically significant interaction difference of water regime by season on panicle length. There was a statistically significant difference (F (2) = 2.791, p < 0.05) in the length of the panicles. There was also a statistically significant difference in the panicle length (F (1) = 17.774, p < 0.001). There was a statistically significant difference (F (2) = 31.240, p < 0.001) between the water regimes and panicle weight; however, there was no statistically significant difference in the water regime by season on the dry panicle weight. There was a statistically significant interaction effect of the water regime and season on biomass (F (2) = 9.626, p < 0.001). There was also a statistically significant interaction effect of the water regime and season on grain weight (F (2) = 3.536, p < 0.05). There was no statistically significant interaction effect of water regime on the 1000-grain weight by season; however, there was a statistically significant effect of water regime (F (2) = 62.411, p < 0.001), variety (F (1) = 5.404, p < 0.05), and season (F (1) = 37.208, p < 0.001). Overall, the water regime influenced all measured parameters; the variety affected panicle length and 1000-seed weight; the season affected the number of productive tillers, panicle diameter, biomass, and 1000-seed weight; and the interaction between water regime and season affected the number of productive tillers, biomass, and grain weight of pearl millet.

3.2.1. Interaction of Water Regime by Season on Pearl Millet Yield and Yield Characteristics

Figure 4 shows the interaction effect of the water regime by season on yield and yield attributes. The seasonal effect model had the lowest AIC compared to other model terms. (a) illustrates the interaction between water regime (ETc) and season on the number of productive tillers in pearl millet. At 50% ETc, both seasons had significantly fewer productive tillers than those under the higher water regime. September–December 2023 showed more productive tillers than January–April 2024 under the 100% and 75% ETc regimes, highlighting that conditions in September–December 2023 favor tiller productivity. Under 75% ETc, there was a similar trend to the 100% ETc regime, with September–December 2023 having more productive tillers than January–April 2024. However, the overall number of tillers was slightly lower than that at 100% ETc. Under 100% ETc, Tukey’s post-hoc test revealed that September to December 2023 had a significantly higher number of productive tillers than January to April 2024. The number of productive tillers increased with a higher water regime, indicating a positive relationship between water availability and pearl millet productivity.
(b) shows the relationship between the water regimes by season and biomass. Both seasons had the lowest biomass at 50% ETc. The seasonal means were not statistically different in this water regime. The biomass decreased as the water regime decreased from 100% to 50% ETc in both seasons. September–December 2023 showed higher biomass than January–April 2024 across all water regimes, but the difference diminished in the 50% ETc water regimes. Under 75% ETc, September–December 2023 biomass was not statistically significantly different from biomass at 100% ETc. The January–April 2024 biomass was statistically different from that of September—December 2023 at 75% ETc. Under 100% ETc, September—December 2023 had the highest biomass compared to January–April 2024. The results show that biomass increased with increased water availability across the two seasons. Additionally, the seasonal effect was observed, which indicates the importance of seasons in evaluating pearl millet productivity.
(c) illustrates the relationship between water regime, season, and grain weight (in grams) for the two seasons. September–December 2023 had a lower grain weight under 50% ETc, whereas January–April 2024 had a slightly higher grain weight. However, the difference between the two seasons was not statistically significant. Grain weight decreased as the water regime decreased from 100% to 50% ETc in both seasons. September–December 2023 maintains a high grain weight at 100% and 75% ETc but significantly drops at 50% ETc. A similar pattern was observed between January and April 2024. Under 75% ETc, the mean was higher in September–December 2023 than in January–April 2024; however, the difference was not statistically significant. Under 100% ETc, September–December 2023 and January–April 2024 had high grain weights, but the results were not statistically different. The results show that grain weight increased with higher water regimes, indicating a positive relationship between water availability and grain weight.

