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

摘要

Mutants with a depigmentated phenotype and characterized by Mendelian inheritance can be useful to analyze the role of nuclear genes inchloroplast biogenesis and photosynthesis. In the important oil crop sunflower, we have recently demonstrated that under dim light conditionsthe xantha1 (xan1) mutant develops true leaves unable to express and retain PSII activity. In order to investigate if this mutation induces anorgan specific effect on photosynthetic activity, the light energy utilization of cotyledons (7, 15 and 18 days after sowing) was characterizedthrough gas exchange and chlorophyll a fluorescence. The xan1 cotyledons are unable to fix CO2 at normal rates. However, rubisco specificactivity is not different in the wild-type (wt) and the xan1 seedlings, indicating that biochemical reactions of the photosynthetic process werenot altered. The photochemical potential of PSII, measured as the dark-adapted quantum efficiency of PSII (Fv/Fm) was strongly decreasedin xan1 cotyledons and is attributable to a significant increase in F0 values. The light-adapted quantum efficiency (phiPSII) and the efficiencyof excitation capture of PSII (phiexc.), were significantly depressed in xan1 cotyledons as compared to the wt. Under light conditions, the mostevident result is the strong decrease of phiPSII, which is linked to a decrease in excitation energy pressure and also to a strong increase in qNP. Insummary, with analogy to results previously obtained in leaves, the xan1 cotyledons have a reduced number of photosynthetic units and highsensibility of the PSII reaction centres to photodamage even in low light conditions. We conclude that the loss-of-function of the sunflowerXAN1 nuclear gene affects the light energy utilization of chloroplasts in an organ-independent way.