A Holistic Investigation into the Leaf Breakdown Condition in Potted Basil (Ocimum basilicum var. Marian)

Basil accounts for 40% of annual potted herb sales in the UK, with the largest commercial grower producing ca. 20 million pots per annum. Narrow per-pot margins necessitate the minimisation of pre-sale waste and post-sale customer complaints in order to maximise the return to the producer. One prominent cause of customer complaints is the foliar spotting condition leaf breakdown which causes lesions typically found on the lamina surface that become visible after pots have been displayed or sold. This project sought to explore the cause of leaf breakdown through transcriptomic, biochemical and microscopy techniques; as well as to identify cost-effective predictors and preventative measures of practical interest to producers and retailers.
Light and electron microscopy revealed severe mesophyll degradation, lamina folding, cuticle flaking, and dehydration symptoms in lesions during leaf breakdown. Lesions were also associated by a pronounced increase in electrolyte leakage, although no reliable metabolite predictors were found. Global transcriptomic and confirmatory RT-qPCR analysis revealed significant reduction in investment in photosynthetic machinery and extensive reprogramming of sugar signalling-associated pathways in a manner consistent with carbon starvation in the low-light environment of supermarket shelves. Genes associated with stress and defence hormone signalling were strongly upregulated; however, a culture-based screening of plant-associated microbes did not reveal the presence of pathogens, suggesting this is associated with wider abiotic stress. Thus, transcriptomic markers suggested that low-light stress on supermarket display shelves was the primary cause of leaf breakdown.
Supplementation of plants in simulated supermarket display conditions with additional light reduced incidence of leaf breakdown as characterised by electrolyte leakage assay but with the trade-off of reduced shelf life due to dehydration. Water is not currently added during supermarket display but supplementation with both additional light and water successfully reduced the incidence of leaf breakdown in a manner that would solve the problem for growers and retailers alike.
The most prudent measure to mitigate leaf breakdown in potted basil is to supplement plants during display with both water and light, in order to limit the extent of causal abiotic stressors.