3.2.2. Changes in Yield and Yield Attributes of Pearl Millet Cultivars During the Two Studied Seasons

Table 4 presents the data on the effect of different water regimes on the yield and yield attributes of the two pearl millet cultivars (Kangara and Okashana 2) across the two seasons (2023 and 2024). Stem thickness decreased with reduced water availability in both seasons; the number of productive tillers was highest at 100% ETc and significantly lower at 50% ETc; and panicle diameter was highest at 100% ETc and decreased with reduced water availability. Panicle length also decreased with reduced water availability at 50% ETc. However, this difference was not statistically significant during the January to April 2024 season. Kangara had significantly shorter panicles than Okashana 2 in both seasons. The dry panicle weight was significantly lower at 50% ETc than at 75% and 100% ETc. Both seasons showed highly significant differences in biomass, with lower biomass observed in reduced water levels. Grain weight was significantly reduced at 50% ETc compared with the higher water regimes. Both seasons showed significant differences, with highly significant differences and significant differences between September and December 2023 and January and April 2024. The 1000-seed weight decreased with reduced water application. Both seasons show significant differences, with highly significant differences between January and April 2024 and significant differences between September and December 2023. Furthermore, there were no significant interactions between the water regime and cultivar for yield and yield attributes in either season.

4. Discussion

The seasonal variations observed in this study significantly affected the morpho-physiological parameters and yield of pearl millet. Adequate water availability (100% ETc) during the vegetative and reproductive phases enhanced photosynthesis, nutrient uptake, and biomass accumulation, supporting vigorous growth and higher yields (Table 3). However, adverse conditions, such as high temperatures from September to December 2023 and reduced water availability (50% ETc), pose stress on the crop, leading to reduced chlorophyll content, leaf numbers, and stomatal conductance, which subsequently limits photosynthesis and assimilate transport to grains. This is in agreement with the findings of [31], who linked seasonal changes to plant growth and productivity variation in pearl millet.
Variations in water regimes further influenced specific growth characteristics, such as plant height, number of leaves and tillers, leaf temperature, chlorophyll content, and stomatal conductance (Table 2). Plants under 100% ETc treatment performed better than those under the 75% and 50% ETc treatments across these parameters. This could be attributed to the fact that adequate water facilitates nutrient mobilization, cell elongation, and efficient transpiration, which collectively reduces heat stress and enhances physiological processes [32]. The findings of this study corroborate those of [33,34], who reported increased plant height with enhanced irrigation and soil moisture availability. Furthermore, the results of this study are consistent with those of [14], who demonstrated a linear increase in leaf number with increasing water availability. Ref. [35] found that lower water availability under rainfed conditions resulted in fewer leaves and reduced yields, which is consistent with the findings of the present study. The reduced number of leaves observed under water stress conditions in this study likely contributed to diminished photosynthetic capacity and, consequently, lower yields. Refs. [36,37] reported similar trends for pearl millet tiller production under well-watered conditions. The physiological basis for this phenomenon is that water stress limits the energy available for tiller initiation because water scarcity reduces photosynthetic activity and carbon dioxide uptake, both of which are essential for tiller development [38]. Additionally, under drought stress, pearl millet exhibits plasticity by delaying tiller development to conserve resources [39].
Water-stressed plants experience reduced transpiration, resulting in higher leaf temperatures and increased susceptibility to heat stress [40]. In this study, as water was reduced (50% ETc), plants showed elevated leaf temperature compared to 100% ETc. Higher leaf temperature at 50% ETc could be attributed to less water evaporating, thereby cooling the leaf surfaces. The findings of this study align with those of [41], who found that soil moisture conservation strategies such as mulching significantly influenced canopy temperature. Elevated leaf temperatures under water stress can result in physiological damage, as demonstrated by [42], who emphasized the critical role of water in sustaining crop productivity under changing environmental conditions. The significant decrease in chlorophyll content under reduced water availability emphasizes the impact of drought stress on photosynthetic capacity and, consequently, crop productivity. These findings are consistent with those of [43,44], who observed reduced chlorophyll contents under drought-stress conditions. Drought-induced reductions in chlorophyll content may result from oxidative damage to chlorophyll molecules because water stress increases the accumulation of reactive oxygen species (ROS) [45]. Additionally, in this study, the stomatal conductance of pearl millet was reduced at 50% ETc. This indicates that pearl millet closes its stomata to minimize water loss through transpiration under severe water stress. While this adaptation conserves water, it can also limit carbon dioxide uptake, potentially reducing photosynthesis and, consequently, plant growth and yield. These findings are consistent with those of [46,47], who reported a higher stomatal conductance under well-watered conditions.
