Light-emitting diodes and gas exchange facilitation minimize hyperhydricity in Lippia grata: Physiological, biochemical and morpho anatomical aspects

Abstract

Lippia grata Schauer is a Brazilian savanna plant that belongs to the Verbenaceae family. Its importance in folk medicine, combined with threats to its natural habitat, gave rise to the necessity to carry out studies involving large-scale propagation. Unlike other species of the same genus, L. grata has shown high rates of hyperhydricity, which reduces survival during acclimatization. The present study aimed to define an efficient protocol that minimizes hyperhydricity. Young cuttings containing nodal segments were cultivated in Murashige-Skoog medium. The culture test tubes were totally sealed with aluminum foil (non-ventilated) or partially sealed (ventilated) with aluminum foil containing one hole. The cultures were maintained in a growth room exposed to four light treatments for 45 days: two combinations of red and blue light-emitting diodes (LED) with red: blue light ratios of 5:1 and 1:1, white-LED, and fluorescent lamps. Non-ventilation under fluorescent lamps represent traditional conditions of tissue culture. The lowest hyperhydricity rates in plants were found in the partially sealed culture test tubes. The combination of partial sealing with red/blue LED at a 1:1 light ratio resulted in 26% hyperhydric leaves in contrast to 87% under traditional conditions. Higher contents of photosynthetic pigments, namely chlorophyll a, chlorophyll b, total chlorophyll, and carotenoids were found in plants under partially sealed test tubes under white-LED illumination, reaching 1.24, 0.56, 1.79, and 412.94 mg g−1 FM, respectively. In addition to uniformity in cell morphology of the various tissues, plants cultivated under partial sealing showed higher content of total soluble carbohydrates and sucrose, i.e., 487.49 and 192.74 mg g−1 FM, respectively. These results demonstrate that micropropagation systems with higher light intensity and quality (wavelengths), combined with a facilitated gas exchange between culture and laboratory air, were efficient in preventing hyperhydricity and in preserving leaf morphophysiology.