Rose development under blue light environment: behaviour of three cultivars of rose bushes

Ornamental plant shape is one of the main criteria of plant sale value. Under horticultural practices, plant shape is classically modulated through the use of growth regulators that enhance plant capacity. Increasing concern about environmental preservation encourages for the search of new environmental-friendly practices. Light is known to be the environmental factor that most affects plant architecture and thus plant shape. Using specific light quality rather than chemicals could not only contribute to preserve the environment, but also cause new aesthetical effects on plant shape. With this objective, we studied the impact of blue light on three rose cultivars, Rosa hybrida 'Radrazz', Rosa hybrida 'Meiratcan' and Rosa chinensis 'Old Blush'. Plants were grown from single node cuttings under white (Mastec 36 Watt; white/33 cool neons), or blue light (Philips TL-D 36, Watt/18, blue neons) at an equal photosynthetic efficiency (110 mu mol.m(-2).s(-1)) until the end of flowering of second order axes. Surprisingly, plants grown under blue light had normal vegetative and floral developments: for the cultivars 'Radrazz' and 'Old Blush', morphological and architectural characteristics as length and diameter of first and second order axes, number and localisation of second order axes, number and length of metamers, were similar between the two light treatments. However, for Rosa hybrida 'Meiratcan', both organogenesis and growth were slightly affected by blue light with a decrease of metamer number, axis length and axis diameter for the order 1 axes and a decrease of the number of order 2 axes. These results thus reveal the strong adaptive properties of rose plants to their light environment, but the effects of blue light are cultivar dependent. As far as horticultural practices are concerned, testing the effect of blue light on a cultivar is necessary before deciding if blue light is an efficient treatment to modulate or change rose plant shape.