Variation in Relative Irradiance of Red, Blue and Far-red Wavebands Modifies Root Morphology and Root-Shoot Ratio in Celery and Lettuce

Leafy salad crops represent one of the largest and most dynamic sectors of horticulture. However, little is known about how rooting behaviours of crops are regulated by changes in the spectral environment. This is important as modification of spectra during propagation of commercial transplants may impact on plant performance upon planting. Little information on rooting of plants grown indoors in so-called ‘plant factories’ under different spectra is also available. This study seeks to understand how variation in the irradiances of red, blue and far-red wavebands influences root morphological traits and root-shoot partitioning in two leafy salad crops.

Two sets of experiments were designed to assess the impact of blue:red ratio (0%, 15%, 35% & 100% blue) and far-red supplementation (0, 5 & 20 µmol m^-2 s^-1) on rooting charcteristics of celery (Apium graveolens) cv. Victoria and Gem lettuce (Lactuca sativa) cv. Maureen. Seeds were sown into sectioned Eppendorf tubes containing small quantities of rockwool and germinated under cool-white LED light at 150 µmol m^-2 s^-1 before transferring to a custom-designed hydroponic setup. All subsequent light treatments were 16 hour days at a PPFD of 210 ± 10 µmol m^-2 s^-1. Plant physiological traits and root / shoot biomass were assessed. Root morphological parameters were estimated via image-analysis and results are discussed with reference to total carbon assimilation.

Root:shoot ratio, length of longest root, lateral root density and the positioning of branching along the primary root were all regulated by blue percentage in both species. Variation in far-red light produced smaller effects, but increases in far-red irradiance reduced root:shoot ratio. Discussion of further work is outlined.