PhD thesis: "To Bloom or Not to Bloom: The Effect of Photoperiod is Spectrum Dependent" – Malleshaiah SharathKumar

What is the impact of the light spectrum on flowering and yield of short-day plants?

The greenhouse and ornamental industries are increasingly embracing LED lighting. But what is the impact of the light spectrum on flowering, and how can it help to help to optimize production processes and maximize yield? A PhD research project of Malleshaiah SharathKumar of Wageningen University, titled "To Bloom or Not to Bloom: The Effect of Photoperiod is Spectrum Dependent," may give an answer. "It gives insights and can guide growers in effectively implementing LED lightning systems, enabling year-round cultivation of short-day plants. I want to share this study as the significance of it extends beyond academic circles", he says.

In this research, Sharath used the obligate short-day plant chrysanthemum (Chrysanthemum morifolium cv. Radost) as a model system to elucidate the role of the light spectrum in chrysanthemum flowering under long days with dynamic LED lighting. The aim of the research was to use the light spectrum to control photoperiodic flowering, specifically to obtain flowering in short-day plants by extending short days to long days by 4h using either sole blue or sole red LED light via dynamic light spectra. Furthermore, he aimed to understand the role of the light spectrum in regulating photoperiodic flowering in chrysanthemum at the physiological and molecular levels.

The full report is not publicly available yet, but Sharath can share the summary of the study, which already entails much information. In this article, just some findings.

It was found that, in the climate chamber, dichromatic red-blue short-day extended with 4h of sole blue light resulted in complete flowering, while extension with 4h of sole red light resulted in floral initiation but no further flower development. "In the greenhouse, plants grown under short days provided by solar light extended with 4h of blue or red light failed to flower. This study shows that not only day length per se, but also the spectral composition of the first 11h of a long photoperiod (dichromatic red-blue vs. full-spectrum solar light) influences the flowering responses of chrysanthemum."

Also, the flowering of chrysanthemum grown under short days with dichromatic RB and broad-spectrum Rbw LED-light extended with 4h of sole blue light was similar to that under short days. "The flowering under blue-extended long days was accompanied by the rapid downregulation of the antiflorigen gene CmAFT and the gradual increase in expression of the florigen gene CmFTL3, regardless of whether dichromatic RB of broad-spectrum Rbw light was used during the 11h of the short day. These results imply that the current molecular model for chrysanthemum flowering (the rapid downregulation of CmAFT and the gradual increase in expression of CmFTL3 during capitulum development) also holds true under blue-extended long days. The results also clearly show that these molecular mechanisms are not solely dependent on the length of the dark period but are also regulated by the light spectrum."

Flowering was completely inhibited in plants grown under dichromatic RB with far-red light followed by 4h of blue light extension, whereas using broad-spectrum Rbw in place of dichromatic RB resulted in partial (only a few plants flowered) flower inhibition. "Plants in the dichromatic RB with far-red light followed by 4h of blue
light extension showed higher expression of the antiflorigen gene CmAFT and no upregulation of the florigen gene CmFTL3, which correlated with a lack of flowering. The presence of far-red in broad-spectrum Rbw that resulted in partial flowering maintained higher expression of CmAFT, while the expression of CmFTL3 remained low. These results indicate that far-red during the first 11h in blue-extended long days hinders flowering, highlighting a previously undiscovered negative role of far-red light on chrysanthemum flowering."

He also evaluated 12 genotypes of nine short-day plants species (kalanchoe - Kalanchoe blossfeldiana, perilla - Perilla frutescens, stevia - Stevia rebaudiana, artemisia - Artemisia annua, chrysanthemum - Chrysanthemum seticuspe and Chrysanthemum morifolium, cosmos - Cosmos bipinnatus, poinsettia - Poinsettia pulcherrima and wild tomato - Solanum habrochaites) under three light conditions. The result? "The flowering response differed among species. Kalanchoe, perilla, and stevia flowered only in red-blue short-day conditions. Artemisia, chrysanthemum, cosmos, poinsettia, and wild tomato flowered red-blue short day, and in blue-extended long days, but not in red-blue long days."

For more information:
Wageningen University
Sharath Malleshaiah

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