RELATIVE GROWTH RATES ON WARM-SEASON, NATIVE AND INTRODUCED PERENNIAL GRASS SPECIES IN RANGELANDS OF THE SOUTH OF THE PHYTOGEOGRAPHICAL PROVINCE OF THE MONTE.

TORRES, Y.A., C.A. BUSSO, O.A. MONTENEGRO, H.D. GIORGETTI, G.D. RODRíGUEZ, T. MONTANI AND A. MAIDANA

Tafí del Valle, Tucumán

Jornada; XXIV Jornadas Científicas de la Asociación de Biología de Tucumán; 2007

Asociación de Biología de Tucumán

Knowledge of relative growth rates (RGR) is important for determining defoliation tolerance, which is given for a rapid photosynthetic surface area reestablishment after a defoliation event. RGR for several parameters were compared for the native species Pappophorum subbulbosum and 2 introduced genotypes from the U.S.A.: Leymus cinereus cultivars ‘Magnar’ and ‘Trailhead’. This study was conducted between October 2006 and February 2007, in the Chacra Experimental de Patagones, south of Buenos Aires Province. Established plants of the same age were used. Half of these plants were defoliated to 5 cm stubble height at the end of November and mid-December 2006. Apical meristems were not removed by the defoliation events. The other half of the plants remained undefoliated (controls). Every 21 days, tiller height, green blade + sheath length, and total (green + dead) blade + sheath length were determined. These data were used to calculate RGR:=  (ln Xt+1 – ln Xt)/Ät, where X is the measured variable, t+1 represents the sampling date immediately after t, and Ät is the time interval between consecutive measurements. Data were analyzed using two-way ANOVA and LSD when F tests were significant (p<0.05). During November 2006, undefoliated tillers of all 3 genotypes  showed greater (p<0.05) RGR for height than defoliated ones. There were no significant differences (p>0.05) neither among genotypes nor between treatments in the remaining sampling dates. During January 2007, defoliated and undefoliated tillers of P. subbulbosum showed greater (p<0.05) RGR for green blade + sheath length than those in the 2 cultivars of L. cinereus. From the beginning of November to mid-December 2006, green blade + sheath lengths were greater (p<0.05) on undefoliated than on defoliated tillers in all 3 genotypes. No differences (p>0.05) were found between treatments in the remaining sampling dates. From the beginning of November to mid-December 2006, RGR for total blade + sheath length were greater (p<0.05) on undefoliated than defoliated tillers in all 3 genotypes; values for this variable were similar (p>0.05) between treatments in the remaining sampling dates. Between mid-December 2006 and the first week of January 2007, RGR for total blade + sheath length were greater (p<0.05)  on defoliated and undefoliated tillers of L. cinereus cv. ‘Trailhead’ than on those of P. subbulbosum . The reverse was true for the next period (first week-end of January 2007), when defoliated and undefoliated tillers of P. subbulbosum showed greater (p<0.05)  RGR for total blade + sheath length than those in L. cinereus cv. ‘Trailhead’. In general, there were few significant differences between treatments in the RGR for the study variables. This would indicate that defoliated tillers reestablished the photosynthetic surface area after defoliation rapidly enough as to compensate growth of undefoliated controls.