Photosynthetic Daily Light Integral During Propagation Influences Rooted Cutting Morphology and Quality

Bedding and garden plants are the most valuable sector of the U.S. floriculture industry, with a wholesale value of $1.81 billion in 2009 (U.S.D.A., 2010).  To meet market demands for spring sales, cuttings must be rooted in late winter and early spring when ambient outdoor photosynthetic daily light integrals (DLIs) are at seasonally low levels. We evaluated the effect of DLI during root development on morphology and quality of nine popular vegetatively propagated annual bedding plant species.  Cuttings of Angelonia angustifolia Benth. ‘AngelMist White Cloud’, Argyranthemum frutescens (L.) Sch. Bip. ‘Madeira Cherry Red’, Diascia barberae Hook. f. ‘Wink Coral’, Lantana camara L. ‘Lucky Pure Gold’, Nemesia fruticans (Thunb.) Benth. ‘Aromatica Royal’, Osteospermum ecklonis (DC.) Norl. ‘Voltage Yellow’, Scaevola L. hybrid ‘Blue Print’, Sutera cordata Kuntze ‘Abunda Giant White’, and Verbena Ruiz ×hybrida ‘Aztec Violet’ were harvested and propagated in a glass glazed greenhouse with 23 °C air and substrate temperature set points.  After callusing (≈5 mol·m^-2·d^-1 for 7 d), cuttings of each species were placed under one of three different shade cloths providing ≈38, 61, or 86% shade, or no shade with 16 h of supplemental irradiance for 14 d.  The experiment was repeated two additional times resulting in 12 different DLIs during root development ranging from 1.2 to 12.3 mol·m^-2·d^-1.  There were no clear trends for shoot length in response to DLI across species.  The stem caliper of Argyranthemum, Diascia, and Nemesia increased by 35%, 119%, and 89%, respectively, as DLI increased from 1.2 to 12.3 mol·m^-2·d^-1.  Depending on the species, the total, shoot, and root dry mass increased by 64% to 465%, 50% to 384%, and 156% to 1137%, respectively, as DLI increased from 1.2 to 12.3 mol·m^-2·d^-1.  The quality index, a new integrated and quantitative measurement of rooted cutting quality for floriculture crops, increased for all species by 176% to 858% as DLI increased from 1.2 to 12.3 mol·m^-2·d^-1. Our results indicate that providing a DLI of ≈8 to 12 mol·m^-2·d^-1 after callusing can increase both growth and quality of rooted cuttings while potentially reducing propagation time.