The seedlings of Oryza sativa L. cv. Nipponbare grown by hydroponic culture for 3 wks were treated with 75, 100, 150 and 200 mM NaCl for 14, 14, 6 and 3 days, respectively, and examined for chloroplast ultrastructure in the region where chlorophyll fluorescence had been recorded. NaCl treatment decreased the ratio of variable to maximum chlorophyll fluorescence yield (Fv/Fm) and caused swelling of thylakoids. The swelling of thylakoids was quantified by the percentage of the length of swollen thylakoids to the total length of thylakoids. This value was increased with increasing NaCl concentration. Although Fv/Fm decreased at all concentrations of NaCl, the minimal fluorescence yield F0 was not increased by the treatment with 75 or 100 mM NaCl. The percentage of the length of swelling was low at 75 and 100 mM NaCl. On the other hand, F0 increased and the swelling of thylakoids was prominent with 150 and 200 mM NaCl treatment. These results suggest that the decrease in Fv/Fm due to the increase in F...

Chloroplast protrusions (CPs) are often observed under environmental stresses, but their role has not been elucidated. The formation of CPs was observed in the leaf of rice plants treated with 75 mm NaCl for 14 d. Some CPs were almost separated from the main chloroplast body. In some CPs, inner membrane structures and crystalline inclusions were included. Similar structures surrounded by double membranes were observed in the cytoplasm and vacuole. Ribulose 1,5‐bisphosphate carboxylase/oxygenase (Rubisco) was detected in CPs and the similar structures in the cytoplasm and vacuole. These results suggest that CP is one of the pathways of Rubisco exclusion from chloroplasts into the cytoplasm under salinity, and the exclusions could be transported to vacuole for their degradation.

Background and Aims Ultrathin sections of rice leaf blades observed two-dimensionally using a transmission electron microscope (TEM) show that the chlorenchyma is composed of lobed mesophyll cells, with intricate cell boundaries, and lined with chloroplasts. The lobed cell shape and chloroplast positioning are believed to enhance the area available for the gas exchange surface for photosynthesis in rice leaves. However, a cell image revealing the three-dimensional (3-D) ultrastructure of rice mesophyll cells has not been visualized. In this study, a whole rice mesophyll cell was observed using a focused ion beam scanning electron microscope (FIB-SEM), which provides many serial sections automatically, rapidly and correctly, thereby enabling 3-D cell structure reconstruction. Methods Rice leaf blades were fixed chemically using the method for conventional TEM observation, embedded in resin and subsequently set in the FIB-SEM chamber. Specimen blocks were sectioned transversely using the FIB, and block-face images were captured using the SEM. The sectioning and imaging were repeated overnight for 200–500 slices (each 50 nm thick). The resultant large-volume image stacks (x = 25 μm, y = 25 μm, z = 10–25 μm) contained one or two whole mesophyll cells. The 3-D models of whole mesophyll cells were reconstructed using image processing software. Key Results The reconstructed cell models were discoid shaped with several lobes around the cell periphery. The cell shape increased the surface area, and the ratio of surface area to volume was twice that of a cylinder having the same volume. The chloroplasts occupied half the cell volume and spread as sheets along the cell lobes, covering most of the inner cell surface, with adjacent chloroplasts in close contact with each other. Conclusions Cellular and sub-cellular ultrastructures of a whole mesophyll cell in a rice leaf blade are demonstrated three-dimensionally using a FIB-SEM. The 3-D models and numerical information support the hypothesis that rice mesophyll cells enhance their CO₂ absorption with increased cell surface and sheet-shaped chloroplasts.