Yield attributes such as stem thickness, panicle size, biomass, grain weight, and 1000-seed weight were also significantly affected by water availability, with well-watered conditions supporting thicker stems, larger panicles, higher dry panicle weight, and better grain development (Table 4). From this study, yield and yield attributes were drastically reduced at 50% ETc and were not statistically different at 75% ETc and 100% ETc. These findings highlight that water availability is crucial for sustaining crop productivity, especially under climate variability, as drought stress negatively affects photosynthetic efficiency, nutrient uptake, and yield. A similar trend was reported by a previous study [48]. High levels of irrigation resulted in high stem diameter. Furthermore, previous studies have shown that moisture stress reduces the number of productive tillers produced by pearl millet [49,50]. Moisture stress also reduces panicle length [14,51]. Photosynthetic activity is also diminished owing to reduced leaf area and stomatal closure, limiting the carbon assimilation and energy production necessary for biomass accumulation [52]. In this study, stomatal conductance was reduced at 50% ETc, which affected the gaseous exchange for biomass accumulation. The results of this study corroborate those of previous studies, which showed that drought or reduced moisture negatively affected biomass accumulation in pearl millet [6,53]. Limited water availability critically affects the grain weight of pearl millet, primarily by disrupting vital physiological processes essential for grain development [54]. During periods of water stress, reduced turgor pressure and impaired photosynthesis lead to lower carbohydrate synthesis and translocation to the developing grains [55]. This resulted in smaller, less dense grains with a diminished weight. Moisture stress during the grain-filling stage is particularly detrimental because it directly curtails starch deposition and other vital nutrients in the grains [56]. Compared with other cereals, a similar trend in maize was observed by previous studies [57,58]. They found that, at reduced moisture levels, maize yields were still higher, highlighting the adaptability of crops to moderate irrigation in semi-arid environments. Ref. [59] also found that deficit irrigation reduced the physiological and phenological parameters of sorghum, consequently affecting yields.
This study emphasizes the importance of sufficient water supply to enhance crop resilience, optimize yield potential, and support sustainable pearl millet production in arid and semi-arid regions. Water availability significantly influences pearl millet growth, necessitating the adoption of efficient irrigation practices such as drip and deficit irrigation to optimize water use without compromising productivity.

5. Conclusions

This study highlights that pearl millet growth and yield are highly sensitive to water availability, with water stress inducing physiological changes that prioritize survival over productivity, leading to reduced morpho-physiological growth and yield. These findings highlight the critical impact of water availability on key morpho-physiological parameters, such as plant height, leaf number, tiller production, chlorophyll content, stomatal conductance, panicle length, biomass, and grain weight. Optimal water conditions (100% ETc) significantly enhance photosynthesis, nutrient uptake, and plant productivity. In contrast, reduced water availability (50% ETc) triggers physiological stress responses, negatively affecting growth and yield by limiting key processes such as stomatal conductance and chlorophyll synthesis. These physiological changes prioritize resource conservation and reduce biomass accumulation and grain production. These results emphasize the importance of effective water management in crop production, particularly in drought-prone regions. Seventy-five percent of ETc is recommended as the most efficient strategy in the cultivating of pearl millet in semi-arid environments. Developing and implementing optimized irrigation strategies and selecting water-efficient crop varieties are essential to enhance the resilience and productivity of pearl millet under varying environmental conditions. This study contributes to the growing literature on improving crop water-use efficiency and adapting agriculture to water-limited environments. Future work should focus on the long-term effects of water regimes, soil moisture depletion patterns, testing the cultivar’s responses in different environments, and the role of soil microbes in pearl millet growth and development under moisture-limiting conditions. Furthermore, studies comparing pearl millet’s morpho-physiological changes to other cereal crops could broaden the applicability of this work. Finally, crop diversification should be encouraged alongside pearl millet to build a more resilient farming system capable of withstanding climatic shocks, especially in semi-arid regions where water availability is unpredictable.

