The sun/nitrogen connection

Plants need nitrogen (N) and sunlight to be healthy. Nitrogen allows leaves packed with chlorophyll to absorb the sun’s energy and ultimately turn CO2 into sugar. Nitrogen also stimulates growth and quality of light impacts turf health. The impact of N and sunlight on turf health is obvious, but there is another sun/nitrogen connection affecting turf health that is not always considered.

Turfgrass roots primarily take up nitrate (NO3-) because it is more commonly available in the soil compared with ammonium (NH4+). Even ammonium-based fertilizers quickly are transformed from NH4+ to NO3- by soil microbes. That soluble soil NO3- moves to the turf roots via mass flow, the process of water being drawn toward the roots as the leaves transpire. The NO3- then is taken up by proteins on the turf roots and moved up to the leaves. This is where the sun/nitrogen connection is important. Light energy from the sun is used to reduce or change the NO3- back to NH4+ before it can finally be used to make protein.

This is applicable because sunlight drives transpiration and mass flow of N to the roots and also is required to reduce or transform the NO3- into a form the plant can use for growth. Shade, low clouds and constant rain reduce solar intensity, which can slow turf green up following N fertilization. We saw this at our plots during cloudy and/or rainy weather.

Nitrate fertilizer also can help with too much light. This occurs on dry and sunny days or during stress in cool-season turf. Warm-season turf also can obtain too much light when it is cool and sunny. This results in chilling injury.

During these conditions the plant is absorbing light faster than it can use the energy. Excess energy then escapes and damages the plant through oxidative stress. Many turf products, from fungicides to pigments, are marketed to reduce oxidative stress. Some of these products can positively affect plant health. Products such as Turf Screen can minimize oxidative stress on our bentgrass greens. This led to improved quality and higher levels of chlorophyll in the leaves (Fig. 1). Fertilizers containing nitrate also can help with this stress because some of the excess sun energy can be used to reduce NO3- to NH4+ and finally protein. Nitrogen will help the plants repair damage caused by light stress.

Bill Kreuser, Ph.D., is a turfgrass scientist at the University of Nebraska-Lincoln. You may contact him at wkreuser2@unl.edu for more information.