Jun-ya Yamazaki

Effects of high light and low temperature during harsh winter on needle photodamage of Abies mariesii growing at the forest limit on Mt. Norikura in Central Japan

Abstract Coniferous evergreen firs Abies mariesii (A. mariesii) growing at the forest limit (near 2500 m altitude) on Mt. Norikura (36°61′N, 137°33′E, 3026 m altitude) in Central Japan are exposed to severe winter-stresses. Under such conditions, only the abaxial side of A. mariesii needles changes from green to reddish–brown in early spring, resulting in the death of the needles. Since this needle damage is only observed in shoots that protrude from the snow surface and not in those buried in snow or located at lower elevations, this phenomenon seems to be caused by the interaction of strong sunlight reflected from the snow surface and low temperature. We found that the damaged needles have increased in the de-epoxidation state of xanthophyll cycle because they contained large amounts of zeaxanthin, which appears when the leaves encounter a strong light stress, leading to the long-term down-regulation of PS II photochemistry. These results indicate that the needles acclimatize against the strong light during severe winter. Furthermore, ascorbate peroxidase (APX, E.C. 1.11.1.11) activity was found to decrease remarkably during critical subzero periods, while superoxide dismutase (SOD, E.C. 1.15.1.1) activity remains constant throughout the year. Based on these results, we discuss herein how A. mariesii growing at the forest limit of temperate zone responds to high light and low temperature in harsh winter conditions.

Changes in photosynthetic characteristics and photosystem stoichiometries in the lower leaves in rice seedlings

Abstract Changes in the Chl a / b ratio, electron transport, electron carriers, and photosystem stoichiometries were examined in rice leaves in the present study. The Chl a / b ratio is known to decrease gradually from the top to the bottom leaves, indicating a increase in the abundance of LHC II relative to the reaction center complexes of the two photosystems. We used juvenile rice canopy and obtained the following results: (1) the photosynthetic activity and Chl content per leaf area decreased from the top to the bottom leaves, the Chl a / b ratio also declined from 3.7 to 3.0; (2) when determined on the basis of Chl content, C-550 and Cyt f content decreased, but there was no loss of P-700, Consequently, the PS II/PS I ratio significantly decreased; (3) on the basis of mmol Chl, the levels of Cyt f dramatically decreased and, therefore, no loss was observed for whole chain electron transport per Cyt f ; and (4) the percentage abundance of PS IIα increased, but the rate constants of Q A photoreduction and P-700 photooxidation gradually decreased. From these results, we hypothesize that there is a compensatory relationship between the decline in the Chl a / b ratio and that in the PS II/PS I ratio in the lower leaves in rice seedlings.

Relationship between photosystem stoichiometries and changes in active oxygen scavenging enzymes in natural grown rice seedlings

The relationship between antioxidative enzymes and photosystemstoichiometries was examined in rice leaves. The photosystem (PS) II/PS Ireaction centre ratio decreased from the top to the bottom leaves, suggestinganimbalance of light absorption and electron flow as well as a probableover-reduction between the two photosystems. We used rice leaves from juvenileseedlings and obtained the following results: (1) Photosynthetic capacitymeasured from the pulse-amplitude modulation (PAM) fluorometer showed the PS IIphotochemistry to have a reduced capacity, but the excitation-energy trappingcapacity at PS II gradually increased from the top to the bottom leaves. (2)Thelevels of Chl, soluble protein, and ascorbate decreased, but those ofH2O2 increased slightly in the bottom leaves. (3) Whendetermined on the basis of leaf area, glycolate oxidase, catalase, ascorbateperoxidase and glutathione reductase degraded in the bottom leaves. (4) On thebasis of unit of Chl, the levels of superoxide dismutase (SOD) and guaiacolperoxidase (GPX) increased dramatically. Based on the above results, we discussthe relationship between the two photosystem stoichiometries and changes in theantioxidative enzymes.

Effects of light on the photosynthetic apparatus and a novel type of degradation of the photosystem I peripheral antenna complexes under darkness.

A novel type of degradation of photosystem I peripheral antenna complexes has been observed in rice leaves under darkness in the present study. Photosynthesis, chlorophyll content, the chlorophyll a/b ratio, and relative amounts of ribulose-1,5-bisphosphate carboxylase/oxygenase decrease during dark treatment. The levels of photosystem II reaction-center complex and cytochrome f on the basis of units of chlorophyll also decline rapidly under darkness. In contrast, the levels of photosystem I reaction-center complex remain stable under darkness for six days. Low-temperature fluorescence emission spectra ascribed to photosystem I antennae clearly show a blue shift. A similar shift is also observed in the photosystem I complexes resolved with dodecyl maltoside-polyacrylamide gel electrophoresis. Moreover, polypeptide analysis of the thylakoids and photosystem I complexes isolated from the green gels shows that some polypeptides originating from photosystem I peripheral antenna complexes disappear during the dark treatment. A curve-fitting method also displays remarkable changes in the chlorophyll components between the light and dark treatments. It is likely that these results indicate the disconnection/disassembly of the photosystem I antenna as well as the photosystem II complexes induced by dark treatment. Moreover, these findings also imply the existence of different degradation mechanisms for the photosystem I and II complexes.

