Photosynthetic acclimation of groundnut (Arachis hypogaea) cultivars to varying light intensities

Introduction

About 28.5 million hectares of groundnut are harvested annually, with over half grown in intercropping systems where light fluctuates with companion crop shading [1] . Photosynthetic acclimation the adjustment of carbon fixation capacity to prevailing light determines genotype performance across variable environments [2] , yet most breeding programmes evaluate lines under full sunlight only [3] . Light response curves yield parameters that describe acclimation: maximum photosynthesis (Pn max), light compensation point (LCP), light saturation point (LSP), apparent quantum efficiency (AQE), and dark respiration (Rd) [4,5] . Low LCP and high AQE favour shade performance; high LSP and Pn max favour open canopies [6,7] . This research compared photosynthetic acclimation of four groundnut cultivars to three light regimes, measuring light response parameters, fluorescence, and carbon allocation at the pod-filling stage.

Materials and Methods

Research area and environmental conditions

The experiment was conducted in the controlled-environment glasshouse complex at the Department of Agronomy, University of Bologna (44°29′N, 11°20′E), during the 2022 growing season (April-September). The facility maintains temperature at 28/22 °C (day/night) with natural photoperiod extended to 14 h using supplemental LED lighting. Mean daytime relative humidity was 55-65%.

Field experimental design

A completely randomised design with factorial arrangement (4 cultivars × 3 light levels) was replicated four times, giving 48 experimental pots. Each pot (15 L, sandy loam) held two plants thinned to one at the V3 stage. Light treatments were imposed from 21 days after sowing using neutral-density shade cloth (200 PAR) and unshaded positions (1200 PAR), with a 50% shade treatment providing 600 PAR. PAR was verified weekly with a line quantum sensor (LI-191R, LI-COR).

Materials

Seeds of cultivars ICGV-91114, TAG-24, GG-20, and TG-37A were obtained from the ICRISAT genebank. A basal dose of 20 kg N ha⁻¹ equivalent and 40 kg P₂O₅ ha⁻¹ equivalent was applied per pot at sowing. Pots were irrigated to field capacity every three days. Rhizobium inoculant was applied at sowing to ensure nodulation.

Methods

Light response curves were generated at the R5 (pod filling) stage using a LI-6400XT (LI-COR) with an LED chamber providing 12 PAR levels (0, 25, 50, 100, 150, 200, 400, 600, 800, 1000, 1200, 1500 µmol m⁻² s⁻¹). Parameters were fitted to the non-rectangular hyperbola model [8] . Chlorophyll fluorescence ETR, NPQ) was recorded with a PAM-2500 (Walz). Chlorophyll was extracted in 80% acetone and measured spectrophotometrically. At harvest, biomass was partitioned into leaves, stems, roots, and pods. Data were analysed by two-way ANOVA in R 4.2 with Tukey HSD separation. superior low-light performance (Figure 2). TAG-24 showed the highest LSP (1342) and Rd (2.41), traits linked to high-light adaptation. 1). Total chlorophyll was 28.3% higher in shade-grown leaves. Fv/Fm stayed above 0.78 in all treatments. NPQ was 43.7% higher under high light in TAG-24 than GG-20 (Table 2).

Results

Discussion

GG-20's combination of high AQE (0.068) and low LCP (33.1) positions it as a shade-tolerant type suitable for intercropping, whereas TAG-24's high Pn max and LSP mark it as sunpreferring [9,10] . The chlorophyll a:b shift from 3.41 to 2.68 under shade reflects antenna remodelling consistent with earlier findings in soybean [11] and common bean [12] . Carbon redistribution from leaves to pods under high light appears source-driven: total biomass exceeded low light by 41.3%, but pod allocation increased disproportionately, suggesting pod fill may be source-limited under shade [13,14] . In intercropping systems, cultivar choice should account for both photosynthetic capacity and carbon allocation flexibility [15,16] .

Conclusion

Groundnut cultivars differed markedly in their photosynthetic acclimation to varying light. GG-20 showed the strongest shade tolerance (highest AQE, lowest LCP), while TAG-24 maximised carbon gain under full light. Carbon allocation shifted toward reproductive sinks as light increased. These cultivar-specific acclimation profiles should inform genotype selection for intercropping and canopy management systems in Mediterranean and subtropical environments.

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