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TERASHIMA Ichiro's studies

I have been studying on several subjects. Here, I summarize these studies briefly referring my papers. Many projects are going on in this laboratory.

(1) Light environment within a leaf and its effects on leaf photosynthesis. (1982 - present)

I examine optical properties of palisade tissue and spongy tissue isolated by means of paradermal sectioning from Camellia japonica leaves. Extinction coefficinet of chlorophyll in situ is greater in the spongy tissue than in palisade tissue (1-1). To have optically different two tissues in one leaf, the leaf can moderate the gradient of light absorption. However, the actual light gradient is considerable and chloroplats acclimate to their local light environments(1-2,-3,-4,and -5, Figs 1, 2).

chloroplast characters
Figure 1 Chlorophyll a/b ratio and the rate of electron transport rate per chlorophyll in 10 serially obtained sections from a spinach leaf.
palisada and spongy tissue chloroplasts
Figure 2 Palisade tissue (left) and spongy tissue chloroplasts from a spinach leaf.

Effects of mesophyll differentiation into palisade and spongy tissue and chloroplast acclimation on the light response curve of leaf photosynthesis was modelled and the difference in curves depending on the illumination direction was discussed (1-6 and 1-7, Fig. 3).

light response curves
Figure 3 Photosynthetic light response curves of a normally oriented leaf of Glycine max obtained by illuminating either adaxial or abaxial surface.

Original Papers

Reviews

(2) Nitrogen use in the leaf and plant individual (1988 - present)

Not only the absolute level of nitrgen but the allocation pattern of nitrogen among photosynthetic components is important. In the shade, nitrogen is to be preferentially used in light harvesting system, while in the sun,it is mostly used to raise the maximum photosynthetic rate. are to beEffects of nitrogen nutrition and growth light environment on leaf photosynthesis (2-1, 2-2).

Hikosaka developed a model predicting apitimal allocation of nitrogen among the photosynthetic componets (2-3). The relevance of the model has been shown with herbaceous plants (2-4) as well as with woody plants (2-5).

There is a PPFD gradient within the plant stand, and old leaves that developed and act as sun leaves are shaded by younger leaves. Nitorgenous compounds in such old leaves are re-allocated to young leaves. The sensecence of old leaves is important in efficient nitrogen use. To examine whether light environmetn or leaf age is regulating sensecenc, Hikosaka devised a system with a vine plant that can be grown horizontally to avoid self-shading. He clearly showed that when nutrient level is sufficient, light environment is the primary determinant of leaf properties (2-6)．It shoule be stressed here that senescence is acclimation process. When the (Hikosaka, K. (1996) Planta 198: 144-150).

Dr. Ono has been studying the mechanisms how a leaf senses its photosynthetic status in a plant. The Demand for photosynthates of the leaf would be sensed by monitoring sugar concentration in the leaf. Thus, she is examining the possibility that sugar-sensing is playing a role (see Ono's page). Araya is currently tackling with this problem. In particular, he is interested in sink-source transition. Tazoe is investigating nitrogen and light effects on organization of the photosynthetic machinery of C4 leaves.

Original papers

Reviews

(3) Photoinhibition (1988 - present)

When I was a postdoctral fellow in the laboratory of Professor Osmond, I started photoinhibition business. At first, we tried to cause of low temperature photoinhibition of photosynthesis. Unexpectedly, PS II in cucumber leaves, atypical chilling-sensitive plant, was resistant. After chilling treatment of the leaves in the light, I isolated thylakoids and found that thylakoids became leaky to protons. In vitro and in vivo studies clearly indicated that CF1 of H+-ATPase detaches from the membrane by chilling treatment in thelight (3-1, 3-2, and 3-3. but see 3-4, Fig. 4).

蛍光誘導期現象
Figure 4 Traces of fluorescence induction curves in cucumber leaves before and after the light-chilling treatment obtained with a PAM system. A, untreated control; B, just after the light-chilling treatment for 5 hours; C, after the light-chilling treatment for 5 hours and the rewarming in the dark for 30 min. Note that the NPQ was lost after the light-chilling treatment. But NPQ was clearly observed after the rewarming. The plastoquinone pool was reduced in B and C due to the damage to PSI.

These studies also clarified that this damage is reversible. However, the inhibition of photosynthesis is irreversible. Therefore, the uncoupling of the thylakoids is not a direct cause of the irreversible inhibition of photosynthesis.

Examining the partial reactions of photosynthesis, we found that PSI is damaged by the chilling treatment in the light (3-5). Dr. Sonoike further characterized this damage biochemically and biophysically (3-6, 3-7). Uncoupling during the chilling treatment in the light completely suppress ATP synthesis. Then Calvin cycle does not work. Furthermore, dissipation of energy as heat does not occur. Thus, electrons preferentially reduce O2. Because activities of scavenging enzymes are suppressed due to low temperature, H2O2 is not perfectly scavenged and by means of Fenton-type reaction is converted to hydroxyl radical. The hydroxyl radical thus produced damages bound-type iron sulfur centers (3-8).

For reviews see Dr. Sonoike's home page.

Dr. Inoue is studying molecular organization of PSII with Drs. Kaz. Sato and Kashino at Himeji Institute of Technology.

Original Papers

(4) CO2 diffusion in leaf photosynthesis. (1988 - present)

I characterized patchy photosynthesis over the leaf area in the presence of ABA (Fig. 5). When analyses using A-Ci relationship are made, some caution is needed (4-1).

starch-iodine test
Figure 5 The starch-iodine test. A sunflower leaf was illuminated in the presence of ABA.

Dr. Hanba constructed an on-line carbon discrimination/gas exchange system. Leaves of evergreen trees (4-2) and alpine plants (4-3), the resistance to CO2 diffusion in the leaf is considerable so that the CO2 concentration in chloroplasts is very low in such plants.

Because of this resistance, sun leaves showing high photosynthetic rate needs to have large internal area for gas exchange (see Dr. Hanba's page). This explains why sun leaves are thicker than shade leaves (4-4, Fig. 6). But, we have not known how. Yano is studying this problem (see his page). In Chenopodium album, the number of cell layer in the palisade tissue increases in response to light environment of the mature leaves but not to that of the developing leaf (4-5, 4-6).

葉の葉緑体の様子
Figure 6 When a thin leaf accommodates a given amount of Rubisco per unit leaf area, chloroplasts are fat and the CO2 concentration in chloroplasts is low. This is why sun leaves having much Rubisco per unit leaf area should be thick.

Recently we have got evidence that water channels in plasma membrane are involved in CO2 transfer (4-7, 4-8).

Original Papers

Reviews

(5) Photosynthesis in alpine plants

We made field measurements of photosynthesis in the East Himalayas (5-1). Because both O2 and CO2 partial pressures decreases with the decrease in atmospheric pressure, decrease in photosynthesis with elevation is small (5-2).

Mesophyll cell walls of alpine plants are thicker. This impose large resistance to CO2 diffusison (5-3).

Original Papers

(6) Architecture of photosynthetic systems of trees.

Determination processes of characteristics of branches and those of leaves in relation to light environment were studied. We are planning to start to study tree architecture extensively. Suzuki is analyzing cost and benefit of tree growth (see Suzuki's page). Sone is intensively revising Shinozaki's pipe model (see Sone's page).

Original Papers

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