日出間 純

Free radicals with a long lifetime were observed in the leaves of two rice cultivars (Oryza sativa L,.), Sasanishiki (UVB resistant) and Norin-1 (UVB sensitive), by electron spin resonance spectroscopy. The leaves of both cultivars grown with visible light show very similar ESR spectra composed of radical 1 (R1) and radical 2 (R2), which may be attributable to P-700 cation radicals in the reaction center of photosystem I, and tyrosine cation radicals in the reaction center of photosystem II, respectively. The ESR spectrum composed of R1 and R2 radicals in the leaves of Sasanishiki grown under visible light with supplemental UVB was similar to that in the plant grown without supplemental UVB. On the other hand, the amount of R2 radicals in the leaves of Norin-1 grown under visible light with supplemental UVB was significantly smaller than that in the plant grown without supplemental UVB. It is suggested that the loss of R2 radicals in Norin-1 upon UVB irradiation is related to the instability of the plant.

We have examined the changes in the steady-state levels of cyclobutyl pyrimidine dimer (CPD) in rice plants (Oryza sativa L.) growing indoors and outdoors with or without supplemental W-B radiation. The CPD levels remain between 3 and 6 CPD Mb(-1) throughout the day in plants grown under sunlight although the levels fluctuate. The CPD levels are slightly elevated with UV-B (1 W m(-2)) supplementation, but remain constant (between 5 and 8 CPD Mb(-1)) throughout the day. In plants grown under visible radiation in a growth chamber, the CPD levels remain constant but are lower compared with those in outdoor-grown plants; they are slightly elevated by supplementation with UV-B radiation. Under exposure to UV-B radiation without visible radiation, the CPD levels gradually increase with lengthening of the irradiation time and reach a level of approximately 35 CPD Mb(-1) at dusk. However, this level decreases during the night and then increases again the next day. It is thus evident that the CPD levels are strictly maintained within constant limits throughout the day in plants growing indoors and outdoors with or without supplemental W-B radiation. It is also found that excision repair occurs during the night in plants having higher CPD levels. (C) 1999 Elsevier Science S.A. All rights reserved.

We have examined the susceptibility to cyclobutyl pyrimidine dimer (CPD) induction by UVB radiation, the ability to photorepair cyclobutyl pyrimidine dimers (CPDs), and the levels of ultraviolet (UV)-absorbing compounds in various developing stages of leaves in UVB-resistant (Sasanishiki) and -sensitive (Norin 1) rices (Oryza sativa L.). UVB produces more CPDs in the second, third and fourth fully expanded leaves than in the fifth and sixth fully expanded leaves. In leaves in which the levels of UV-absorbing compounds are lower, more CPDs are produced. The ability to photorepair CPDs in fully expanded leaves of different stages of each cultivar remains approximately constant, although the photorepair capacity in the fourth leaf rapidly increases from leaf emergence until full expansion. The photorepair capacity is lower in Norin 1 than that in Sasanishiki in all the leaf stages or different aged leaves. These results therefore suggest that lice plants could be more susceptible to CPD induction at the stage when the second, third or fourth leaves are developing, and UVB sensitivity correlates with low CPD photorepair.

Repair of cyclobutyl pyrimidine dimers (CPDs) in DNA is essential in most organisms to prevent biological damage by ultraviolet (UV) light. In higher plants tested thus far, UV-sensitive strains had higher initial damage levels or deficient repair of nondimer DNA lesions but normal CPD repair. This suggested that CPDs might not be important for biological lesions. The photosynthetic apparatus has also been proposed as a critical target. We have analyzed CPD induction and repair in the UV-sensitive rice (Oryza sativa L.) cultivar Norin 1 and its close relative UV-resistant Sasanishiki using alkaline agarose gel electrophoresis. Norin 1 is deficient in cyclobutyl pyrimidine dimer photoreactivation and excision; thus, UV sensitivity correlates with deficient dimer repair.

The effects of UVB radiation on the growth of two cultivars of Japanese lowland rice (Oryza sativa L.), Sasanishiki and Norin 1, were examined in a phytotron. Supplementation of visible radiation with UVB radiation reduced plant length, tiller number, the fresh and dry weights of the aboveground parts of plants, and the amounts of total leaf nitrogen, chlorophyll, soluble protein and ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) in the eighth leaf, the youngest fully expanded leaf. By contrast, UVB radiation significantly increased the accumulation of UV-absorbing compounds. There was a difference between the two cultivars in the resistance to the effects of UVB radiation. The reduction in the amounts of Rubisco was smaller in Sasanishiki, while the increase in the accumulation of UV-absorbing compounds was greater in Sasanishiki. Parameters of plant growth, with the exception of the amount of Rubisco, decreased in direct proportion to decreases in total leaf nitrogen in plants grown under lower or higher doses of UVB radiation. However, the decrease in the Rubisco content of Norin 1 grown under a high dose of UVB radiation was exceptionally marked, and was not observed similarly in Sasanishiki. These results suggest that the remarkable reduction in Rubisco content in Norin 1 might have been due to the specific effects of UVB radiation. It is also suggested that the difference between cultivars in the resistance to UVB radiation might be due to the differences in the levels of Rubisco and in UV-absorbing compounds that are induced by UVB radiation.

