A proteomic analysis combining photosynthesis-related key enzymes measurement and mRNA level analysis was used to investigate the variations of proteins involved in the photosynthetic apparatus in flag leaves of high-yield rice LYPJ during senescence under field conditions. The stability of chlorophyll-protein complexes of thylakoid membranes was measured by BN-PAGE system. Our findings revealed that the LHCII complexes were more stable than F0F1-ATPase and Cyb
 6
 /f complexes. Thylakoid membranes proteins during the senescence stage were separated by two-dimensional gel electrophoresis. We identified 13 photosynthesis-related proteins (a fivefold or greater variation), including electron transport, Calvin–Benson cycle and chloroplast protease during senescence of flag leaves. Moreover, our data showed that Ca2+-ATPase was more stable during senescence of flag leaves in rice, compared to Mg2+-ATPase. In enzymes activities analyses, it was observed that a little increment of C4 photosynthetic enzymes activities at the beginning of flag leaves senescence might compensate, to some degree, for the larger reductions in Rubisco activity, confirmed by quantitative RT-PCR analysis.

Analysis of thylakoid membrane protein and photosynthesis- related key enzymes in super high-yield hybrid rice LYPJ grown in field condition during senescence stage

Publisher Summary This chapter discusses the recent findings of chloroplast degradation and provides an overview of organelle senescence. Senescence is the last phase of development of a whole organism, organ, cell, or organelle. It is a degenerative process that leads to the death of a living system. Different tissues and cells of the leaves have their own pattern and timing of senescence. The initiation of senescence in mesophyll cells may not necessarily be synchronized with the process in vascular or epidermal tissues. Even different organelles of a single leaf cell––namely, chloroplasts, mitochondria, endoplasmic reticulum, ribosomes, and the nucleus do not show synchrony in the induction and progress of senescence. The chapter discusses a general pattern of temporal changes in the fine structure of different cellular organelles, including chloroplasts. Chloroplasts are the first organelles to show symptoms of disorganization when all other organelles are normal, followed by a change in the structure of the endoplasmic reticulum and loss of ribosomes. The loss of intrinsic electron transport components should follow the senescence-induced damage to the water-splitting system.

Ultrastructural modifications and biochemical changes during senescence of chloroplasts

Ultrastructural and biochemical changes in chloroplasts during Brassica napus senescence

Light treatment markedly accelerated the chlorophyll loss in senescing leaves of Hydrilla verticillata [(L.f.) Royle] as compared to dark treatment, whereas such acceleration could not be observed in senescing spinach (Spinacia oleracea L.) leaves. The light-induced cholorophyll loss in Hydrilla was retarded slightly by chloramphenicol and markedly by cycloheximide. Catalase (EC 1.11.1.6) activity did not change appreciably in Hydrilla leaves either in light or in darkness, while in spinach it declined markedly in the dark, and light retarded such decline. Peroxidase activity in Hydrilla showed faster increase in light than in darkness, while in spinach it increased only in light during senescence. The activity of phenol(pyrogallol)-specific peroxidase increased markedly in light, and that of ascorbate-specific peroxidase decreased slightly both in light and darkness during senescence of Hydrilla leaves. This rise in phenolspecific peroxidase activity was prevented by cycloheximide treatment. Pretreatment of Hydrilla leaves with monophenol (2,4-dichlorophenol) and o-diphenol (hydroquinone) accelerated and retarded, respectively, the light-induced cholorophyll loss. Pretreatment of Hydrilla leaves with H2O2 augmented the chlorophyll loss more markedly in light than in darkness. The endogenous level of H2O2 increased more in light than in dark during senescence of Hydrilla leaves. Treatment of Hydrilla leaves with 3-(3.4-dichlorophenyl)-l,l-dimethylurea. a photosystem II inhibitor, prevented both light-induced rise in H2O: level and chlorophyll loss, but it was without effect in the dark. Retardation of light-induced chlorophyll loss occurred during senescence of Hydrilla leaves when light was given in different photoperiods in a 24-h daily cycle for 6 days instead of as continuous irradiance. There was a negative correlation between the length of the photoperiod and the extent of cholorophyll loss.

Possible mechanisms of light-induced chlorophyll degradation in senescing leaves of Hydrilla verticillata

Prasanna K. Mohanty, a great scientist, a great teacher and above all a great human being, left us more than a year ago (on March 9, 2013). He was a pioneer in the field of photosynthesis research; his contributions are many and wide-ranging. In the words of Jack Myers, he would be a ''photosynthetiker'' par excellence. He remained deeply engaged with research almost to the end of his life; we believe that generations of researchers still to come will benefit from his thorough and enormous work. We present here his life and some of his contributions to the field of Photosynthesis Research. The response to this tribute was overwhelming and we have included most of the tributes, which we received from all over the world. Prasanna Mohanty was a pioneer in the field of ''Light Regulation of Photosynthesis'', a loving and dedicated teacher—unpre- tentious, idealistic, and an honest human being.

