Leaf gas exchange and water use efficiency of millets under deficit irrigation regimes

Introduction

While maize and rice command vast irrigation infrastructure across the tropics, millets-arguably better adapted to heat and drought-are still grown mainly under rainfed conditions with minimal management inputs [1] . This contrast is partly historical (colonial-era research prioritised export cereals) and partly economic (millets fetch lower market prices), but it leaves a major knowledge gap about how these crops respond to controlled water supply [2] . With irrigation expanding rapidly into semi-arid zones of West Africa, understanding how millets handle regulated deficit irrigation is overdue. Deficit irrigation deliberately supplies less water than full crop evapotranspiration, accepting a small yield penalty in exchange for substantial water savings [3] . The physiological basis of this trade-off lies in stomatal behaviour: partial closure under mild stress reduces transpiration faster than photosynthesis, lifting the ratio of carbon fixed to water lost-the instantaneous water use efficiency [4] . C4 millets, with their high intrinsic WUE, may tolerate deficit irrigation better than C3 cereals, but direct comparisons among millet species under identical field conditions are scarce [5] . This research measured gas exchange parameters across three millet species and three irrigation levels over two seasons in southern Ghana.

Climatic Conditions

The Kpong Irrigation Research Station experiences a tropical savanna climate (Aw) with bimodal rainfall peaking in June and October. During the two research seasons, total rainfall was 1,024 mm (2020) and 963 mm (2021). Mean growing-season temperature was 28.7 °C, and midday vapour pressure deficit averaged 2.1 kPa. Reference evapotranspiration (ET₀) ranged from 4.2 to 5.8 mm day⁻¹, calculated using the Penman-Monteith equation from on-site weather station data.

Material and Methods

Material

Pearl millet (Pennisetum glaucum cv. Tifton Late), finger millet (Eleusine coracana CV.

KAT/FM-1), and foxtail millet (Setaria italica cv. Prasad) were sown in plots of 5 × 6 m at the Kpong station (6°09′N, 0°04′E). Soils were Chromic Luvisols with 31% clay, pH 6.1, and organic carbon 1.4%. Drip lines (Netafim, 16 mm, 2 L h⁻¹ emitters at 30 cm spacing) delivered water at three levels: 100% ETc (full), 75% ETc (moderate deficit), and 50% ETc (severe deficit). Basal fertiliser was applied uniformly at 60-30-30 kg NPK ha⁻¹.

Methods

Irrigation treatments were imposed from 21 days after sowing until physiological maturity. Soil moisture was monitored by TDR probes (Campbell CS616) at 20 and 40 cm depths. Leaf gas exchange was measured on the youngest fully expanded leaf using a LI-6400XT (LI-COR, Lincoln, USA) at 1,200 μmol m⁻² s⁻¹ PPFD and ambient CO₂ between 08:30 and 11:00 h. Stomatal conductance (gₛ), transpiration (E), net photosynthesis (A), and instantaneous WUE (A/E) were recorded on six leaves per species per treatment at fortnightly intervals, yielding 1,296 observations per season. A split-plot ANOVA (irrigation as main plot, species as subplot) with Tukey's HSD was applied in R v4.2.1.

Field experimental design

The trial followed a split-plot layout in a randomised complete block design with four replications. Irrigation regime was the main-plot factor and millet species the subplot factor. A 2 m buffer separated main plots, and each subplot was bordered by guard rows. A = net photosynthesis (μmol CO₂ m⁻² s⁻¹); gₛ = stomatal conductance (mol m⁻² s⁻¹); E = transpiration (mmol m⁻² s⁻¹); WUE = A/E (μmol mmol⁻¹); Ci/Ca = intercellular-to-ambient CO₂ ratio. All three species showed significant reductions in A, gₛ, and E under deficit irrigation (p<0.001), while WUE increased significantly (Table 1). Finger millet achieved the highest WUE at every irrigation level, and pearl millet maintained the highest photosynthetic rate. At 75% ETc, yield losses ranged from only 6.9 to 8.2% while water productivity improved by 22.5-24.2% (Table 2). Under severe deficit (50% ETc, not shown), yield losses exceeded 22% across all species, confirming 75% ETc as the practical optimum.

Results

Comprehensive interpretation

Across all species, instantaneous WUE correlated negatively with stomatal conductance (r = -0.88, p<0.001) and positively with the Ci/Ca drawdown (r = 0.83, p<0.001), confirming that stomatal regulation was the main driver of WUE shifts under deficit irrigation.

Discussion

The 7-8% yield penalty at 75% ETc is well within the range that economic analyses consider compensated by 25% water savings in semi-arid contexts [3,6] . Pearl millet's high absolute photosynthetic rate under full irrigation reflects its vigorous C4 metabolism, consistent with Sahelian data [7] . Finger millet's superior WUE under stress aligns with its reputation as the most drought-hardy cultivated millet, linked to tighter stomatal control [8,9] . All three millets displayed anisohydric stomatal behaviour where partial closure preferentially limits water loss before carbon gain [4] , though finger millet showed the most conservative response, suggesting near-isohydric tendencies [10] . Foxtail millet occupied an intermediate position, making it versatile for areas where water availability fluctuates unpredictably [11] .

Conclusion

Moderate deficit irrigation at 75% ETc reduced millet grain yields by only 6.9-8.2% while improving water productivity by 22-24%. Pearl millet produced the highest absolute yields, finger millet achieved the best WUE under stress, and foxtail millet balanced both traits. Irrigation advisory services in semiarid West Africa should consider 75% ETc as a default scheduling target for millets, adjusting downward only when water allocation is critically constrained.

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