PHOTOASSIMILATE ALLOCATION AND PHOTOSYNTHETIC AND BIOCHEMICAL CHARACTERISTICS OF 2 ALFALFA (MEDICAGO-SATIVA) CULTIVARS OF DIFFERENT OZONE SENSITIVITIES

Ozone (O-3) is a major air pollutant that has been reported to affect growth of alfalfa (Medicago sativa L.). In the present study, an O-3-s ensitive cultivar, 'Apica', and an O-3-tolerant cultivar, 'Team', of a lfalfa were exposed to multiple levels of ambient O-3 concentrations ( i.e., 0x, 1x, 1.5x, and 3x) using open-top chambers. The objectives we re to characterize O-3 sensitivities at the leaf level and to determin e whether stomatal or mesophyll conductance could account for the diff erences in O-3 sensitivity. Differences in photoassimilate allocation were also determined using (CO2)-C-14,. Effects of O-3 exposures on le af characteristics such as leaf area and dry matter; leaf concentratio ns of chlorophyll, starch, proteins, amino acids, and nutrients; and s tomatal density were evaluated. Gas exchange measurements were also pe rformed. Results showed that O-3 exposures produced a greater decline in leaf biomass, chlorophyll concentration, and net assimilation rate for 'Apica' compared with 'Team'. For O-3 concentrations of 20-40 nL L -1, a 7-15% reduction in net assimilation rate was observed for 'Apica ' only. With an increase in O-3 concentrations, stomatal limitation di d not increase correspondingly, and a strong correlation between net a ssimilation rate, stomatal conductance, and carboxylation efficiency w as observed, suggesting a physiological coordination of the O-3 stress response at the leaf level. Possibly the decrease in assimilation rat e induced by O-3 could be associated with the O-3 effects at the mesop hyll level. For both cultivars, O-3 increased the CO2 compensation poi nt of the leaves, and some effects were also noted in terms of leaf mi neral concentrations. O-3 did not alter photoassimilate allocation bet ween shoots and roots but stimulated their retention in leaves compare d with stems. These results suggest that O-3 has a major impact on the carbon metabolism of leaves, which could explain differences in O-3 s ensitivities.