There are many reasons why sports turfgrasses in New England decline in the summer, such as too much use, disease, insects and too much or too little water.  There is also a physiological reason called photorespiration.  Understanding photorespiration can help sports turf managers have a healthier plant in the summer that can better withstand other stresses. 

During photosynthesis, plants take in carbon dioxide (CO2), break it apart and use the carbon to produce organic compounds.  These compounds (carbohydrates) are used mostly as fuel for the plant’s cells, as cellulose that is used to make cell walls or it is stored in stems for future energy needs.

There is one major problem with this process in cool season grasses.  An enzyme called rubisco does the work of breaking apart the carbon dioxide.  This enzyme has a site that the carbon dioxide binds to, but unfortunately the site can not distinguish between CO2 and oxygen (O2).  When the rubisco works on the O2 it uses energy and carbon and produces nothing for the plant, essentially undoing photosynthesis.  This is called photorespiration.

Photorespiration occurs during all growing seasons in cool season grasses, but is more important during hot, dry weather.  In these environmental conditions, the grass plant can sometimes only manufacture half the carbohydrates it could potentially produce.  The hot temperatures make it so there is a depletion of carbon dioxide inside the leaves, so more photorespiration occurs. 

So in the summer, the grass plants keep on growing in the warm environment, but the plant can not produce enough carbohydrates to keep up with the amount being used.  The leaves making the limited carbohydrates, keep the carbohydrates for themselves and other parts of the plant suffer, especially the roots. 

Carbohydrate reserves are essential when the plant is using more than it can produce.  These reserves are made at times when the plant has a high rate of photosynthesis and can create more carbohydrates than it needs to grow, such as in the spring and fall.  If the plant’s reserves are depleted, the roots will begin to shrink, picking up less nutrients and water.  The plant will also not be able to grow rapidly and recover from wear or pest stresses.

Photorespiration is going to occur in your Kentucky bluegrass, ryegrass and other cool season grasses no matter what you do.  Your best defense against it is to have an ample carbohydrate reserve in your plants.  This can be done by using cultural practices that promote a healthy plant without excessive growth.    A way to do this is to fertilize adequately in spring, late summer and late fall, with very small amounts in the summer.

Following is a sample fertility program for a native soil, irrigated, high use field in New England, which requires 3-5 pounds of nitrogen per thousand square feet per year.  Notice that it does not include any mid summer applications of nitrogen that might encourage excessive growth and use of limited carbohydrates. 

Date                 lbs. nitrogen/1000 sq. ft.
May 1                          .75
June 1                          .75
August 20                     1.0
November 5                 1.0

There are many factors involved in developing a fertility program for your field and providing a specific program for everyone is impractical, but this example illustrates a program that assists the turfgrass in dealing with photorespiration by fertilizing wisely.

References
Turgeon, A.J. 1991, Turfgrass Management
Hopkins, William G. 1995, Introduction to Plant Physiology