Morphological plasticity of greengram (Vigna radiata L.) under different photovoltaic system

Introduction

Agriphotovoltaic systems (APV)-the simultaneous utilization of land for solar photovoltaic power generation and agricultural production-have emerged as a sustainable strategy to address land competition. However, overhead solar structures create heterogeneous spatial microclimates characterized by reduced Photosynthetically Active Radiation (PAR) and altered light quality. Crops under partial or intermittent shade exhibit distinct morpho-physiological responses, collective known as the Shade Avoidance Syndrome (SAS). Key indicators of SAS include hypocotyl and stem elongation, petiole extension, leaf movements (steeper leaf angles). In pulse crops like greengram (Vigna radiata L.), canopy architecture directly regulates light interception efficiency, photosynthate partitioning and mechanical stability. Understanding how panel height, module technology (monofacial vs. bifacial) and spatial sowing distances influence canopy geometry is essential for designing crop-compatible APV installations.

Materials and Methods

Experimental site and treatment layout

The field investigation was conducted using a Randomized Block Design (RBD) with seven treatments replicated three times. The treatment details were as follows: T 1: 1.75m Monofacial panel with 0.5 m sowing distance away from panel. T 2: 1.75m Monofacial panel with 1.5m sowing distance away from panel.

Recording of data

Three plants were selected at random from each replication for recording morpho-architectural data at 60 DAS: Hypocotyl Length (cm) measured from the ground line to the cotyledonary node using a scale. Petiole Length (cm) measured from the nodal attachment point on the main stem to the leaflet base using a scale. Stem Thickness (mm) recorded at the middle of the second internode above ground level using a digital vernier caliper. Leaf Angle ( o ) measured relative to the horizontal main axis using a protractor positioned along the central leaf vein. Leaf Area (cm 2 ) total leaf surface area per plant was determined using a portable leaf area meter.

Result and Discussion

Marked differences in hypocotyl length were observed among the agriphotovoltaic treatments. The longest hypocotyl (13.17 cm) was recorded in T 1 , comprising a 1.75 m monofacial panel height with a 0.5 m sowing distance, whereas the shortest hypocotyl (6.17 cm) was recorded in T 6 , consisting of a 1.25 m bifacial panel height with a 1.5 m sowing distance. The difference between T 1 and T 6 (7.00 cm) was greater than the CD at 5% (1.49 cm), indicating a significant difference between these treatments. The greater elongation observed in T 1 associated with the lower light environment prevailing close to the PV structure. Under reduced irradiance, plants can modify their architecture by allocating relatively greater growth towards vertical elongation in an attempt to improve access to available light. The relatively shorter hypocotyl under T 6 suggests comparatively lower shade-induced elongation at the greater horizontal distance from the panel. Similar shade-responsive elongation patterns have been reported under reduced light availability by Fan et al. (2019) [1] . Lyu et al. (2023) [2] observed that low-light stress induced rapid hypocotyl extension while suppressing internode expansion in the middle and upper main stem, ultimately decreasing mature plant height in greengram. (fig. 1)

Petiole length differed significantly among the evaluated agriphotovoltaic treatments. The maximum petiole length (11.67 cm) was recorded in T 7 (Non-PV system) which was significantly superior to the other treatments, while the minimum value (2.13 cm) was observed in T 5 , where greengram was cultivated beneath a 1.25 m bifacial panel height at a 0.5 m sowing distance. The difference of 9.54 cm between these treatments was substantially greater than the CD at 5% (1.50 cm), confirming a significant treatment effect. The greater petiole development under the Non-PV system related to the higher and more uniform availability of solar radiation, which favoured normal leaf and supporting-structure development. In contrast, the strongly modified light environment beneath the low-clearance PV structure have restricted petiole development. Differences in petiole morphology under contrasting irradiance regimes are consistent with the concept that plants adjust supporting tissues and leaf architecture according to their light environment (Niinemets et al., 2002) [3] (fig. 2).

