Gas exchange analysis and chlorophyll a fluorescence in cotyledons of the xan1 sunflower mutant with defects in light energy utilization

Mutants with a depigmentated phenotype and characterized by Mendelian inheritance can be useful to analyze the role of nuclear genes in chloroplast biogenesis and photosynthesis. In the important oil crop sunflower, we have recently demonstrated that under dim light conditions the xantha1 (xan1) mutant develops true leaves unable to express and retain PSII activity. In order to investigate if this mutation induces an organ specific effect on photosynthetic activity, the light energy utilization of cotyledons (7, 15 and 18 days after sowing) was characterized through gas exchange and chlorophyll a fluorescence. The xan1 cotyledons are unable to fix CO2 at normal rates. However, rubisco specific activity is not different in the wild-type (wt) and the xan1 seedlings, indicating that biochemical reactions of the photosynthetic process were not altered. The photochemical potential of PSII, measured as the dark-adapted quantum efficiency of PSII (Fv/Fm) was strongly decreased in xan1 cotyledons and is attributable to a significant increase in F0 values. The light-adapted quantum efficiency (ΦPSII) and the efficiency of excitation capture of PSII (Φexc.), were significantly depressed in xan1 cotyledons as compared to the wt. Under light conditions, the most evident result is the strong decrease of ΦPSII, which is linked to a decrease in excitation energy pressure and also to a strong increase in qNP. In summary, with analogy to results previously obtained in leaves, the xan1 cotyledons have a reduced number of photosynthetic units and high sensibility of the PSII reaction centres to photodamage even in low light conditions. We conclude that the loss-of-function of the sunflower XAN1 nuclear gene affects the light energy utilization of chloroplasts in an organ-independent way.