The spinach said ‘Let there be light’
Plants eat light. Animals eat plants. And we humans eat plants and animals, so sunlight – or a really good substitution for it – is essential for life.
One important plant in a healthy human diet is spinach. It’s high in vitamins A, C and K and iron and magnesium. But to get the most from those minerals, spinach needs not only to grow well but to finish strong.
Studying end-of-production lighting effects on spinach growth, nutritional quality and post-harvest longevity are Jacob Friedrich, Sarah P. Armstrong and Kellie J. Walters from the University of Tennessee-Knoxville and W. Garrett Owen of Ohio State. They shared their findings at an American Society for Horticultural Science conference.
The team looked specifically at the ‘Space’ cultivar with the goal of reducing energy costs and improving its growth and post-harvest longevity with end-of-production (EOP) lighting. Light intensity, quality and duration impacts on growth, nutritional quality and post-harvest characteristics were measured.
The spinach was grown in a controlled environment agriculture system with five different light treatments: 100 moles of broad spectrum; 200 moles of broad spectrum; 400 moles of broad spectrum; 600 moles of broad spectrum; and 200 moles of red/blue.
The light treatments were applied for seven days, three days or just one day before harvest.
When it came time for harvest, the researchers looked at leaf number, growth index, fresh mass, dry mass and photosystem efficiency. Post-harvest, they looked at fresh mass, colorimetry and qualitative visual rank.
There were no significant differences in color or visual quality of the spinach at harvest. “Leggier” growth was seen in the lower light intensities and with the shorter durations of light treatments, though.
When it came to yield, fresh and dry mass both tended to increase with higher light intensity and longer EOP lighting duration. There was no significant difference between the red/blue and broad spectrum light qualities in yield.
For health purposes, vitamin C concentrations did increase with light intensity. However, EOP lighting did not influence the carotenoids lutein, alpha-carotene and beta-carotene, neoxanthin, violaxanthin, zeaxanthin or chlorophyll.
Anyone who has grown and/or purchased spinach knows it has a very finite time it’s still good after being harvested (before the sliming begins). The good news with this study is that the spinach treated with 600 moles for seven days maintained the highest quality over time. No difference was observed between 200 moles and red/blue. The silver lining? Phytonutrients were similar between harvest and two weeks post-harvest.
So, what do the experiment results say? At harvest, increasing EOP light intensity and duration boosted yield, influenced growth habit and enhanced photosystem efficiency in spinach. At 200 moles, light quality had no impact on yield, color, water-soluble vitamins or carotenoids. The highest yield came from 600 moles applied for seven days. Higher light intensity also increased vitamin C concentrations when applied one and three days prior to harvest.
After harvest, spinach’s visual quality improved with higher light intensity and a longer EOP duration. Water-soluble vitamins and carotenoid levels remained stable between harvest and two weeks post-harvest.
The research team noted these findings support optimizing EOP lighting in CEA to enhance yield, shelf life, nutritional value and marketability of hydroponic spinach.
by Courtney Llewellyn
