EFFECTS OF OZONE ON YIELD, GROWTH, AND ROOT STARCH CONCENTRATIONS OF 2 ALFALFA (MEDICAGO-SATIVA L) CULTIVARS

Ozone (O-3) is considered to be a major air pollutant that affects the yield of several sensitive crop species. Its concentration may reach phytotoxic levels several times during the growing season in Eastern C anada. This study was initiated to evaluate the O-3 effects on alfalfa , a major crop species. The objective was to compare the yield and gro wth parameters of the main alfalfa cultivar used in Quebec, Apica, to a cultivar more tolerant to O-3, Team. Effects on root starch concentr ations were also examined as this parameter is an important indicator of alfalfa perennity. The results obtained have shown that the forage yield of Apica was more reduced by O-3 during two growing seasons than the yield of Team. For O-3 concentrations of 20 to 40 nl liter(-1), y ield reductions were 14-26% for Apica and 0-20% for Team. Whereas Apic a could be considered more susceptible to O-3 than Team, the latter ha s shown contrasting responses from year to year. This fact suggests th at the mechanisms involved in O-3-tolerance could be modulated by envi ronmental conditions. At low O-3 levels, Apica has shown reduced root growth in terms of dry matter and length. However, contrary to the cur rent hypothesis that O-3 would affect more root than shoot growth, we were unable to show a consistent alteration of the biomass allocation between the two. Ozone seems to reduce globally the growth of the whol e plants. The greater O-3-tolerance of Team could partly be associated to its capacity to maintain more leaves, to delay their senescence, o r to keep a larger leaf:stem ratio under increasing levels of O-3. At the end of the two growing seasons, the amount of starch reserves stor ed below ground was shown to be reduced by the current O-3 levels. Thi s reduction was mainly associated with a decrease in root biomass unde r O-3 stress. This result support the hypothesis that O-3 may accelera te alfalfa decline under field conditions. (C) 1997 Elsevier Science L td.