SUPRAOPTIMAL TEMPERATURE STRESS AND NITROGEN NUTRITION IN KENTUCKY BLUEGRASS (POA PRATENSIS L.)

Supraoptimal summer soil temperatures reduce the root activity of cool season turf. This condition is aggravated by the high nitrogen rates often applied to well maintained turf. To evaluate management strategies or select cultivars to reduce supraoptimal temperature stress, a quantitative measure of root growth status is required. Root function is comprised of biochemical processes to utilize shoot derived sucrose and soil derived nitrogen to synthesize new root material. Therefore, root content of primary carbon and nitrogen metabolites should be a quantitative indicator of root function.; Methods have been described to measure expediently carbohydrates in turf tissue and as part of this study a method to quantify ammonium, glutamine, and asparagine in turf root tissue was developed. The method is easy to use for the many analyses required in an agronomic situation, is specific, and is free from significant interference by substances in a root extract.; Root growth and nitrogen assimilation compete for the same carbohydrate resource. Since nitrate uptake is essentially equivalent to nitrogen assimilation, a measure of nitrate uptake should also be a measure of root growth. This idea was investigated in a greenhouse hydroponic turf culturing system with controlled root temperatures of 20 and 28 C and two nitrate levels of 2 and 10 mM (air temperatures varied with the season). Clipping yields, root mass and tensile strength, and solution nitrate were determined in spring and summer trials. During the summer trial root mass and tensile strength, clipping yield, and nitrate uptake were decreased by supraoptimal temperatures especially at 10 mM nitrate. The spring trial was similar but no effect of temperature on the two 2 mM nitrate treatments were detected. In the spring trial, root content of reducing sugars, sucrose, fructans, ammonium, glutamine, asparagine, and activity of radiolabeled water soluble compounds from a shoot applied ('14)CO2 pulse were also determined and found not to vary significantly with temperature.; Two experiments were conducted with sand cultured turf given a range of nitrate fertility from 2 to 10 mM. One experiment was run in a growth chamber under optimum conditions and another in the greenhouse in spring and summer. Clipping weight response to nitrogen was similar in the growth chamber and spring greenhouse conditions. As temperature became supraoptimal in the greenhouse, the optimum nitrate level occurred at a lower level and clipping yields were reduced at moderate and high nitrate levels. Root metabolites were measured in the greenhouse turf on 9 April, 18 June, and 28 June after 10 days of high temperatures. Supraoptimal temperature stress increased asparagine, glutamine, and reducing sugar content at high nitrate levels, increased sucrose content at moderate nitrate levels and decreased it at low nitrate levels, and decreased glutamine at moderate nitrate levels. Temperature had little effect on fructan and ammonium content.; We conclude that measuring metabolite content in turf roots can be a practical indicator of root status and that measurement of nitrate uptake in a hydroponic system can be used for controlled studies of root growth and metabolite content.