UVA radiation promotes tomato growth through morphological adaptation leading to increased light interception

UVA radiation (315 - 400 nm) is the main component of solar UV radiation. Although it shares photoreceptors (i.e. cryptochromes and phototropins) with blue light (400-500 nm), its function in plant biology is unclear to a large extent. This study aimed at exploring how UVA radiation affects plant morphology and physiology, and at distinguishing to what extent these effects differ from those of blue light. Tomato plants were grown under monochromatic red (R), dichromatic red and blue (R/B = 7:1), as well as red and two different levels of UVA radiation (R/UVA = 7:1 and 15:1, respectively), with identical photon flux density (250 mu mol.m(-2).s(-1)). Peak intensities of UVA, B and R were 370, 450 and 660 nm, respectively. We showed that replacing blue by UVA (in a background of red light) induced plant morphological modifications, as reflected by larger leaf area, steeper leaf angles, flatter leaves and longer stems. UVA had reduced effects on leaf secondary metabolism compared to blue light, resulting in significantly lower total phenolics and flavonoid contents, as well as concentrations of UV-absorbing compounds. In addition, UVA had a similar function as blue light in shaping the development of the photosynthetic apparatus, as both wavebands alleviated the 'red light syndrome' (i.e. low photosynthetic capacity, reduced photosynthetic electron transport, and unresponsive stomata). We conclude that: 1) UVA promotes tomato growth through morphological adaptation leading to increased light interception; 2) UVA affects leaf secondary metabolite accumulation less strongly than blue light; 3) UVA functions similarly to blue light in maintaining leaf photosynthetic functioning. Thus, unlike previously suggested, UVA cannot be unequivocally considered as an abiotic stress factor. This research adds to the understanding of plant processes in response to UVA radiation and provides a basis for future recipes for growing plants with artificial light.