Photosynthesis rates in situ may significantly deviate from measured rates of photosynthesis due to the measuring-light spectrum used

Despite the progress that has been made in biophysical methods, such as chlorophyll fluorescence, gas exchange techniques are still the gold standard for measuring photosynthesis. Until recently the most commonly used light sources for photosynthesis measurements were broad-band spectrum quartz-halogen lamps. More recently, red LEDs or mixed red and blue LEDs have also been used. LEDs emit a relatively narrow-band spectrum. Although it is well known that the spectral quality of irradiance has an influence on photosynthesis, the consequences of a difference between the light spectrum during plant growth and during measuring photosynthesis is generally not taken into account. However, accurate measurements of photosynthesis are essential for research comparing different light-spectrum conditions or in models estimating crop growth based on photosynthesis measurements. Cucumber was grown under different combinations of red and blue irradiance. We measured photosynthesis/irradiance responses using both the same and different red/blue combinations as during growth. Actual differences in assimilation due to acclimation to the different spectra during growth, measured with the same spectrum as during growth, were up to 26%. The differences measured on a single leaf using different measuring-light spectra were up to 30%. Measurements of light absorptance and ¿PSII for the different measuring-light qualities revealed that the commonly used expression for calculation of electron transport rate (ETR) is not valid if photosynthesis parameters are measured under a different light spectrum to that used during growth. (ETR=¿PSII*PPF*a*PSII/(PSI + PSII)where a=absorbed light fraction and PSII/(PSI + PSII)= the absorptance by PSII divided by the absorptance of PSI + PSII).