Identification of morphological characteristics and light environment strategies to mitigate intumescence in water spinach cultivated in a plant factory with artificial lighting

Abstract

Water spinach (Ipomoea aquatica Forrsk.) is a productive aquatic plant extensively cultivated in tropical and subtropical areas; nonetheless, it is susceptible to intumescence, a physiological condition that reduces growth and commercial value, particularly in plant factories employing artificial lighting. This study investigated the characteristics of intumescence and explored light strategies to mitigate its incidence. Seedlings were grown under white LEDs with a light intensity of 100 μmol m⁻² s⁻¹ for seven days during seedling production. Air temperature and relative humidity were maintained at 24/20 °C and 65/85 % (photo-/dark periods), respectively, and the CO₂ concentration was maintained at 800 μmol mol⁻¹. Seedlings were transplanted at planting densities of 105.8, 211.6, and 846.6 plants/m² and exposed to photosynthetic photon flux density (PPFD) levels of 100, 200, and 300 μmol m⁻² s⁻¹ for 21 days. Another experiment maintained the same daily light integral (DLI) of 11.52 mol m⁻² d⁻¹ across all treatments, which consisted of three combinations of PPFD (320, 200, and 160 μmol m⁻² s⁻¹) and photoperiod (10, 16, and 20 h d⁻¹). The third experiment used high-density planting (846.6 plants/m²) under a PPFD of 300 μmol m⁻² s⁻¹ with supplementary UV-B radiation (0.0, 1.0, 1.5, and 2.0 W m⁻²) for four hours daily. In water spinach, intumescence symptoms under these conditions involved hypertrophy of spongy parenchyma cells in leaves and hyperplasia of collenchyma cells in stems, accompanied by cuticular abnormalities that are likely to impair photosynthesis and reduce the commercial value of the crop. Higher PPFD increased biomass at lower densities, whereas mutual shading at denser planting reduced the light availability. Longer photoperiods with lower light intensities reduced intumescence severity while maintaining yield at the same DLI. UV-B supplementation at 1.0 W m⁻² effectively mitigated intumescence without reducing growth, while higher UV-B intensities negatively impacted growth. Cultivating water spinach at 300 μmol m⁻² s⁻¹ PPFD with high planting density and 1.0 W m⁻² UV-B radiation minimizes intumescence while ensuring high yield and quality.