Author Contributions

O.M.: Conceptualization, Data Curation, Formal Analysis, Investigation, Methodology, Validation, Visualization, Writing Original draft, G.K.: Conceptualization, Methodology, Supervision, Writing—Review and Editing, V.C.: Conceptualization, Methodology, Supervision, Writing—Review and Editing, M.Z.: Formal Analysis, Resources, Visualization, Supervision, and M.A.W.: Conceptualization, Methodology, Resources, Supervision, Validation, Visualization, Writing—Review and Editing. All authors have read and agreed to the published version of the manuscript.

Funding

The authors acknowledge funding for this project by the Southern African Science Service Centre for Climate Change and Adaptive Land Management (SASSCAL), sponsored by the German Government through the Federal Ministry of Education and Research (BMBF) with funding no. 01LG2091A.

Data Availability Statement

The original contributions presented in the study are included in the article, further inquiries can be directed to the corresponding author.

Acknowledgments

Acknowledgements were also extended to the Ministry of Agriculture, Water and Land Reform, Crop Research and Production, and the Directorate of Agricultural Research and Development Division, Namibia, for the support and offering of land to carry out this research.

Conflicts of Interest

The authors declare no conflicts of interest.

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Agronomy 15 00381 g001
Figure 1. A map showing the study area.
Figure 1. A map showing the study area.
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Figure 2. Experimental design for September–December 2023 and January–April 2024 growing seasons.
Figure 2. Experimental design for September–December 2023 and January–April 2024 growing seasons.
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Figure 3. Interaction effect of water regime by season on pearl millet length (a) and number of tillers (b). Means are separated by Tukey’s Honestly Significant Difference (Tukey’s HSD) test at p = 0.05. Different superscript letters explain mean differences. Season 1 is September–December 2023, and Season 2 is January–April 2024. Whiskers represent standard errors.
Figure 3. Interaction effect of water regime by season on pearl millet length (a) and number of tillers (b). Means are separated by Tukey’s Honestly Significant Difference (Tukey’s HSD) test at p = 0.05. Different superscript letters explain mean differences. Season 1 is September–December 2023, and Season 2 is January–April 2024. Whiskers represent standard errors.
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Agronomy 15 00381 g004aAgronomy 15 00381 g004b
Figure 4. Effect of water regime by season on pearl millet’s (a) number of productive tillers, (b) biomass, and (c) grain weight. Means are separated by Tukey’s Honestly Significant Difference (Tukey HSD) test at p = 0.05. Means with the same letters are not significantly different. Season 1 is September–December 2023, and Season 2 is January–April 2024. Whiskers represent standard errors.
