Role of Root Temperature on Shoot Growth of Two Kentucky Bluegrass Cultivars1

Abstract

AbstractShoot growth inhibition and changes in carbohydrate levels occur in cool‐season grasses under midsummer conditions. Cultural practices, such at thatch removal or topdressing, could alter high temperature extremes of the crown and root. Thus, the objective of this study was to determine the role of root temperature on shoot growth rate and water soluble carbohydrate (WSC) levels in plant parts of ‘Merion’ and ‘Nugget’ Kentucky bluegrass (Poa pratensis L.).The turfgrasses were established from seed on coarse sand in the greenhouse. After 12 weeks the pots were transferred to an environmental growth chamber where air temperatures were increased from 22 to 38 C in 4 C increments every 2 weeks. The effects of root zone temperatures controlled at 22 C (CRT) were compared to noncontrolled temperatures (NRT) in which the root zone was allowed to equilibrate with the air temperature. Shoot growth above 4.0 cm. was clipped at the end of each temperature increment from 22 to 34 C. After the 38 C exposure plants were separated into verdure (live leaf tissue), “thatch” (dead leaf tissue), crowns, and roots.Shoot growth of Merion Kentucky bluegrass grown at NRT, was greatest at 26 C and was significantly reduced at 30 and again at 34 C. Shoot carbohydrate levels were lowest at 26 and 30 and highest at 34 C. At 38 C, verdure and crown carbohydrate levels were significantly higher than those of the roots.The growth rate of Nugget (NRT) at 26 C was not as rapid as for Merion. Shoot WSC levels were high at 26, 30, and 34 C compared to Merion Kentucky bluegrass. At 38 C, root WSC levels were significantly higher in roots than verdure or crown. The greater high temperature tolerance of Nugget was documented through a higher verdure‐thatch ratio.Survival of both cultivars was enhanced by controlling the root temperature at 22 C (CRT). Shoot WSC levels at all temperatures were significantly lower at CRT compared to NRT in Nugget. This was true in Merion only at 34 C. Root WSC levels in Merion at CRT were at the high level found for Nugget at both NRT and CRT. Shoot growth cessation at supraoptimal temperatures may result from a high temperature‐labile process in the root. Loss of root carbohydrates appears to increase the temperature lability of the process.