Y Yoshimura

Structural and biochemical bases of photorespiration in C4 plants: quantification of organelles and glycine decarboxylase

In C4 plants, photorespiration is decreased relative to C3 plants. However, it remains unclear how much photorespiratory capacity C4 leaf tissues actually have. We thoroughly investigated the quantitative distribution of photorespiratory organelles and the immunogold localization of the Pxa0protein of glycine decarboxylase (GDC) in mesophyll (M) and bundle sheath (BS) cells of various C4 grass species. Specific differences occurred in the proportions of mitochondria and peroxisomes in the BS cells (relative to the M cells) in photosynthetic tissues surrounding a vein: lower in the NADP-malic enzyme (NADP-ME) species having poorly formed grana in the BS chloroplasts, and higher in the NAD-malic enzyme (NAD-ME) and phosphoenolpyruvate carboxykinase (PCK) species having well developed grana. In all C4 species, GDC was localized mainly in the BS mitochondria. When the total amounts of GDC in the BS mitochondria per unit leaf width were estimated from the immunogold labeling density and the quantity of mitochondria, the BSs of NADP-ME species contained less GDC than those of NAD-ME or PCK species. This trend was also verified by immunoblot analysis of leaf soluble protein. There was a high positive correlation between the degree of granal development (granal index) in the BS chloroplasts and the total amount of GDC in the BS mitochondria. The variations in the structural and biochemical features involved in photorespiration found among C4 species might reflect differences in the O2/CO2 partial pressure and in the potential photorespiratory capacity of the BS cells.

Estimation of Photosynthetic Activity from the Electron Transport Rate of Photosystem 2 in a Film-Sealed Leaf of Sweet Potato, Ipomoea batatas Lam

In order to evaluate the photosynthetic activity of a C3 leaf from the electron transport rate (ETR) of photosystem 2 (PS2), a new method was devised and examined using leaves of sweet potato. In this method, both surfaces of a leaf were sealed with transparent films to stop the gas exchange between the leaf and the atmosphere; hence the functions of both photosynthetic assimilation (CO2 uptake) and photorespiration (CO2 release) are restricted to the inside of the leaf. After both functional rates became equally balanced, ETR of the sealed leaf (ETRseal) was determined from the chlorophyll fluorescence. The measurements were conducted at different irradiances and leaf temperatures and by using leaves of different age. Under each measurement condition, ETRseal showed a close positive relationship with the photosynthetic potential, or the gross photosynthetic rate measured in the air of 2 % O2 (PG2%) before sealing. ETRseal may become an indicator to estimate or evaluate the photosynthetic activity of C3 leaves.

Estimation of Photorespiration Rate by Simultaneous Measurements of CO2, Gas Exchange Rate, and Chlorophyll Fluorescence Quenching in the C3 Plant Vigna Radiata (L.) Wilczek and the C4 Plant Amaranthus Mongostanus L.

So far the photorespiration rate (RP) in a leaf has been determined as the difference between the net photosynthetic rates (PN) measured in 21 % O2 air (PN21%) and 3 % O2 air (PN3%). In the C3 plant Vigna radiata and the C4 plant Amaranthus mongostanus L., PN and chlorophyll fluorescence quenching in leaves were monitored simultaneously. RP of leaves in situ was estimated as termed RPE from the electron transport rates through photosystem 2 (PS2), and compared with RPO (PN3% − PN21%). In V. radiataRPO was 11.9 µmol(CO2) m−2 s−1 and the ratio of RPO to PN21% was 42.2 %, whereas the ratio of RPE to PN21% was 25.7 %. This suggests that RPO may be over-estimated for the real RP in normal air. In A. mongostanus, PN was almost not changed with a decrease in O2 concentration from 21 to 3 %, whereas the quantum yield of PS2 was evidently affected by the change in O2 concentration. This fact shows the presence of photorespiration in this C4 species, where RPE was equivalent to 3.8 % of PN21%.

Long-term monitoring of feral genetically modified herbicide-tolerant Brassica napus populations around unloading Japanese ports.

Genetically modified, herbicide-tolerant (GMHT) Brassica napus plants originating from seed spill have recently been found along roadsides leading from Japanese ports that unload oilseed rape. Such introductions have potential biodiversity effects (as defined by the Cartagena Protocol): these include replacement of native elements in the biota through competitive suppression or hybridization. We conducted surveys in the period 2006–2011 to assess such threats. We examined shifts in the population distribution and occurrence of GMHT plants in 1,029 volunteer introduced assemblages of B. napus, 1,169 of B. juncea, and 184 of B. rapa around 12 ports. GMHT B. napus was found around 10 of 12 ports, but its proportion in the populations varied greatly by year and location. Over the survey period, the distributions of a pure non-GMHT population around Tobata and a pure GMHT population around Hakata increased significantly. However, there was no common trend of population expansion or contraction around the 12 ports. Furthermore, we found no herbicide tolerant B. juncea and B. rapa plants derived from crosses with GMHT B. napus. Therefore, GMHT B. napus is not invading native vegetation surrounding its populations and not likely to cross with congeners in Japanese environment.

Concurrent Monitoring of Oxygen Evolution and Chlorophyll Fluorescence in Mungbean Leaves with a Liquid-Phase Oxygen Electrode

In the liquid-phase oxygen electrode, photosynthetic rate is determined by the rate of oxygen evolution from sliced leaf pieces stirred in CO2-saturated reaction solution. The slicing and stirring treatments promote CO2 absorption of leaf pieces, but there is some difficulty in determining fluorescence emittance from the moving objectives. The barrier of CO2 uptake into leaf was removed by peeling its epidermis, and oxygen evolution rates from peeled leaves were readily measured (Yatomi, M. et al. 1992). A liquid-phase oxygen electrode was partly improved to make concurrent measurements of oxygen evolution rate and chlorophyll fluorescence quenching. In this study, we used this tool and observe the responses of gross oxygen evolution rate (Og), the quantum yield of PSII ( e) and photorespiration rate (Pr) to the changes of photosynthetic environment.

Concurrent Monitoring of Oxygen Evolution and Chlorophyll Fluorescence in Peeled Leaves with a Liquid-phase Oxygen Electrode

Concurrent monitoring of photosynthetic rate and chlorophyll fluorescence is one of the useful methods to understand the chemical energy balance in photosynthesis. The liquid-phase oxygen electrode is widely employed for determining photosynthetic rate by monitoring oxygen evolution from leaf segments in reaction solution. If it is possible to measure the emittance of chlorophyll fluorescence concurrently with oxygen evolution rate in leaf by the oxygen electrode method, it may become a valid technique for studying the energy flow in photosynthesis of leaf in various physical and chemical solution conditions. We set up a system for concurrent measurement of gross oxygen evolution rate (Og) and chlorophyll fluorescence by partially remodeling a conventional type of liquid-phase oxygen electrode. A piece of 0.785 cm2 leaf with the abaxial epidermis peeled off was fixed in solution of the electrode, and Og and quantum yield of PSII (Fe) were concurrently measured. By the addition of ATP to solution Fe was temporarily decreased, whereas Og was unchanged. Change in osmotic potential from ?0.3MPa to ?1.5 MPa did not affected Og in a leaf, but Fe was gradually reduced with a decrease in the osmotic potential of solution. It may be expected that the concurrent measurement by this remodeled electrode is widely used for analysis of energy balance in photosynthesis and energy utilization efficiency in crop leaves.