Figure 4. Effect of water regime by season on pearl millet’s (a) number of productive tillers, (b) biomass, and (c) grain weight. Means are separated by Tukey’s Honestly Significant Difference (Tukey HSD) test at p = 0.05. Means with the same letters are not significantly different. Season 1 is September–December 2023, and Season 2 is January–April 2024. Whiskers represent standard errors.
Agronomy 15 00381 g004aAgronomy 15 00381 g004b
Table 1. The total amount of irrigation water supplied to Pearl Millet.
Table 1. The total amount of irrigation water supplied to Pearl Millet.
Treatment (ETc)Total Water Applied (mm)—Season 1 (September to December 2023)Total Water Applied (mm)—Season 2 (January to December 2024)
50%304.77239.34
75%457.16359.02
100%609.55478.69
Table 2. Analysis of variance (mean squares) for physiological and vegetative traits of pearl millet.
Table 2. Analysis of variance (mean squares) for physiological and vegetative traits of pearl millet.
Model TermDFPlant Height (cm)No. of Leaves No. of TillersChlorophyll Content (SPAD value)Stomatal Conductance (mmol_m_2_s_)Leaf Temperature (°C)
Water Regime (WR)23078.48 (<0.001)131.94 (<0.001)81.39 (<0.001)687.61 (<0.001)90,047.00 (<0.001)398.58 (<0.001)
Variety (V)1349.25 (0.05)0.15 (0.807)0.18 (0.72)0.47 (0.88)4178.00 (0.26)0.25 (0.84)
Season (S)11333.47 (<0.001)234.53 (<0.001)201.38 (<0.001)1021.36 (<0.0010)2346.00 (0.40)47.73 (0.005)
WR × V271.67 (0.43)0.26 (0.90)0.08 (0.94)24.75 (0.27)2068.00 (0.53)0.15 (0.97)
WR × S21199.87 (<0.001)4.63 (0.172)13.47 (<0.001)35.35 (0.16)2300.00 (0.49)21.66 (0.03)
V × S1191.82 (0.13)1.02 (0.53)2.12 (0.21)0.02 (0.93)1100.00 (0.56)0.43 (0.78)
WR × V × S276.71 (0.40)1.61 (0.53)0.17 (0.88)0.75 (0.96)334.00 (0.90)0.92 (0.85)
Residual3681.942.511.3218.423201.005.56
Grand Mean 149.1810.975.4344.45297.6134.27
CV (%) 6.114.421.19.719.06.9
S.E± 9.051.581.154.2956.582.36
Note: DF: degrees of freedom brackets (p-values). Means were separated using Tukey’s Honestly Significant Difference (Tukey’s HSD) test at p = 0.05.
Table 3. Analysis of variance (mean square) of yield and yield attributes of pearl millet varieties evaluated at different water regimes in the 2023 and 2024 seasons.
Table 3. Analysis of variance (mean square) of yield and yield attributes of pearl millet varieties evaluated at different water regimes in the 2023 and 2024 seasons.
Model TermDFST (cm)NPPD (cm)PL (cm)DPW (g)BM (g)GW (g)1000-SW (g)
Water Regime (WR)20.12 (<0.001)18.51 (<0.001)0.34 (<0.001)5.71 (0.07)10,043.0 (<0.001)99,558 (<0.001)4521.0 (<0.001)80.58 (0.001)
Variety (V)10.01 (0.28)0.81 (0.15)0.01 (0.46)36.38 (<0.001)109.0 (0.56)2576 (0.25)21.0 (0.71)6.98 (0.03)
Season (S)10.0008 (0.76)7.32 (<0.001)0.58 (<0.001)1.08 (0.47)391.0 (0.28)41,167 (<0.001)169.0 (0.30)48.04 (<0.001)
WR × V20.004 (0.64)0.60 (0.22)0.002 (0.91)0.50 (0.78)80.0 (0.78)4814 (0.08)72.0 (0.63)3.24 (0.10)
WR × S20.009 (0.31)4.67 (<0.001)0.020.325.36 (0.09)681.0 (0.14)17863 (<0.001)533.0 (0.04)0.39 (0.74)
V ×S 10.01 (0.22)0.03 (0.79)0.04 (0.13)2.05 (0.32)196.0 (0.44)68 (0.85)26.0 (0.68)1.67 (0.26)
WR × V × S20.001 (0.88)0.34 (0.42)0.02 (0.40)0.26 (0.88)416.0 (0.29)90 (0.95)138.0 (0.41)0.37 (0.75)
Residual360.0080.380.022.05321.01856151.01.29
Grand Mean 0.582.952.5120.7170.75164.1748.889.72
