Plants increase in mass, or dry weight, through photosynthesis. Photosynthesis is the process that harvests light energy to drive critical processes resulting in plant growth. Why is increasing photosynthesis important? Increased photosynthesis is associated with increased flowering, increased plant quality and increased post-harvest life. How much, or how fast, a plant can photosynthesize depends on light intensity, carbon dioxide and temperature.
Although much work was done during the 1960s and ’70s, there is little recent research on how changing light intensity, carbon dioxide and/or temperature affect photosynthesis of commonly grown greenhouse crops today. For this reason, we started a research project at the University of Minnesota with support from the Floriculture and Nursery Research Initiative and the American Floral Endowment to provide growers with answers to questions such as: How much supplemental light should I add to maximize photosynthesis? What is the impact on photosynthesis of adding supplemental carbon dioxide? When light levels decrease, should I reduce temperature? When should I close a light reduction curtain so it does not reduce photosynthesis and plant quality?
In this article, we will briefly go over preliminary results that answer these questions: How does increasing light intensity affect photosynthesis of some common greenhouse crops, and do optimal light intensities change in a plant canopy? We will also translate that data into recommendations on how you can use this new information in your facility. We believe growers can use this new information to help 1) increase crop quality, 2) decrease energy input costs and 3) minimize unnecessary plant stress that could decrease flowering.
What We Learned
In all cases, photosynthesis increased as light intensity increased up to some maximum light level. As light intensity increased further, photosynthesis did not increase further (i.e. it is light-saturated).
What differs between species is the light intensity at which photosynthesis saturates. There are plants in which photosynthesis saturates at low light intensities, such as 200 umol m¯² s¯¹ (approximately 1,000 footcandles). In other plants, photosynthesis saturates at intermediate light levels, such as 400 umol m¯² s¯¹ (2,000 footcandles). Lastly, there are plants in which photosynthesis saturates at high light levels (600 um¯² s¯¹, 3,000 footcandles). Examples of approximate light levels when photosynthesis saturates are shown in Table 1.
Light Intensities In Canopies
The maximum light intensities for photosynthesis in Table 1 are for an unshaded, completely exposed leaf. As our crops grow, the canopy closes if plants are not regularly spaced. This means you will have to provide higher light levels at the top of the canopy in order to achieve optimal light levels and maximum photosynthesis in the middle of the crop canopy. How much a canopy reduces light transmission through shading depends on many things, including species, spacing and leaf number.
For instance, we looked at how leaves of different species reduce light to lower leaves. Table 2, online at GreenhouseGrower.com, shows how much photosynthetically active radiation (PAR) different greenhouse crops transmit–half an inch to 1 inch below a fully expanded leaf on a clear day at noon outdoors. Note that light transmission varies from 5 to 12 percent (i.e. some species filter twice as much light as others). Those species that filter more light will require increased light intensity as a canopy closes compared to those that transmit more PAR.
We also measured how light intensity is reduced as you move down through a canopy of two species: coleus and Panicum virgatum (switchgrass). You can see that coleus filters light more than the grass as you move down through the canopy. This means you will need more light at the top of a coleus canopy than at the top of a panicum canopy to achieve 2,000 footcandles at mid- or lower leaves in a canopy.
Do Your Own Tests
Interested in doing similar tests at your operation? Let’s get you started:
1) Buy a light meter that can measure light in micromoles (umol m¯² s¯¹). Micromoles are the units that are associated with photosynthesis. Units commonly used in greenhouse control systems that measure light in joules, watts or footcandles do not accurately measure only light that plants use for photosynthesis. Photosynthetic light is the only light that we should be concerned with for supplemental lighting for photosynthesis in greenhouse crops!
2) Install light meters/sensors that control supplemental lighting and/or automated light reduction curtains at plant level inside the greenhouse. The standard procedure of mounting light meters outdoors above a greenhouse is effectively useless for growing crops indoors.
3) Group crops by how much light they can use (low, medium, high light-saturating crops, see Table 1) and provide supplemental lighting based on this.
4) See how much your light intensity decreases as you go down through your crop canopy. Increase light intensity at the top of the plant as a canopy closes to achieve maximum photosynthesis levels in the middle of the canopy. Again, this can be done automatically if light meters are placed in a canopy indoors in the middle of a canopy!
5) Use of greenhouse coverings (removal, shading, etc.) should be based on light effects on crop photosynthesis (i.e. high light requiring crops may need to be in greenhouses with high light transmission, have coverings replaced more often or shading sprayed on greenhouses later than low light requiring crops). In contrast, low light requiring crops could stay in darker, older greenhouses.