Leaf age and light quality influence the basal resistance against Botrytis cinerea in strawberry leaves

In this study, the effects of both leaf age and light quality on leaf resistance against Botrytis cinerea were investigated. Strawberry plants were grown in a growth chamber equipped with white, blue, red or red + blue light-emitting diodes (LEDs) at a photosynthetic photon flux (PPF) of 100 μmol m−2 s−1. The effect of leaf age and light quality on leaf morphology, resistance to B. cinerea, hydrogen peroxide levels, metabolites and pigments was studied. Leaf number increased under red light, while leaf petiole length was reduced by red + blue light. Leaf resistance to B. cinerea dramatically increased with leaf age from 1- to 4-week-old leaves but decreased again in 5-week-old leaves. Red light significantly improved leaf resistance, while white and blue-light treated leaves were the most susceptible to B. cinerea at all leaf ages. Hydrogen peroxide levels positively correlated with disease severity and were influenced by both leaf age and light quality. They were lowest in 4-week-old leaves and in red light-grown leaves, irrespective of leaf age. Chlorophyll and carotenoids levels negatively correlated with disease severity and increased with leaf age but were lowest in blue light-grown leaves. Total phenolics and flavonoid levels were very high in the very susceptible 1-week-old leaves and considerably lower in the older leaves. Red light stimulated total phenolics in 1-4-week-old leaves. Proline levels were strongly stimulated by blue light, especially in 1 and 5-week-old leaves. Overall, low hydrogen peroxide levels and high chlorophyll and carotenoids levels appear to be the best indicators for leaf resistance to B. cinerea in strawberry leaves. Moreover, leaf age should be taken into account when assessing the effect of light quality on disease resistance.