Supplemental and Sole-source Light and Root-zone Temperature Influence Cutting Physiology, Morphology, and Rhizogenesis of Herbaceous Perennials
Herbaceous perennial propagative material in the United States has a wholesale value of $120 million, representing 30% of the total propagative material category. While perennials can be successfully propagated by seed, cuttings, divisions, and tissue-cultured plantlets; unrooted shoot-tip, stem, and basal cuttings are recommended to maintain genotypic and phenotypic uniformity and hasten production. Propagation of unrooted herbaceous perennial cuttings occurs year-round under seasonal average daily temperatures (ADT) and photosynthetic propagation daily light integrals (PDLIs), thereby hindering the ability of propagators to maintain consistent environmental parameters. Callus and adventitious root formation (ARF) and development of cuttings (liners) are independent of each other; however both are dependent upon temperature and light. Independently, root-zone temperature (RZT) and PDLI during propagation has been shown to influence ARF and cutting growth, development, morphology, and physiology of herbaceous annuals and woody ornamentals, though little literature exists for herbaceous perennials. Furthermore, photomorphogenic responses to monochromatic and dichromatic light spectra during seedling (plug) and in vitro propagation have been previously reported. However, literature regarding photomorphogenesis of cuttings during ex vitro callusing and ARF is limited and exists for few herbaceous annuals and woody ornamentals. Therefore, we were interested in improving rhizogenesis of herbaceous perennials with RZT and PDLI and minimizing production time of rooted cuttings. The specific objectives of this study were therefore to 1) quantify growth, development, and morphology during rhizogenesis of purple fountain grass [ Pennisetum × advena Wipff and Veldkamp (formerly known as P. setaceum Forsk. Chiov. ‘Rubrum’)] propagated from single-internode culm cuttings (Expt. 1); 2) characterize the effects of RZT and PDLI on biomass accumulation and allocation, photosynthesis, and starch status during rhizogenesis of blanket flower (Gaillardia aristata Pursh ‘Gallo Red’), coral bells (Heuchera hybrida L. ‘Black Beauty’), and wand flower ( Gaura lindheimeri Engelm. and A. Gray ‘Siskiyou Pink’) cuttings (Expt. 2); and 3) quantify photomorphogenic responses during callusing and ARF of perennial sage (Salvia nemorosa L. ‘Lyrical Blues’) and wand flower cuttings propagated under SSL red:blue (R:B) light-emitting diodes (LEDs; Expt. 3). In Expt. 1, RZT set points of 23 and 25 °C under PDLIs of 8 to 10 mol·m–2·d–1 resulted in the greatest root and culm growth and development, while suboptimal (21 °C) and supraoptimal (27 °C) RZTs resulted in less or reduced rhizogenesis and liner quality, respectively. In Expt. 2, species-specific responses to increasing PDLI at RZTs of 20, 24, and 28 °C during rhizogenesis of blanket flower, coral bells, and wand flower cuttings were observed. Increasing PDLIs during ARF increased root biomass of blanket flower by up to 395%, 255%, and 295% and by up to 142%, 602%, and 762% for wand flower at RZTs of 20, 24, and 28 °C, respectively, while root biomass of coral bells was unaffected by PDLI. Additionally, photosynthesis of wand flower increased by up to 187% as PDLI increased from ≈1 to 10 mol·m–2·d –1, while total starch increased by 456% under PDLIs of ≈1 to 15 mol·m–2·d–1. Finally, in Expt. 3, callus growth of perennial sage and wand flower cuttings were unaffected by light quality, but under SSL R50:B50 LEDs, cuttings were 21 and 30% shorter and accumulated 50% and 8% more root biomass, respectively, and were of similar or greater quality compared to cuttings propagated under supplemental lighting in the greenhouse. Based on these results, it is recommended to maintain RZTs of 23 to 25 °C and target a PDLI of 8 to 10 mol·m–2·d –1 during propagation of purple fountain grass, blanket flower, and wand flower cuttings, while higher RZTs (28 °C) and PDLIs (15 mol·m –2·d–1) during summer months will not negatively affect ARF of blanket flower and wand flower. Additionally, callusing and rooting perennial sage and wand flower cuttings in controlled-environments, such as a growth chamber, under SSL R50:B50 LEDs will result in greater or comparable liner quality than those propagated in a greenhouse. Furthermore, the scope of the research provides a deeper insight into the physiological mechanisms related to ARF, growth, and development, thereby characterizing responses to improve ARF during herbaceous perennial propagation.
Mitchell, Purdue University.
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