Threshold or not: Spectral composition and light-intensity dependence of growth and metabolism in tomato seedlings

Application of LED lighting technology in indoor cultivation systems enables to improve vegetable cultivation, including tomato plantlet production. Most studies investigated the effects of light intensity and spectral composition separately and most of them used low light intensities. The present study examined the effects of light intensity and spectral composition together on the growth, flowering and accumulation of primary and secondary metabolites in tomato leaves. The most important light factors affecting the growth and metabolism in tomato plants were determined together with the roles of photoreceptor phytochromes and cryptochromes. The changes of some morphological, physiological and metabolic parameters, such as plant height, photosynthetic activity, stomatal conductance and sugar accumulation showed a linear correlation with the increased proportion of blue light within 5–30% blue light range. Others, including stem diameter, biomass, leaf area and protein content grew up only to a point. Above this threshold, the blue light had adverse effects. Red or green light fluence stimulated the biomass production, flowering and the accumulation of carbohydrates in which phytochromes (B1 and B2) are involved. The proportion of red light influenced the specific area of leaves and also the chlorophyll biosynthesis. High amount of blue light stimulated the protein, carotenoid, anthocyanin and flavonoid biosynthesis in tomato leaves. It may have happened through CRY2-mediated regulation. The optimal blue proportion was around 15% for the growth of young tomato plants, however, it can also shift to higher blue proportion if the growth light intensity was low indicating that the optimal level of blue light depends also on the light intensity. The results showed that light intensity and spectral composition regulate the growth and metabolism of tomato plants in interaction.