Seasonal changes in the excess energy dissipation from Photosystem II antennae in overwintering evergreen broad-leaved trees Quercus myrsinaefolia and Machilus thunbergii.

We monitored chlorophyll (Chl) fluorescence, pigment concentration and the de-epoxidation state of the xanthophyll cycle (DPS(1)) in two warm temperate broad-leaved evergreen species (Quercus myrsinaefolia and Machilus thunbergii). Reduction of the maximal quantum yield of Photosystem II (PSII) (calculated from Fv/Fm, variable to maximal Chl a fluorescence) and retention of a high DPS were observed in both species in the winter, and can be interpreted as acclimation to winter. In particular, the acclimation of PSII in these species can be chiefly attributed to thermal dissipation, which is correlated with the retention of high zeaxanthin. Furthermore, we attempted to divide the fate of the absorbed light energy by the PSII antennae into three components: (i) PSII photochemistry (represented by its quantum yield, ΦPSII), (ii) dissipation by down-regulation via non-photochemical quenching (ΦNPQ) and (iii) other non-photochemical processes (ΦONP). The estimated energy allocation of the absorbed light indicated that the proportion of ΦPSII decreased, whereas that of ΦNPQ+ΦONP increased during winter. This result suggests that the excess energy absorbed in the PSII complexes is safely dissipated from the PSII antennae. Based on these results, we conclude that thermal dissipation from the PSII antennae plays an important role in two overwintering broad-leaved evergreen trees growing in Japan.

CHANGES IN PHOTOSYNTHETIC APPARATUS IN THE JUVENILE RICE CANOPY AND A POSSIBLE FUNCTION OF PHOTOSYSTEM I IN THE BOTTOM LEAVES

Abstract Changes in the photosynthetic apparatus and relative antenna sizes of photosystem (PS) I and PS II were measured in the rice canopy. We used juvenile rice seedlings to examine light utilization and its absorption in the bottom leaves and obtained the following results: (1) When referred to chlorophyll (Chl), levels of the electrochromic shift at 550 nm and cytochrome ƒ decreased from the sixth to the third leaves, but there was no loss of pigment (P)-700. As a consequence, the PS II/PS I ratio significantly decreased from 1.5 in the sixth leaves to 0.9 in the third leaves. (2) The electron transport capacity in the sixth leaves was 1.5-times larger than that in the third leaves. (3) The levels of cytochrome b6 referred to Chl were almost constant from top to bottom. (4) The photosynthetic performance of the leaf decreased concomitant with the depth, whereas the respiration was slightly increased. From these results, we hypothesize that there are maintenance mechanisms when the imbalances of light absorption and electron transport capacity occur in the bottom leaves.

Responses of the photosynthetic apparatus to winter conditions in broadleaved evergreen trees growing in warm temperate regions of Japan

Photosynthetic characteristics of two broadleaved evergreen trees, Quercus myrsinaefolia and Machilus thunbergii, were compared in autumn and winter. The irradiance was similar in both seasons, but the air temperature was lower in winter. Under the winter conditions, net photosynthesis under natural sunlight (Anet) in both species dropped to 4 μmol CO2 m(-2) s(-1), and the quantum yield of photosystem II (PSII) photochemistry in dark-adapted leaves (Fv/Fm) also dropped to 0.60. In both species the maximum carboxylation rates of Rubisco (V(cmax)) decreased, and the amount of Rubisco increased in winter. A decline in chlorophyll (Chl) concentration and an increase in the Chl a/b ratio in winter resulted in a reduction in the size of the light-harvesting antennae. From measurements of Chl a fluorescence parameters, both the relative fraction and the energy flux rates of thermal dissipation through other non-photochemical processes were markedly elevated in winter. The results indicate that the photosynthetic apparatus in broadleaved evergreen species in warm temperate regions responds to winter through regulatory mechanisms involving the downregulation of light-harvesting and photosynthesis coupled with increased photoprotective thermal energy dissipation to minimize photodamage in winter. These mechanisms aid a quick restart of photosynthesis without the development of new leaves in the following spring.