Abstract: The present invention relates to the use of a Raman spectral signature for detection of plant metabolites, specifically carotenoids, in tissue of a plant leaf. Carotenoids are used as a biomarker for an early, real-time diagnosis of shade avoidance syndrome (SAS) in growing plants in a non-invasive or non-destructive way in order to detect the adverse effect of the SAS upon their health, and ultimately their yield. The early, real-time diagnosis of SAS provides a window period within which further adverse effects of SAS may be slowed or prevented without negatively affecting the yield of growing plants or leafy vegetables.