Photoassimilate allocation and photosynthetic and biochemical characteristics of two alfalfa (Medicago sativa) cultivars of different ozone sensitivities

Abstract

Ozone (O3) is a major air pollutant that has been reported to affect growth of alfalfa (Medicago sativa L.). In the present study, an O3-sensitive cultivar, 'Apica', and an O3-tolerant cultivar, 'Team', of alfalfa were exposed to multiple levels of ambient O3 concentrations (i.e., 0x, 1x, 1.5x, and 3x) using open-top chambers. The objectives were to characterize O3 sensitivities at the leaf level and to determine whether stomatal or mesophyll conductance could account for the differences in O3 sensitivity. Differences in photoassimilate allocation were also determined using 14CO2. Effects of O3 exposures on leaf characteristics such as leaf area and dry matter; leaf concentrations of chlorophyll, starch, proteins, amino acids, and nutrients; and stomatal density were evaluated. Gas exchange measurements were also performed. Results showed that O3 exposures produced a greater decline in leaf biomass, chlorophyll concentration, and net assimilation rate for 'Apica' compared with 'Team'. For O3 concentrations of 20-40 nL ·L-1, a 7-15% reduction in net assimilation rate was observed for 'Apica' only. With an increase in O3 concentrations, stomatal limitation did not increase correspondingly, and a strong correlation between net assimilation rate, stomatal conductance, and carboxylation efficiency was observed, suggesting a physiological coordination of the O3 stress response at the leaf level. Possibly the decrease in assimilation rate induced by O3 could be associated with the O3 effects at the mesophyll level. For both cultivars, O3 increased the CO2 compensation point of the leaves, and some effects were also noted in terms of leaf mineral concentrations. O3 did not alter photoassimilate allocation between shoots and roots but stimulated their retention in leaves compared with stems. These results suggest that O3 has a major impact on the carbon metabolism of leaves, which could explain differences in O3 sensitivities.Key words: gas exchange, ozone tolerance, alfalfa cultivars, stomata, photoassimilate allocation.