JXB Advance Access originally published online on April 23, 2009
Journal of Experimental Botany 2009 60(8):2283-2290; doi:10.1093/jxb/erp112
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This article appears in the following Journal of Experimental Botany issue: Special Issue: Mesophyll conductance to CO2: mechanisms, modelling, and ecological implications [View the issue table of contents]
Methodological Aspects of gm and Implications for Modelling |
The impact of blue light on leaf mesophyll conductance
1Consiglio Nazionale delle Ricerche–Istituto di Biologia Agroambientale e Forestale, Via Salaria Km 29,300, 00015 Monterotondo Scalo, Roma, Italy
2Bulgarian Academy of Sciences, Institute of Plant Physiology, Sofia, Bulgaria
* To whom correspondence should be addressed. francesco.loreto{at}ibaf.cnr.it
Blue light has many direct and indirect effects on photosynthesis. The impact of blue light on mesophyll conductance (gm), one of the main diffusive limitation to photosynthesis, was investigated in leaves of Nicotiana tabacum and Platanus orientalis, characterized by high and low gm, respectively. Leaves were exposed to blue light fractions between 0% and 80% of incident light intensity (300 µmol photons m–2 s–1), the other fraction being supplied as red light. Leaves exposed to blue light showed reduced photosynthesis and unaltered stomatal conductance. The gm, measured using the chlorophyll fluorescence-based method, was strongly reduced in both plant species. Such a reduction of gm may not be real, as several assumptions used for the calculation of gm by fluorescence may not hold under blue light. To assess possible artefacts, the electron transport rate measured by fluorescence (Jf) and by gas-exchange (Jc) were compared in leaves exposed to different fractions of blue light under non-photorespiratory conditions. The two values were only equal, a prerequisite for correct gm measurements, when the illumination was totally provided as red light. Under increasing blue light levels an increasing discrepancy was observed, which suggests that Jf was not correctly calculated, and that such an error could also upset gm measurements. Blue light was not found to change the absorbance of light by leaves, whereas it slightly decreased the distribution of light to PSII. To equate Jf and Jc under blue light, a further factor must be added to the Jf equation, which possibly accounted for the reduced efficiency of energy transfer between the pigments predominantly absorbing blue light (the carotenoids) and the chlorophylls. This correction reduced by about 50% the effect of blue light on gm. However, the residual reduction of gm under blue light was real and significant, although it did not appear to limit the chloroplast CO2 concentration and, consequently, photosynthesis. Reduction of gm might be caused by chloroplast movement to avoid photodamage, in turn affecting the chloroplast surface exposed to intercellular spaces. However, gm reduction occurred immediately after exposure to blue light and was complete after less than 3 min, whereas chloroplast relocation was expected to occur more slowly. In addition, fast gm reduction was also observed after inhibiting chloroplast movement by cytochalasin. It is therefore concluded that gm reduction under blue light is unlikely to be caused by chloroplast movement only, and must be elicited by other, as yet unknown, factors.
Key words: Chlorophyll fluorescence, diffusive limitations to photosynthesis, electron transport rate, light-emitting diodes, light spectrum, photosystems
Received 2 January 2009; Revised 12 March 2009 Accepted 17 March 2009