MODIFIED GIBBERELLINS RETARD GROWTH OF COOL-SEASON TURFGRASSES

Several modified gibberellins which inhibit shoot elongation have been assessed as turfgrass growth retardants. Dichloro-methano 16,17-dihyd ro GA(5) (DMDGA(5)) effectively inhibited growth of the cool season gr asses, Festuca arundinacea Schreb., Poa pratensis L., and Lolium peren ne L. Turf growth (weekly cutting dry weight) was reduced by up to 70% in three separate field trials over three seasons. This inhibition la sted up to 5 wk before recovery to control growth rates and, often, th ere was then a transient enhancement of growth. DMDGA, inhibited turf growth to the same extent as did the commercial turfgrass retardant, T rinexapac-ethyl (commercially formulated as Prime, Novartis, Inc., Bas el, Switzerland). Daily water use was also 25 to 30% less over at leas t 4 wk in studies with mini-swards growing in controlled environment c onditions. With isolated plants of two cultivars of P. pratensis (cvs Holt and Bronco) grown in controlled conditions leaf elongation rates were more than three-fold greater in long than short daylengths. Both Trinexapac-ethyl and DMDGA, blocked most of this growth increase as di d a related derivative, 16,17-dihydro GA(5). Since these compounds inh ibit gibberellin biosynthesis it appears that the extra growth in long days arises from an increase in gibberellin content.