Changes in carbon fixation rate and the levels of photosynthetic proteins were measured in fourth leaves of Lolium temulentum grown until full expansion at 360 μmol quanta m^<-2> s^<-1> and subsequently at the same irradiance or shaded to 90 pmol m^<-2> s^<-1>. Ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco), light-harvesting chlorophyll a/b protein of photosystem II (LHCII), 65 kDa protein of photosystem I (PSI), cytochrome f (Cyif) and coupling factor 1 (CF_1) declined steadily in amount throughout senescence in unshaded leaves. In shaded leaves, however, the decrease in LHCII and the 65 kDa protein was delayed until later in'senescence whereas the amount of Cytf protein decreased rapidly following transfer to shade and was lower than that of unshaded leaves at the early and middle stages of senescence. Decreases in the Rubisco and CF_1 of shaded leaves occurred at slightly reduced rates compared with unshaded leaves. These results indicate that chloroplast proteins in fully-expanded leaves are controlled individually, in a direction appropriate to acclimate photosynthesis to a given irradiance during senescence.

Effects of irradiance on changes in the amounts of chlorophyll (Chl) and light-harvesting chlorophyll a/b protein of PS II (LHCII) were examined in senescing leaves of rice (Oryza sativa L.). Results of treatments at two irradiances (100% and 20% natural sunlight) were examined after the full expansion of the 13th leaf throughout the course of senescence. With 20% sunlight, the Chl content decreased only a little during leaf senescence, while with 100% sunlight it decreased appreciably. Similarly, the amount of LHCII protein during treatment with 20% sunlight remained almost constant. However, the ratio of Chl a/b during the shade treatment decreased significantly and the rate of decrease was greater than during the full-sunlight treatment. The ratio of Chl a/b for Chl a and b bound to LHCII was about 1.2, irrespective of leaf age or irradiance treatment. When the amounts of Chl bound to LHCII were calculated from the total leaf content of Chl and the ratio of Chl a/b, assuming a ratio of Chl a/b bound to LHCII of 1.2, they were well correlated with the amounts of LHCII protein. Changes in the amounts of LHCII synthesized during the two irradiance treatments were examined using an N-15 tracer. Incorporation of N-15 into LHCII declined dramatically during both treatments from full expansion through senescence, suggesting that there was little synthesis of LHCII protein during that time. In addition, the amount of LHCII synthesized during senescence was lower during the shade treatment than during the 100% sunlight treatment. These results indicate that the absence of an apparent change in levels of LHCII with shade treatment during senescence was caused by the very low rate of turnover of LHCII protein.

The amounts of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco), total chlorophyll (Chl), and total leaf nitrogen were measured in fully expanded, young leaves of wheat (Triticum aestivum L.), rice (Oryza sativa L.), spinach (Spinacia oleracea L.), bean (Phaseolus vulgaris L.), and pea (Pisum sativum L). In addition, the activities of whole-chain electron transport and carbonic anhydrase were measured. All plants were grown hydroponically at different nitrogen concentrations. Although a greater than proportional increase in Rubisco content relative to leaf nitrogen content and Chl was found with increasing nitrogen supply for rice, spinach, bean, and pea, the ratio of Rubisco to total leaf nitrogen or ChI in wheat was essentially independent of nitrogen treatment. In addition, the ratio of Rubisco to electron transport activities remained constant only in wheat. Nevertheless, gas-exchange analysis showed that the in vivo balance between the capacities of Rubisco and electron transport in wheat, rice, and spinach remained almost constant, irrespective of nitrogen treatment. The in vitro carbonic anhydrase activity in wheat was very low and strongly responsive to increasing nitrogen content. Such a response was not found for the other C3 plants examined, which had 10- to 30-fold higher carbonic anhydrase activity than wheat at any leaf-nitrogen content. These distinctive responses of carbonic anhydrase activity in wheat were discussed in relation to CO2-transfer resistance and the in vivo balance between the capacities of Rubisco and electron transport.

Effects of irradiance on photosynthetic characteristics were examined in senescent leaves of rice (Oryza sativa L.). Two irradiance treatments (100 and 20% natural sunlight) were imposed after the full expansion of the 13th leaf through senescence. The photosynthetic rate was measured as a function of intercellular CO2 pressure with a gas-exchange system. The amounts of cytochrome f, coupling factor 1, ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco), and chlorophyll were determined. The coupling factor 1 and cytochrome f contents decreased rapidly during senescence, and their rates of decrease were much faster from the 20% sunlight treatment than from the full sunlight treatment. These changes were well correlated with those in the photosynthetic rate at CO2 pressure = 600 microbars, but not with those under the ambient air condition (350 microbars CO2) and 200 microbars CO2. This suggested that the amounts of coupling factor 1 and cytochrome f from the full sunlight treatment cannot be limiting factors for the photosynthetic rate at ambient air conditions. The Rubisco content also decreased during senescence, but its decrease from the 20% sunlight treatment was appreciably retarded. However, this difference was not reflected in the photosynthetic rates at the ambient and 200 microbars CO2. This implied that in vivo Rubisco activity may be regulated in the senescent leaves from the 20% sunlight treatment. The chlorophyll content decreased most slowly. In the 20% sunlight treatment, it remained apparently constant with a decline in chlorophyll a/b ratio. These photosynthetic characteristics of the senescent rice leaves under low irradiance were discussed in relation to acclimation of shade plants.