Florida is the Sunshine State. Most days of the year, the sun is bright in the sky and we assume plants have the light they need to grow. Recent research in my lab is leading me to challenge this assumption. I think we are taking light for granted.

Plants use light in two different ways: to power photosynthesis and to get information about objects around them. The light plants use for photosynthesis is usually called photosynthetically active radiation (abbreviated PAR). The PAR intensity in the middle of a summer day is approximately 2,000 micromoles per meter square per second. That is a lot of light when we consider that the average blueberry leaf reaches its maximum photosynthetic rate with approximately 800 micromoles per meter square per second. But not all leaves have the same exposure to light. While the top and exterior leaves in a blueberry bush usually face high PAR intensities, the interior and lower parts of the canopy are exposed to very low PAR intensities. 

In our first experiment in this subject, we tracked light interception in ‘Farthing’ and ‘Optimus’ southern highbush blueberry. We found that leaves that are 2 feet from the ground receive only 25% of the total light intensity the top of the canopy receives. These leaves are effectively resting instead of working! This brought us to look at a larger set of varieties. 

We surveyed 27 varieties and advanced selections from the UF Blueberry Breeding program. Some varieties, like ‘Patrecia’ and ‘Sweetcrisp’ make sparse canopies that allow high light intensities to reach lower leaves. In these varieties, all leaves are at work. In contrast, varieties like ‘Colossus’, ‘Optimus,’ and ‘Sentinel’ make dense canopies that intercept all the light at the top of the canopy. In these varieties, lower leaves do not get enough PAR to have high photosynthetic rates. As you can imagine, we started asking ourselves how to increase light interception in these lower leaves. We have tested reflective mulches and winter pruning as management tools to accomplish this. At this point, we are not ready to recommend either of these practices.

The second way plants use light does not focus on intensity but instead in composition. Light composition refers to the specific wavelengths of light that reach the leaves. Far-red light (wavelengths 700 nm to 800 nm) lets plants know that there are leaves or other objects above them. When plants get excessive far-red light, they grow long internodes, making them tall and spindly. We have all seen those “primocanes” that sprout in the middle of the spring. They grow tall very fast because they develop in an environment that has abundant far-red light (the interior of the canopy). Other wavelengths also affect plant responses. For example, fruit coloration is affected by UV light (315 nm to 400 nm) and flower bud initiation is affected by red light (620 nm to 700 nm). Changing light composition around plants is much more challenging than changing light intensities. 

We still have a lot to learn about how light affects blueberry plants, but our first years of research in this area have given us plenty of ideas to explore in the future.

CREDIT

GERARDO H. NUNEZ, assistant professor, University of Florida