On the photosynthetic and devlopmental responses of leaves to the spectral composition of light

Key words: action spectrum, artificial solar spectrum, blue light, Cucumis sativus, gas-exchange, light-emitting diodes (LEDs), light interception, light quality, non-photosynthetic pigments, photo-synthetic capacity, photomorphogenesis, photosystem excitation balance, quantum yield, red light.

A wide range of plant properties respond to the spectral composition of irradiance, such as photosynthesis, photomorphogenesis, phototropism and photonastic movements. These responses affect plant productivity, mainly via changes in the photosynthetic rate per unit leaf area, light interception, and irradiance distribution through the canopy. The spectral environment of plants is dependent on location (e.g. latitude), changes over time (e.g. Sun-angle), shading by other leaves, and, in the case of protected cultivation, the use of growth lamps. Therefore, not only the acclimation of developing leaves to light spectrum is important for plant productivity and survival, but also the capability of mature leaves to respond to changes in spectrum. This thesis focuses on the acclimation of photosynthesis per unit leaf area to the growth-light spectrum, the consequences of spectral acclimation for the wavelength dependence of photosynthetic quantum yield, and photomorphogenetic versus leaf photosynthetic acclimation in relation to biomass production. Cucumis sativus is used as a model plant. Additionally, the consequences of the choice and quality of the actinic light used during photosynthesis measurements are explored.
By growing plants under seven different combinations of red and blue light, blue light is shown to have both a qualitative and a quantitative effect on leaf development. Only leaves developed under red alone (0% blue) displayed a dysfunctional photosynthetic operation, which was largely alleviated by only 7% blue. Quantitatively, leaf responses to an increasing blue light percentage resembled responses associated with an increase in irradiance.
Next, the wavelength dependence of the quantum yield for CO2 fixation (α) is analysed in detail. Leaves grown under artificial shadelight, which overexcites photosystem I (PSI), had a higher α at wavelengths overexciting PSI (≥690 nm) and a lower PSI:PSII ratio compared with artificial sunlight and blue light grown leaves. At wavelengths overexciting PSII, α of the sun and blue grown leaves was higher. The photosystem excitation balance is quantitatively shown to determine α at those wavelengths where absorption by carotenoids and non-photosynthetic pigments is insignificant (≥580 nm). The wavelength dependences of the photosystem excitation balance calculated via an in vivo and an in vitro approach were substantially in agreement with each other, and where not, carotenoid absorption and state transitions are likely to play a role.
Not only is the photosynthetic rate per unit leaf area is important for plant productivity, but also photomorphogenesis. We have engineered an artificial solar (AS) spectrum under which plants produced a dry weight that was, respectively, 2.3 and 1.6 times greater than that of plants grown under fluorescent tubes and high pressure sodium light. This striking difference was due to a morphology of the AS-plants that was more efficient in light interception, and not related to photosynthesis per unit leaf area. These results highlight the importance of a spectrum that is more natural than that of usual growth-lamps for research and possibly also for horticultural production.
A technically orientated part of this thesis presents a simple method to quantify the light distribution in leaf chambers, which is shown to be important for the accuracy of photosynthesis measurements by gas-exchange. The match between growth-light and measuring-light spectrum is likewise shown to be important. A mismatch can have significant consequences for the estimate of α in situ, but only minor consequences for the estimate of the light-saturated photosynthetic rate. The relationship between the electron transport rate calculated using chlorophyll fluorescence measurements and the CO2 fixation rate also changed considerably with changes in measuring-light spectrum. The use of erroneous estimates of α as input for crop growth models is shown to have disproportionately large consequences for predictions of plant growth.