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Light spectrum has a significant impact on a wide variety of plant processes including photosynthesis and photomorphogenesis which in turn impact plant productivity. The choice of supplementary light spectrum in greenhouses is therefore not trivial, and this is especially so given the diverse spectral possibilities afforded by increasingly popular LED supplementary lighting. The main focus of this thesis is on spectral impacts of light-limited quantum yield of CO2 fixation in leaves of tomato, a common greenhouse-grown crop. Some attention is also given to the photomorphogenetic impacts of spectrum as well as the impact of temperature on ΦCO2.
Enhancement of photosynthesis is examined using diverse actinic spectra. Leaves were grown in either an artificial daylight or an artificial shade spectrum. Gas exchange was measured using each of 17 narrowband irradiances, a nearly identical irradiance to growth irradiance, and a combination of the narrowband irradiances and growth irradiance in a 1:1 ratio on an absorbed PAR basis. Enhancement was calculated to be 23% in the shade spectrum whereas enhancement did not occur in the sun spectrum. In the spectral combination experiments, maximum enhancement of 76% occurred when 720 nm was combined with the shade spectrum; enhancement across all other wavelengths added to the shade spectrum ranged between 17 and 27%. No, or negligible, enhancement occurred in the daylight spectrum or when wavelengths of 660 nm or shorter were combined with the daylight spectrum. However, enhancement of 4.7, 7.6 and 43% was observed when 680, 700 and 720 nm were combined with the daylight spectrum, respectively. Taken together, a commonality of instances where enhancement was found to occur was a spectrum rich in far-red light (>700 nm) which highlights the inadequacy of the PAR definition.
Another experiment examined how leaves acclimate to spectral fluctuations in the short-term and what impact this may have on ΦCO2. State transitions were induced by subjecting leaves grown in an artificial daylight irradiance or artificial shade irradiance to alternating irradiances which are extreme in terms of their PSII and PSI over-excitation. A relationship between ΦCO2 and the occurrence of state transitions is revealed in a higher plant for the first time. The accompanying increase in ΦCO2 from commencement to completion of a state transition was found to be between 10-13%.
A third experiment examines the combined spectral impacts of blue light on photosynthesis, plant morphology, and whole-plant light interception. The wide variety of leaf-level and whole-plant impacts regulated by blue light makes it an interesting candidate to explore these relationships, further stimulated by the absence of study of blue light doses in a more natural daylight background. Blue light inhibition of etiolation was preserved under a simulated daylight background and this inhibition increased with blue light fraction while whole-plant light absorption decreased. Photosynthetic effects of blue light fraction were unremarkable, with ΦCO2 and maximum photosynthetic capacity (Amax) negligibly affected, the latter of which may already have been saturated by the blue fraction of the daylight spectrum itself. With limited impact on ΦCO2, the primary impact of blue light fraction on plant biomass appears to be through whole-plant light absorption.
A final experiment examines the impact of temperature on ΦCO2, ΦPSII, ΦPSI, and the electrochromic shift at 2% and 21% O2 and between 15 and 35 °C. ΦCO2 was predictably greater at 2% O2 at all temperatures due to the suppression of photorespiration. ΦCO2 showed a temperature optimum at 18 °C in 2% and 21% O2 but the extent of the temperature dependency at 2% O2, being 29% lower at 35 °C, was unexpected given the practical absence of photorespiration. Whereas ΦPSII was mostly independent of temperature at 2% O2, it showed a parabolic response in 21% O2 with a temperature optimum at 25 °C. Surprisingly, ΦPSII and qP were lower in 2% O2 than 21% O2, which may be the result of cyclic electron transport to accommodate for shortfalls in ATP production.
|Qualification||Doctor of Philosophy|
|Award date||31 May 2022|
|Place of Publication||Wageningen|
|Publication status||Published - 2022|