Abstract
A suitable light quantity and quality is essential for optimal photosynthetic metabolism. Using combinations of three lamp types, the impact of the quality of artificial light conditions on the photosynthetic apparatus of leaves developed in growth chambers was analysed. The VIALOX-Planta lamps are quite poor outside the green to orange (520 - 620 nm) wavelength range, while the HQI-BT lamps present a more uniform spectral intensity between 425 and 650 nm (blue to red). The halogen lamps are particularly rich in the red and far red range of the electromagnetic spectra. The lamps also differ in the red : far red ratio, which were 3.07 (VIALOX), 2.06 (HQI-BT) and 1.12 (halogen). Clear positive effects were detected in most of the photosynthetic parameters in relation to light quality, both at stomatal and mesophyll levels. Despite some species-dependent sensitivity to blue and red/far red wavelengths, observed among the studied parameters, the best photosynthetic performances of the test plants (Packyrhizus ahipa and Piatã, a hybrid of Coffea dewevrei × Coffea arabica) were obtained almost always with the reinforcement of blue (HQI-BT lamps), red and far red (halogen lamps) wavelengths and with a red : far red ratio closer to that observed in nature. This suggests the involvement of more than one photoreceptor family in photosynthetic performance. Under such light conditions, increases in net photosynthesis and stomatal conductance were observed and, despite the moderate effects on photosynthetic capacity, strong effects were observed in the capture and transfer of light energy in the antennae pigments, photochemical efficiency of photosystem II and electron transport. This was related to the striking quantitative and qualitative impacts observed on total chlorophylls and carotenoids, which reached, in some cases, increases of 100 and 200 %, respectively. Among carotenoids, increases as high as 9-fold for α-carotene were observed (P. ahipa), with chlorophyll (a/b), total (chlorophyll/carotenoid) and carotene (α/β) ratios also strongly affected. This would have affected the structure and stability of photosynthetic membranes which, in turn, affected photosynthetic-related processes (e.g., antennae pigments, photosystem II and electron transport efficiencies). This was particularly clear in the HQI + halogen treatment. The results unequivocally show that light quality could remain a clear limiting factor for leaf/plant development under artificial light conditions, which could be overcome using more than one lamp type, with complementary emission spectra.
Abbreviations and Symbols
Pn: net photosynthetic rate at ambient CO2
Amax: photosynthetic capacity at CO2 and light saturating conditions
B: blue light
Chl: chlorophyll
DEPS: de-epoxidation state
F
o: minimal fluorescence of antennae in dark-adapted leaves
F
m: maximal fluorescence in dark-adapted leaves
FR: far red light
F
v/F
m and F
v′/F
m′: photochemical efficiency of PSII in dark-adapted leaves and under photosynthetic steady state conditions, respectively
gs: stomatal conductance to water vapour
LHCII: light harvesting complexes of PSII
PPFD: photosynthetic photon flux density
PSII: photosystem II
QA: quinone primary electron acceptor from PSII
qNP: non-photochemical quenching
qP: photochemical quenching
R: red light
rubisco: ribulose-1,5-bisphosphate carboxylase/oxygenase
φe: estimation of the quantum yield of photosynthetic electron transport
Key words
Artificial light - chlorophyll a fluorescence -
Coffea sp. - gas exchange - light quality -
Packyrhizus ahipa
- photosynthesis - pigments
