SCIENCE TO THE GROWER: Who is lighting whom?

by Richard Evans

In the Fall 2014 issue, I wrote about a Nobel Prize that ushered in the opportunity to use light-emitting diode (LED) lamps for greenhouse lighting (see References). I’m back to write about both lighting and the Nobel Prize again — a group at Utah State University present an interesting economic analysis of greenhouse lighting that could help growers decide which light fixtures are worth investing in, and a research group in Sweden report on electronic circuits they fabricated in cut roses that make leaf cells light up like pixels in a computer screen.

The Utah State study (Nelson and Bugbee 2014) is a comparison of the efficiency and distribution pattern of seven high-pressure sodium (HPS) fixtures, ten LED fixtures, three ceramic metal halide fixtures and two fluorescent fixtures. The authors begin with three important observations. First, the desired light distribution depends on the configuration of plants and benches. Extensive, uniform blocks of plants in greenhouses with narrow aisles benefit from broad, uniform lamp output because much of the emitted light can be captured by the plant canopy. In contrast, LED fixtures may deliver more focused light in small facilities that have widely spaced benches. Their second observation is that photosynthesis and plant growth depend mainly on the quantity of visible light intercepted by leaves. Light quality (color) can affect plant shape, but has a minor effect on overall growth. Third, they note that LED fixtures usually produce no ultraviolet (UV) light, and that the absence of UV can cause disorders, such as intumescences (a topic I wrote about last Spring).

Nelson and Bugbee compared the cost of light output in three scenarios: radiation captured at all downward angles (appropriate for large, uniform blocks of plants), radiation captured within a 100° arc, and radiation captured within a 68° arc (suitable for more smaller, more focused applications of light). The operating cost of LED fixtures is at least five times more than double-ended, electronic ballast HPS fixtures in the scenario where all light is captured. When light is focused within a 68° arc, the operating cost of some LED fixtures is comparable to that of HPS fixtures. Ceramic metal halide and fluorescent fixtures tend to be more expensive to operate. The authors conclude that the better HPS and LED fixtures are equally efficient, but the higher initial cost of LED fixtures results in a much higher five-year cost.

The Swedish study at Linköping University is based on work that was published by another group of scientists back in 1977 announcing the discovery of electrically-conductive polymers; three of the researchers who conducted this original work (Heeger, MacDiarmid and Shirakawa) received the Nobel Prize in Chemistry 23 years later. Earlier this year, the Swedish scientists, who continued this research, reported that a molecule taken up in a solution by cut rose stems polymerized to form an electrically-conductive polymer in the xylem (Stavrinidou and others 2015). The xylem vessels containing the polymer functioned like a conductive wire, and the “wired” stems could function as living electrical circuits. They also infused attached leaves with the polymer. Cavities in the leaves functioned like pixels, which lit up and changed color when a voltage was applied.

The creation of these electronic plants has excited scientists in the field of bioelectronics, who imagine future gardens of fruit, vegetable and flowers that function as computers. Perhaps someday greenhouse plants will light us, instead of the other way around.

Richard Evans is UC Cooperative Extension Environmental Horticulturist, Department of Plant Sciences, UC Davis.

References

Evans R. Science to the Grower: A Nobel idea for plant lighting. UCNFA News 18(3):7.
http://ucanr.edu/sites/UCNFAnews/Science_to_the_Grower/A_Nobel_idea_for_plant_lighting/

Evans R. Science to the Grower: No matter how you spell edema, it’s an excrescent intumescence on the plant leaf. UCNFA News 19(1):8-9. http://ucnfanews.ucanr.edu/Science_to_the_Grower/SCIENCE_TO_THE_GROWER__edema_on_plant_leaf_/

Nelson JA, Bugbee B. 2014. Economic analysis of greenhouse lighting: light emitting diodes vs. high intensity discharge fixtures. PLoS ONE 9(6): e99010. doi:10.1371/journal.pone.0099010.

Stavrinidou E, Gabrielsson R, Gomez E, Crispin X, Nilsson O, Simon DT, Berggren M. 2015. Electronic plants. Science Advances 1(10): e1501136. doi: 10.1126/sciadv.1501136.