The impact of blue light on leaf mesophyll conductance

Abstract

Blue light has many direct and indirect effects on photosynthesis. The impact of blue light on mesophyll conductance (g(m)), one of the main diffusive limitation to photosynthesis, was investigated in leaves of Nicotiana tabacum and Platanus orientalis, characterized by high and low g(m), respectively. Leaves were exposed to blue light fractions between 0% and 80% of incident light intensity (300 micromol 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 g(m), measured using the chlorophyll fluorescence-based method, was strongly reduced in both plant species. Such a reduction of g(m) may not be real, as several assumptions used for the calculation of g(m) by fluorescence may not hold under blue light. To assess possible artefacts, the electron transport rate measured by fluorescence (J(f)) and by gas-exchange (J(c)) 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 g(m) measurements, when the illumination was totally provided as red light. Under increasing blue light levels an increasing discrepancy was observed, which suggests that J(f) was not correctly calculated, and that such an error could also upset g(m) 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 J(f) and J(c) under blue light, a further factor must be added to the J(f) 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 g(m). However, the residual reduction of g(m) under blue light was real and significant, although it did not appear to limit the chloroplast CO(2) concentration and, consequently, photosynthesis. Reduction of g(m) might be caused by chloroplast movement to avoid photodamage, in turn affecting the chloroplast surface exposed to intercellular spaces. However, g(m) 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 g(m) reduction was also observed after inhibiting chloroplast movement by cytochalasin. It is therefore concluded that g(m) reduction under blue light is unlikely to be caused by chloroplast movement only, and must be elicited by other, as yet unknown, factors.

Chloroplast Photorelocation Movement: A Sophisticated Strategy for Chloroplasts to Perform Efficient Photosynthesis

“…Additionally, they could found no difference in the distance between the centers of the chloroplasts and the closest air spaces between two samples (Gorton et al, 2003). Another group found that blue light rapidly reduced CO 2 diffusion from intercellular air spaces to the chloroplasts in both Nicotiana and Platanus leaves and that this reduction was completed before chloroplasts finished the avoidance movement to the aniticlinal walls (Loreto et al, 2009). Importantly, the blue-lightinduced reduction of CO 2 diffusion was still normal in samples treated with an anti-actin inhibitor cytochalasin, which completely inhibits chloroplast movement (Loreto et al, 2009).…”

“…Senn (1908) hypothesized that this positioning might result from the chemotaxis of chloroplasts to the CO 2 in air spaces. Using different approaches and plant species, three groups examined whether chloroplast photorelocation movement, especially the avoidance response, influenced CO 2 diffusion in leaves (Gorton et al, 2003;Loreto et al, 2009;Tholen et al, 2008). One research group hypothesized that chloroplast distribution on the anticlinal walls by the avoidance response facilitated CO 2 utilization by shortening the CO 2 diffusion path length from air spaces to the mesophyll chloroplasts (Gorton et al, 2003).…”

“…Another group found that blue light rapidly reduced CO 2 diffusion from intercellular air spaces to the chloroplasts in both Nicotiana and Platanus leaves and that this reduction was completed before chloroplasts finished the avoidance movement to the aniticlinal walls (Loreto et al, 2009). Importantly, the blue-lightinduced reduction of CO 2 diffusion was still normal in samples treated with an anti-actin inhibitor cytochalasin, which completely inhibits chloroplast movement (Loreto et al, 2009). Thus, results by two independent groups indicated that chloroplast movement did not significantly change the efficiency of CO 2 diffusion from intercellular air spaces to the chloroplasts.…”

“…Collectively, these results suggested that the chloroplast avoidance response was not involved in CO 2 diffusion or that it possibly decreased, rather than increased, the diffusion rate. However, these three groups examined the contribution of chloroplast movement to CO 2 diffusion using different plant species and techniques: pulsed photoacoustics (Gorton et al, 2003), a chlorophyll fluorescence-based method (Loreto et al, 2009) and a carbon isotope discrimination method (Tholen et al, 2008). The examination of one plant species using different techniques and/or that of various plant species by one technique are required to uncover whether chloroplast photorelocation movement influences CO 2 diffusion.…”

“…Shade plants (such as Oxalis oregana and Tradesdantia albiflora) showed a greater avoidance response than that of non-shade plants (such as sunflower and pea) (Brugnoli & Björkman, 1992;Park et al, 1996), partly explaining why T. albiflora was more tolerant of strong light stress than pea plants (Park et al, 1996). Furthermore, cytochalasin-treated Platanus leaves, whose chloroplast movements were inhibited by the drug, but not untreated leaves showed a strong inhibition of photochemical efficiency (Loreto et al, 2009). Clumping of chloroplasts in succulent plants (Kondo et al, 2004) and the aggregative movement of C4 mesophyll chloroplasts were induced by drought stress in a light-dependent fashion, and these Fig.…”