Fig. 1. The quantity of photosynthetically active radiation (PAR) is measured instantaneously as micromoles per square meter per second (µmol·m-2·s-1; top); cumulative PAR, the daily light integral, is measured using moles per square meter per day (mol·m-2·d-1; bottom).
Graphic: Christopher J. Currey

Light is one of the most important factors in crop production. However, light is a multi-faceted parameter. There are three properties of light that greenhouse producers should consider: quality, duration, and quantity. The quantity of light impacts crop quality and yield; higher light tends to improve the growth and quality of containerized crops and the yield of food crops, whereas low light results in poorer quality crops and lower yields. One challenge growers face is how best to measure the quantity of light in their greenhouse.

Light intensity is commonly measured in the greenhouse in foot candles. While this is better than no measurement, a better way to quantify light for growing plants is to measure photosynthetically active radiation (PAR), light between 400 and 700 which is most useful for driving photosynthesis. When measuring PAR, quantities are expressed as the micromoles per square meter per second (µmol·m-2·s-1; Fig. 1). Though light does have wave-like properties, as evident when measuring light quality, it also has particle-like properties and can be quantified using the mole, which is 6.022 × 1023 individual particles. To adjust for the number of photons commonly measured in one second, we use the micromole, with one micromole equal to 0.000001 moles.

The most fundamental relationship between light intensity and plant growth is its effect on photosynthesis; as light intensity increases so does photosynthesis, but up to a point. Photosynthesis doesn’t increase linearly as light intensity increases. Photosynthesis initially increases in a linear fashion as light intensity increases, then the increase in photosynthesis to increasing light starts to diminish until it plateaus off. The light intensity at which photosynthesis no longer increases in response to increasing light and plateaus is called the light saturation point.

It is common to see recommendations for managing light for greenhouse crop production provided in instantaneous units. But consider light in the greenhouse throughout the day. When the sun first rises, the intensity is low and then it increases throughout the course of the day before falling again in the afternoon and evening. Clouds will come and go throughout the day, changing the intensity. Light is very dynamic. As a result, we should consider measuring photosynthetic light cumulatively over the course of the day. In a way, it is like rain. When it is raining, you are not concerned about the number of rain drops falling per second; instead, you are most concerned with the total amount of rain that fell during the event. We can measure light like this, and the daily light integral (DLI) reflects the total amount of PAR over the course of the day and is expressed as moles per square meter per day (mol·m-2·d-1). You will notice that this is similar to the unit used for measuring instantaneous light, except that the “micromole” has been replaced by “mole” since we are measuring many more photons, and “per second” is replaced with “day” since we are measuring light over the entire course of the day.

Which is more useful for managing light in the greenhouse — instantaneous or cumulative light measurements? There is a place for both of these in managing our environments. Instantaneous light intensity is useful for making daily light management decisions. For example, if you use supplemental lights in your greenhouse, the lights may turn on at 6 a.m. and turn off at 10 p.m. But what about the middle of the day? It is unlikely that you need to use supplemental light from, say, 11 a.m. to 2 p.m. because that is the brightest part of the day. Between ambient and supplemental light, you may be above the light saturation point and just wasting electricity with your lights. However, since every day is not bright and sunny, you don’t want to program your lights to automatically shut off between 11 a.m. and 2 p.m. Instead, you can program your environmental control to turn on lights at 6 a.m. and turn off lights when the ambient light reaches a certain ambient intensity. Then, as the sun starts to go down in the afternoon, you can have your lights come back on when the light ambient intensity drops below a certain point before turning off at 10 p.m. Similarly, if you have retractable shade cloth you can use instantaneous measurements of light intensity to determine when a shade should be pulled over a crop as the light gets brighter throughout the day, as well as when to pull it back when light intensity drops in the afternoon.

The DLI is a useful measurement for making bigger-picture light management decisions. First, it is a good guideline for how much light a crop should receive. For example, 6 to 10 mol·m-2·d-1 is the recommended DLI for producing high-quality quality African violets, while 10 to 20 mol·m-2·d-1 is required to grow high-quality poinsettias. You can use also the ambient DLI to determine when you need to start using supplemental lighting as the summer turns to fall and the natural DLI is no longer sufficient for your crops, or when to stop using supplemental lighting during the spring because ambient light is providing enough light to meet your crops’ needs. Similarly, if you are using permanent shade such as white wash or black saran on your glazing material, the DLI can indicate when it is time to apply or remove the shade.

Light is an important factor in producing any ornamental or food crop and proper management will increase quality and yield. Photosynthetic light can be quantified differently, both instantaneously and cumulatively, and both have their place. Instantaneous measurements are useful for making daily light management decisions, whereas cumulative measurements are useful for making seasonal light management decisions.