Keach Murakami

Light-induced systemic regulation of photosynthesis in primary and trifoliate leaves of Phaseolus vulgaris: effects of photosynthetic photon flux density (PPFD) versus spectrum.

The objectives of this work using Phaseolus vulgaris were to examine whether the light spectrum incident on mature primary leaves (PLs) is related to leaf-to-leaf systemic regulation of developing trifoliate leaves (TLs) in photosynthetic characteristics, and to investigate the relative importance of spectrum and photosynthetic photon flux density (PPFD) in light-induced systemic regulation. Systemic regulation was induced by altering PPFD and the spectrum of light incident on PLs using a shading treatment and lighting treatments including either white, blue, green or red light-emitting diodes (LEDs). Photosynthetic characteristics were evaluated by measuring the light-limited and light-saturated net photosynthetic rates and the amounts of nitrogen (N), chlorophyll (Chl) and ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco; EC 4.1.1.39). Shading treatment on PLs decreased the amounts of N, Chl and Rubisco of TLs and tended to decrease the photosynthetic rates. However, we observed no systemic effects induced by the light spectrum on PLs in this study, except that a higher amount of Rubisco of TLs was observed when the PLs were irradiated with blue LEDs. Our results imply that photoreceptors in mature leaves have little influence on photosynthetic rates and amounts of N and Chl of developing leaves through systemic regulation, although the possibility of the action of blue light irradiation on the amount of Rubisco cannot be ruled out. Based on these results, we concluded that the light spectrum incident on mature leaves has little systemic effect on developing leaves in terms of photosynthetic characteristics and that the light-induced systemic regulation was largely accounted for by PPFD.

Interaction between the spectral photon flux density distributions of light during growth and for measurements in net photosynthetic rates of cucumber leaves.

The net photosynthetic rate of a leaf becomes acclimated to the plants environment during growth. These rates are often measured, evaluated and compared among leaves of plants grown under different light conditions. In this study, we compared net photosynthetic rates of cucumber leaves grown under white light-emitting diode (LED) light without and with supplemental far-red (FR) LED light (W- and WFR-leaves, respectively) under three different measuring light (ML) conditions: their respective growth light (GL), artificial sunlight (AS) and blue and red (BR) light. The difference in the measured photosynthetic rates between W- and WFR-leaves was greater under BR than under GL and AS. In other words, an interaction between supplemental FR light during growth and the spectral photon flux density distribution (SPD) of ML affected the measured net photosynthetic rates. We showed that the comparison and evaluation of leaf photosynthetic rates and characteristics can be biased depending on the SPD of ML, especially for plants grown under different photon flux densities in the FR waveband. We also investigated the mechanism of the interaction. We confirmed that the distribution of excitation energy between the two photosystems (PSs) changed in response to the SPD of GL, and that this change resulted in the interaction, as suggested in previous reports. However, changes in PS stoichiometry could not completely explain the adjustment in excitation energy distribution observed in this study, suggesting that other mechanisms may be involved in the interaction.

Time course of the photochemical reflectance index during photosynthetic induction: its relationship with the photochemical yield of photosystem II

Time courses of photochemical reflectance index (PRI) of an attached cucumber leaf during a dark-light transition were compared with those of photochemical yields of photosystem II (YII ) to discuss the feasibility of PRI imaging for estimating the efficiency of photosynthetic light use. YII and PRI were simultaneously evaluated with a pulse-amplitude modulation chlorophyll fluorometer and a low-cost imaging system consisting of digital cameras and band-pass filters, respectively. YII decreased immediately after the transition and then increased under various photon flux densities. Although PRI exhibited delayed time courses with respect to YII under low light conditions, PRI decreased monotonically under high light conditions. There was no correlation between YII and the changes in PRI (ΔPRI) immediately after the transition but YII was correlated with ΔPRI under the steady-state photosynthesis. These results indicate that the use of PRI to estimate YII under fluctuating light based on the regression obtained at steady state can overestimate YII . The imaging system was also applied to evaluate the spatial PRI distribution within a leaf. While PRI of leaf areas that remained untreated, or had been treated with H2 O again, first dropped and then rose under low light and monotonically decreased under high light conditions, leaf areas treated with inhibitor (dichlorophenyl dimethylurea) did not exhibit any changes. It is likely that the inhibitor suppressed lumen acidification, which triggers a decrease in PRI. It was suggested that YII of leaves with malfunctions in the photosynthetic electron transport can be overestimated by the PRI-based estimation.

Optical and Physiological Properties of a Leaf

The most important role of a leaf is capturing light energy and fixing CO2 into carbohydrates (i.e., photosynthesis). Fundamental knowledge on the optical and physiological properties of an individual leaf of a C3 plant is summarized below. A leaf adjusts light absorption, at the scales of both whole-leaf and intra-leaf, in order to efficiently capture light energy and to avoid photodamage caused by excessive light energy. Several interacting factors involved in orchestrating these optical properties, such as leaf orientation, mesophyll structure, chloroplast movement, and the absorption properties of phytopigments, are outlined. Photosynthesis consists of two reactions that are spatially separated within the chloroplast. Light energy is converted into reducing power and chemical energy via the electron transport chain. These are then consumed during CO2 fixation in the carbon assimilation process. The electron transport reaction is affected significantly by the spectral distribution of light due to the optical properties of the leaf. Photosynthesis is closely related to other physiological processes. CO2 uptake accompanies water vapor release (transpiration). Produced photosynthates are transported to the other plant organs (translocation). Brief information about the significance and the machinery of these photosynthesis-related processes is provided.

Light- and CO2-Dependent Systemic Regulation of Photosynthesis

Plant leaves do not only sense and respond to their local environment but also the environment experienced by the other leaves within the same plant. This long-distance signaling is involved in the systemic regulation of various photosynthesis-related phenotypic features of leaves. Here, we summarize the recent research on light- and CO2-dependent, leaf-to-leaf systemic regulation. In the short term, leaves can pre-acclimate to excess light at the transcriptional level, in response to systemic signals from other leaves. Several substances, including reactive oxygen species and phytohormones, have been suggested to play key roles in the signaling pathway. In the long term, the light and CO2 environment around mature leaves systemically regulates stomatal development, anatomical structure, and photosynthetic characteristics of young leaves. Possible mechanisms underlying the systemic regulation and the potential importance of systemic regulation in horticultural crop production are discussed.