THE MEASUREMENT OF CHLOROPHYLL FLUORESCENCE AS A TOOL TO EVALUATE THE PHOTOSYNTHETIC PERFORMANCE OF GRAPEVINE LEAVES

J_t = [4ΔF/F_m'-b)/K] ^* PFD

where PFD is the photosynthetic irradiance, the constant 4 is the number of electrons needed per CO₂ molecule fixed. The letter b represents the y-axis intercept and K the slope of the regression line that fits the relationship of ΔF/F_m' and the quantum yield of carbon fixation (CO₂); K depends on leaf reflectance, chlorophyll content and the spectral composition of the incident light.

Even though the relationship between ΔF/F_m' and CO₂ becomes curvilinear under field (photorespiratory) conditions it is possible to estimate J_t from it. J_t may be considered as uniquely devoted to carboxylation and oxygenation cycles driven by Rubisco and therefore, in C₃ plants, comprises the dissipation energy due to photosynthesis and photorespiration. Using a portable pulse modulated fluorometer J_t can be rapidly determined in the field.

Over the course of several years, involving a number of diverse field and glasshouse experiments, J_t was determined from chlorophyll fluorescence measurements conducted under non-photorespiratory and photorespiratory conditions. Concurrently with chlorophyll fluorescence, the rate of leaf net photosynthesis (P_n) was measured using gas exchange. It was thus possible to assess the correlation between the two methods in determining photosynthetic performance of single leaves.

The field and pot experiments that were evaluated included treatments relating to vine training, leaf age, soil water and nitrogen supply. Results showed that, regardless of the diversity in experimental treatment factors, J_t was always strongly correlated with P_n. This suggests that changes in photorespiration, as a result of the wide range of experimental treatments, did not alter the relationship between J_t and P_n. The suitability of J_t as a rapid probe of photosynthetic leaf performance under drought, heat and light stress and implications for its use in experimental viticulture is discussed.

Download Adobe Acrobat Reader (free software to read PDF files)