Stem thickness exhibited considerable variation among the agriphotovoltaic treatments. The thickest stem (4.34 mm) was recorded in T 4 , representing the 3.75 m bifacial panel height with a 1.5 m sowing distance, whereas the thinnest stem (1.56 mm) occurred in T 1 under the 1.75 m monofacial panel height at a 0.5 m sowing distance. The difference between T 4 and T 1 was 2.78 mm, which exceeded the CD at 5% (0.65 mm), indicating a significant difference between the two treatments.

Figure 1. Hypocotyl Length at 60 DAS influenced by different PV systems

The reduced stem thickness under T 1 associated with shade-induced morphological adjustment, in which plants favour longitudinal extension to improve their position within the available light environment at the expense of radial stem development. Conversely, the greater stem thickness observed in T 4 indicates that the combination of greater panel clearance and increased horizontal distance provided a more favourable light environment for structural growth. Similar reductions in stem girth associated with shade-avoidance responses have been reported in crops grown under agriphotovoltaic or shaded conditions (Lee et al., 2022) [4] . (fig. 3)

Significant differences in total leaf area per plant were observed among the agriphotovoltaic treatments. The greatest total leaf area (78.66 cm 2 ) was recorded under T 4 , comprising a 3.75 m bifacial panel height with a 1.5 m sowing distance, whereas the lowest leaf area (11.9 cm 2 ) was recorded under T 5 , where plants were grown at a 0.5 m distance from the 1.25 m bifacial panel height. The difference of 66.76 cm 2 was considerably greater than the CD at 5% (1.91 cm 2 ), demonstrating a significant treatment effect on leaf development.

The comparatively larger canopy development under T 4 suggested that the greater panel clearance and horizontal spacing created a more favourable light environment for leaf production and expansion. Although plants frequently respond to shade by increasing individual leaf expansion as an acclimation mechanism for improving light interception, severe or prolonged shading can restrict overall canopy de-velopment when carbon assimilation becomes limiting. The reduced total leaf area under T 5 may therefore reflect the stronger shading imposed by the low horizontal clearance from the PV structure. Comparable responses of leaf development to altered light environments have been reported by Yang et al. (2014) [5] and Wu et al. (2017) [6] . (fig. 4)

Leaf angle (o)

Leaf angle differed significantly among the treatments. The maximum leaf angle was recorded in T 1 (73.00°), whereas the minimum was observed in T 4 (41.00°). The difference of 32.00° between T 1 and T 4 was substantially greater than the CD at 5% (1.75°), indicating a significant difference. However, T 4 (41.00°) was statistically at par with T 3 (41.11°), as the difference between these treatments was only 0.11° which was lower than the CD at 5%. The greater leaf angle under T 1 indicates a pronounced alteration in canopy architecture under the more restricted light environment. Modification of leaf orientation is an important mechanism through which plants regulate light interception within the canopy. Under contrasting radiation conditions, leaves can alter their orientation to balance light capture, radiation penetration and thermal load. The relatively lower leaf angles observed under T 3 and T 4 suggest a more favourable canopy arrangement under the elevated bifacial configuration, potentially allowing efficient interception of available radiation without excessive vertical orientation. Such plasticity in leaf orientation represents an important component of plant acclimation to changes in the light environment (Taiz et al., 2015) [7] . (fig. 5)

Acknowledgement

AcknowledgementWe sincerely acknowledge GIZ, Germany, for funding and supporting this collaborative initiative alongside VNMKV, Parbhani, which enabled the execution of this research and covered processing expenses. Data collection was carried out at the Agrophotovoltaic Research Project site in Manwat, Parbhani District, while data analysis and laboratory work utilized facilities provided by the Department of Plant Physiology, College of Agriculture, VNMKV Parbhani. Disclaimer The opinions expressed in this publication are solely those of the authors and do not necessarily represent the official position of their affiliated organizations. Although reasonable steps were taken to guarantee the accuracy and completeness of the data presented, the authors disclaim all liability for any direct or indirect consequences arising from its application.

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