Chapter 3PHOTOSYNTHESIS OF SUPINA BLUEGRASS (POA SUPINA SCHRAD ) ANDKENTUCKY BLUEGRASS (P. PRATENSIS L.) IN REDUCED LIGHTCONDITIONS AS AFFECTED BY NITROGEN AND TRINEXAPAC-ETHYLINTRODUCTIONTurfgrass perfonllance in reduced light conditions (RLC;<300/0sunlight) is oftenpoor due to insufficient light for photosynthesis and normal turf growth. Turfgrassspecies and cultivars n1a)'vary widely in their tolerance to RLC although all may exhibitreduced tillering. reduced rooting, and an upright spindly growth (Beard, 1973). Supinabluegrass(Poo slipinoSchrad.), a stoloniferous grass native to the sub-alpine regions ofEurope. has been developed in Germany as a turfgrass with purportedly good to excellentshade and traffic tolerance (Berner, 1984; Nonn, 1994; Pietsch, 1989; Skirde, 1971).Prelin1inary research supports the hypothesis that Supina bluegrass is more tolerant ofRLC than Kentucky bluegrass(P.pratensisL.) (Stier and Rogers, 1995) which iscomn10nly used in the United States but has poor shade tolerance (Beard, 1973). Then1echanisn1(s) for the apparent shade tolerance of Supina bluegrass is/are unknown.In addition to the use of shade-tolerant turfgrasses proper management techniques arealso in1portant for turf performance in RLC. Previous research has indicated the potentialfor plant gro\vth regulators (PGRs) which inhibit gibberellic acid (GA) biosynthesis toimprove turf quality in reduced light conditions (Rogers et aI., 1996; Stier and Rogers,1011021995). Turf treated with GA-inhibitors in RLC was more uniform with darker color andincreased density compared to untreated turf. While the GA-inhibitors effectivelysuppressed shoot elongation, other mechanisms by which the GA-inhibitors improvedturf quality were unknown. Possibilities range from enhanced photosynthetic rates(Gausnlan et aI., 1991), increased carbohydrate production or partitioning (Hanson andBranham. 1987; Wang et al. 1985), increased chlorophyll levels (Wang et a11985,Archbold and Houtz, 1988), increased protein/enzyme levels and/or activity (Wang et ai.1985), altered hormonal levels affecting foliar production (Gausman et aI., 1991), to geneexpression (Gausman et aI., 1991). Conversely, Archbold and Houtz (1988) reportedflurprinlidol and paclobutrazol decreased photosynthetic rates and Rubisco activities instra\vberry plants. Dejong and Doyle (1984) found paclobutrazol reduced shoot growthof nectarine trees but did not affect photosynthesis. Mefluidide, generally considered amitotic inhibitor which also may inhibit GA-biosynthesis (Wilkenson, 1982), consistentlyreduced photosynthetic rates of 'Baron' Kentucky bluegrass while amidochloroccasionally enhanced photosynthesis (Spokas and Cooper, 1991).In the early 1990' s a new GA-inhibitior, trinexapac-ethyl (TE), was labeled for use onturfgrasses, primarily to decrease mowing requirements by suppressing shoot growth(Vitolo et aI., 1990). The potential side effects ofTE on plant physiology are relativelyunknown due to its recent release but may be different than other turf GA-inhibitors. TEapparently blocks3-poxidation of the biologically inactive GA20 to form the biologicallyactive GAl as opposed to flurprimidol and paclobutrazol which inhibit ent-kaurene103oxidation oxidative steps earlier in the biosynthetic pathway (Coolbaugh et aI., 1982;Rademacher, 1991).In normal (full sun) conditions GA-inhibitor effects on turfgrass can vary withnitrogen (N) rate and turf species or cultivars. Watschke (1981) found differences inresponses of two Kentucky bluegrass culitivars ('Merion' and 'Pennstar') topac1obutrazol and flurprimidol. Other studies showed high N rates reduced the effects offlurprimidol on common bermudagrass [Cynodon dactylon (L.) Pers.] (Devitt and Morris,1988) but not on 'Tifway' hybrid bermudagrass [Cynodon transvaalensis Burtt-Davy xc.dactylol1 (L.) Pers.] (Johnson, 1988). Johnson (1994) corroborated the differences inresponse to trinexapac-ethyl between common bermudagrass and 'Tifway' hybridbermudagrass. In RLC of approximately 5-6 mol photosynthetically active radiation(PAR) day"I,medium to high N rates (48 and 96 kg ha-Iat four to six week intervals)resulted in significantly better quality Kentucky bluegrass compared to low N rates (24kg ha-Iat four to six week intervals) when flurprimidol was applied, although low andnledium N rates provided superior turf in the absence of flurprimidol (Chapter 1).Due to the demand for improved turfgrasses and management schemes for turf inRLC, studies were initiated to examine the effects ofN rate, trinexapac-ethyl, and specieson turf photosynthesis in RLC. Two hypotheses were tested: 1) Supina bluegrass wasmore tolerant of RLC compared to Kentucky bluegrass due to a greater carbon exchangerate (CER), i.e., enhanced photosynthetic rate, and 2) Trinexapac-ethyl improvedturfgrass quality in RLC by enhancing CER. The objectives of this research were todetermine if differences in CER existed between Supina bluegrass and Kentucky104bluegrass and to determine the effects oftrinexapac-ethyl on CER of the two species. Asecond set of objectives were to determine the influence of nitrogen rate and trinexapac-ethyl on the CER of Supina bluegrass.MATERIALS AND METHODSPlot establishment and testingExperinlent I: Species x PGR studyPortable plots \vere established outside in full sun conditions. Wooden boxes (1.2 x].2 x 0.] 5 m depth) were filled with a sand:peat mixture (80:20 v/v) (Table 78,Appendix). The pH was 7.8 with initial P and K levels of 85 kg ha-land 90 kg ha-l,respectively. Sixteen holes (0.6 cm diam) were drilled on approximately 23 cm spacingsin the bottom of each box to provide drainage. Starter fertilizer (13-25-12) was applied tothe soil and supplied 76 kg N ha-l,64 kg P ha-I,and 58 kg K ha-l.Ten plots each weresodded in Sept. 1995 with Supina bluegrass 'Supra' or Kentucky bluegrass 'Blacksburg'.The sod had been grown in a composted wood mulch on polyethylene sheeting during thesmnmer of ]995 (Cairol and Chevallier, 1981). Plots were mowed two to three times\veekly to 3 cm height and irrigated as necessary to prevent moisture stress. Plots werefertilized bimonthly with 48 kg ha-lN, 3 kg ha-lP, and 40 kg ha-lK. To prepare plots fortesting in reduced light conditions (RLC), plots were fertilized with 48 kg N ha-l,41 kg Pha-I,and 38 kg K ha-lon 26 Aug. 1996. Plots were fertilized thereafter on a biweeklybasis with 37 kg N ha-l,3 kg P ha-l,and 30 kg K ha-l.105On 18 Sept. 1996 trinexapac-ethyl (0.19 kg ha-1)was applied