CV (%) 15.021.05.16.925.326.225.211.7
S.E ± 0.090.620.131.4317.9343.0812.301.14
Note: DF, degrees of freedom; ST, stem thickness; NP, number of productive tillers; PD, panicle diameter; PL, panicle length; DPW, dry panicle weight; BM, biomass; GW, grain weight; SW, seed weight; brackets (p-values). Means were separated using Tukey’s Honestly Significant Difference (Tukey SD) test at p = 0.05.
Table 4. Effect of water regime on pearl millet cultivars and their interaction on yield and yield attributes during the two studied seasons.
Table 4. Effect of water regime on pearl millet cultivars and their interaction on yield and yield attributes during the two studied seasons.
ParameterStem Thickness (cm)Number of Productive TillersPanicle Diameter (cm)Panicle Length (cm)
Season12121212
A. Water Regime (ETc)
1000.69 b0.66 b4.27 b3.14 b2.57 b2.75 b21.27 b20.71
750.55 a0.59 b4.24 b2.60 ab2.37 a2.67 b21.19 b21.03
500.52 a0.48 a1.50 a1.93 a2.28 a2.46 a19.21 a20.83
F-test0.004<0.001<0.0010.0020.003<0.0010.020.90
B. Cultivar
Kangara0.560.583.492.662.422.5819.4820.19
Okashana 20.620.583.182.452.392.6721.6421.52
F-test0.150.910.280.360.630.080.0020.04
C. Interaction
A × B0.660.920.160.880.800.380.890.78
ParameterDry Panicles Weight (g)Biomass (g)Grain Weight (g)1000-Seed Weight (g)
Season12121212
A. Water Regime (ETc)
10089.65 b88.89 b278.90 b187.59 b63.16 b57.52 b10.26 b12.04 b
7590.98 b70.48 b232.40 b129.68 a63.83 b49.61 b9.69 b12.05 b
5040.16 a44.31 a69.1 a87.38 a25.27 a33.90 a6.19 a8.07 a
F-test<0.001<0.001<0.001<0.001<0.001<0.0010.001<0.001
B. Cultivar
Kangara74.1164.36184.94128.7552.1646.939.28 10.91
Okashana 273.0971.42201.98141.0249.3547.088.1510.52
F-test0.900.300.420.380.640.970.020.42
C. Interaction
A × B0.350.780.420.130.410.890.130.59
Means were separated using Tukey’s Honestly Significant Difference (Tukey SD) test at p = 0.05. Means with the same letters are not significantly different. Season 1 is September–December 2023, and Season 2 is January–April 2024.
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Moseki, O.; Kangueehi, G.; Chiteculo, V.; Zink, M.; Wanga, M.A. Agro-Physiological and Morphological Responses of Pearl Millet to Varying Water Regimes in Semi-Arid Conditions of Namibia. Agronomy 2025, 15, 381. https://doi.org/10.3390/agronomy15020381

AMA Style

Moseki O, Kangueehi G, Chiteculo V, Zink M, Wanga MA. Agro-Physiological and Morphological Responses of Pearl Millet to Varying Water Regimes in Semi-Arid Conditions of Namibia. Agronomy. 2025; 15(2):381. https://doi.org/10.3390/agronomy15020381

Chicago/Turabian Style

Moseki, Ofentse, Grace Kangueehi, Vasco Chiteculo, Matthias Zink, and Maliata Athon Wanga. 2025. "Agro-Physiological and Morphological Responses of Pearl Millet to Varying Water Regimes in Semi-Arid Conditions of Namibia" Agronomy 15, no. 2: 381. https://doi.org/10.3390/agronomy15020381

APA Style

Moseki, O., Kangueehi, G., Chiteculo, V., Zink, M., & Wanga, M. A. (2025). Agro-Physiological and Morphological Responses of Pearl Millet to Varying Water Regimes in Semi-Arid Conditions of Namibia. Agronomy, 15(2), 381. https://doi.org/10.3390/agronomy15020381

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