Prasanna K. Mohanty (1934–2013): a great photosynthetiker and a wonderful human being who touched the hearts of many

The possibilities of an interaction between light and the growth regulator benzimidazole in retarding senescence-induced changes in activity of chloroplasts isolated from detached wheat (Triticum aestivum L. cv. Kalyansona) leaves have been investigated. The effect of benzimidazole on the rate of degradation of chlorophylls in light depended on the quality of light used. Far-red irradiation given to detached leaves in the presence of benzimidazole retarded the pigment loss more significantly than did white or red light of similar intensity. Senescence-induced loss in chloroplast photochemical activity was higher than the loss of chlorophylls. Loss of photosystem I activity was greater than photosystem II activity. Benzimidazole maintained the gramicidin-mediated enhancement in whole chain electron transport uniformly throughout the incubation period irrespective of the light quality. There was no effect of light and benzimidazole in retarding the loss in photochemical activity, although the same preserved the chlorophyll contents as well as characteristics of the chloroplast absorption spectrum.

Interaction of light quality and benzimidazole on pigment contents and on photochemical activity of chloroplasts isolated from detached senescing wheat (Triticum aestivum) leaves

Chloroplasts isolated from the primary leaves of wheat seedlings grown under different light in the laboratory as well as from the leaves of field grown seedlings exhibited distinct rhythmic oscillations in photosynthetic electron transport activities. Chlorophyll content in the primary leaves of continuous light grown wheat seedlings did not oscillate. Whole chain electron transport and partial reactions measured as photosystem II and photosystem I activities estimated at regular intervals oscillated with a periodicity of about 12–16 h. The periodicity of less than 22 h suggested that the rhythms are of a non-circadian type. The oscillations in photochemical activity persisted whether the seedlings were raised under continuous light or under light-dark cycles, indicating their endogenous nature. The photochemical activities attained maximal levels during both the light and dark phases of the cycles. The rhythm pattern persisted in the presence of photophosphorylation uncouplers. A time dependent sensitivity of uncouplers to stimulate electron transport activity was observed. The timings of the activity maxima for photosystem II and photosystem I mediated reactions alternated each other during the day. The ratios of photosystem II to photosystem I activities were out of phase of photosystem I and closely resembled the rhythm pattern in photosystem II activity indicating a possible oscillation in the population of active photosystem units. The results suggest that the temporal modulation of the photochemical activity in wheat chloroplasts is brought about by some sort of reversible conformational change in the thylakoid membranes.

Rhythmic oscillations in wheat chloroplast photochemical activity. I. Oscillations in whole chain, photosystem II and photosystem I electron transport activities

Effect of sugar mill effluent on oxygen uptake and carbon dioxide output of rice (Oryza sativa L.c.v. Mushoori) seedlings

Chloroplasts isolated at defined times from the primary leaves of wheat seedlings grown under continuous light as well as 12 h light-dark cycles exhibited endogenous oscillations in photosystem II activity monitored as photoreduction of 2,6-dichlorophenol indophenol with water as electron donor. The 12 h rhythmicity in photoreduction of 2,6-dichlorophenol indophenol resembled the oscillation pattern in photosystem II supported O2 evolution. Room temperature chlorophylla fluorescence intensity ratios (F+DCMU/F−DCMU) yielded a rhythmicity closely comparable to that of 2,6-dichlorophenol indophenol photoreduction rhythm. Photosystem II mediated photoreduction of 2,6-dichlorophenol indophenol monitored with the exogenous electron donor hydroxylamine which bypasses the electron donation by water also exhibited similar 12 h oscillations. These results suggest that the O2 evolving complex does not entirely contribute to the origin of the rhythmicity in photosystem II. Instead, periodic changes in the reaction centre-antenna complexes may represent the rate limiting step for the generation of the time triggered rhythms in chloroplast photochemical activity. It is postulated that a temporal reversible control over the functional coupling and uncoupling between the reaction centres and antenna complexes or temporal changes in active and inactive reaction centres may induce the periodic oscillations in PS II activity.

Rhythmic oscillations in wheat chloroplast photochemical activity. II. Further characterization of the rhythm in photosystem II photoelectron transport activity

Temporal oscillations in morphological, physiological and biochemical parameters of plants are ubiquitous. Rhythmic phenomena in photosynthesis have been, found to be endogenous in nature and have been studied in algae as well as in higher plants (Edmunds, 1984; Sweeney, 1979; Lonergan, 1984b; Sweeney and Prezelin, 1978). Among the O2 evolving photosynthetic organisms, with the exception or cyanobacteria a large number of algal species representing almost all major taxonomic groups seem to exhibit rhythmic property in their photosynthetic capacity. However, the number of reports on photosynthesis rhythms in higher plants is limited to only a few species. Observations on the rhythmicity in photosynthesis have been made both under field and laboratory growth conditions. Time-dependent reversible variations in the chloroplast shape, movement and in the ultrastructure of thylakoid membranes in the case of algal cells have been investigated in detail (Sweeney, 1979).

Shabbir A. Sayeed

Papers

Interaction of light quality and benzimidazole on pigment contents and on photochemical activity of chloroplasts isolated from detached senescing wheat (Triticum aestivum) leaves

Rhythmic oscillations in wheat chloroplast photochemical activity. I. Oscillations in whole chain, photosystem II and photosystem I electron transport activities

Effect of sugar mill effluent on oxygen uptake and carbon dioxide output of rice (Oryza sativa L.c.v. Mushoori) seedlings

Rhythmic oscillations in wheat chloroplast photochemical activity. II. Further characterization of the rhythm in photosystem II photoelectron transport activity

Oscillations in Wheat Chloroplast Photochemical Activity: Effect of Uncouplers