Long-term imbalance in light absorption and electron transport by photosystem I (PSI) and photosystem II (PSII) in chloroplasts brings about changes in the composition, structure, and function of thylakoid membranes. The response entails adjustment in the photosystem ratio, which is optimized to help the plant retain a high quantum efficiency of photosynthesis (W.S. Chow, A. Melis, J.M. Anderson [1990] Proc Nat Acad Sci USA 87: [7502][7503][7504][7505][7506]. The dynamics of photosystem ratio adjustment were investigated upon the transfer of pea (Pisum sativum) plants from a predominantly PSI-light to a predominantly PSII-light environment and vice versa. The concentration of functional components (primary electron accepting plastoquinone of PSll [QA], P700) and that of constituent proteins were monitored during acclimation by A difference spectrophotometry and immunoblot analysis, respectively. Fully reversible changes in photosystem ratio occurred with a half-time of about 20 h. They involved closely coordinated changes in the concentration of the QA, reaction center protein D1, D2, and the 9-kD apoprotein of the cytochrome b,,, for PSII. Similarly, closely coordinated changes in the relative concentration of P700 and reaction center proteins of PSI were observed. The leve1 of chlorophyll b and that of the light-harvesting complex II changed in accordance with the concentration of PSll in the acclimating thylakoids. Overall, adjustments in the photosystem ratio in response to PSI-or PSII-light conditions appeared to be a well-coordinated reaction in the chloroplast. The response was absent in the chlorophyll b-less chlorina f2 mutant of barley (Hordeum vulgare) and in a phycobilisomeless mutant of Agmenellum quadruplicatum, suggesting that photosystem accessory pigments act as the lightquality perception molecules and that PSI and PSll themselves play a role in the signal transduction pathway.Under limiting intensity of illumination, the efficiency of photosynthesis depends on the coordinated interaction of two photosystems in the electron-transport chain. PSII is involved in the oxidation of water and reduction of plastoquinone, whereas PSI enables electron transport from plastohydroquinone and from the Cyt b-f complex to Fd. The quantum yield of photosynthesis in many plant species from diverse light habitats is about 0.106 & 0.001 mo1 of O2 evolved mol-' of photons absorbed (Ley and Mauzerall, 1982; Bjorkman and Demmig, 1987; Evans, 1987). This value is very close to a theoretical upper limit of 0.125 mo1 of O2 evolved mol-' of photons absorbed, translating in a quantum efficiency of about 85%, independent of the light climate in ' This work was supported by a grant from the National Science * Corresponding author; fax 1-510-642-4995.Founda tion. which plants grow. This is a remarkable feature of the photosynthetic apparatus, given the contrasting light qualities that prevail in different plant ecosystems (Bjorkman and Ludlow, 1972; Kirk, 1983;Terashima and Saeki, 1983) and the fact that substantially d...

We have therefore investigated the changes that occur at the organelle level during the chloroplast/chromoplast transition in tomato. In our previous work we have shown that most of the mRNAs for PSI and PSII, as well as stromal peptides, have decreased to nondetectable levels in chromoplasts (25). In this report we correlate the changes of mRNA levels with alterations that occur at the photosynthesis and polypeptide level. We present evidence that chloroplasts in green tomato fruits are photosynthetically active since substantial levels of the large subunit of ribulose 1,5-bisP carboxylase, the reaction center proteins of PSI and PSII, the light harvesting complex proteins of PSII, plastocyanin and Fd-NADP-oxidoreductase are detected in green fruit pericarp. MATERIALS AND METHODSThe ripening process of tomato fruits is characterized by a number of major changes, such as the loss of Chl, the accumulation of lycopene, fruit softening, and alterations in the metabolism of organic acids and monosaccharides (6,12). During the early stages of fruit ripening, tomato chloroplasts differentiate into chromoplasts. In general, such differentiation of organelles result in plastids of various shapes and functions (31). The chloroplast/chromoplast transition in tomato fruits is accompanied by major ultrastructural changes such as the breakdown of thylakoid membranes (9,10,20, 29). The most evident changes are the degradation of Chl and increasing synthesis and accumulation of lycopene (4, 27). Plastids in green tomato fruits contain high levels of starch and therefore resemble amyloplasts more closely than chloroplasts (29). It has been reported, however that thylakoid membrane systems are present and that green fruit chloroplasts are able to incorporate CO2 (5,16). However, CO2 fixation activity, the relative quantities ofvarious lipids, and the total protein and Chl concentrations in the thylakoid mem-

Ozone induces reductions in net photosynthesis in a large number of plant species. A primary mechanism by which photosynthesis is reduced is through impact on carbon dioxide fixation. Ozone induces loss in Rubisco activity associated with loss in concentration of the protein. Evidence is presented that ozone may induce oxidative modification of Rubisco leading to subsequent proteolysis. In addition, plants exposed to ozone sustain reduction in rbcS, the mRNA for the small subunit of Rubisco. This loss in rbcS mRNA may lead to a reduced potential for synthesis of the protein. The regulation of O3-induced loss of Rubisco, and implications of the decline in this protein in relation to accelerated senescence are discussed.

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The mesoscale packing and crystal structure of cellulose microfibrils as well as temporal changes in cell wall composition and hydration during the development of cotton fibers from two species, Gossypium hirsutum and G. barbadense were studied using vibrational sum frequency generation (SFG), attenuated total refection infrared (ATR-IR), Fourier transform Raman (FT-Raman) spectroscopy and X-ray diffraction (XRD). The developmental stages analyzed (13-60 days post anthesis) included primary wall synthesis, transitional cell wall remodeling, secondary wall thickening via synthesis of nearly pure cellulose, and fiber maturation. ATR-IR and FT-Raman combined with principle component analysis revealed that fibers of both species undergo abrupt changes in the cellulose and matrix polymer contents during the transition to secondary cell wall synthesis. XRD revealed that cellulose crystal size and crystallinity increase similarly over time in both species. SFG analysis of fibers from un-opened bolls, which were stored in water then air dried, showed subtle differences between two species in the mesoscale ordering of cellulose microfibrils in the maturing secondary walls. In the samples of mature fibers dried on the plant after the boll split opened naturally, the difference in SFG spectra between species was negligible. Collectively, the results show that (a) SFG can uniquely reveal differences in cellulose fibril ordering in maturing cotton fibers before boll opening; and (b) illustrate the comparative usefulness of other commonly used spectroscopic analytical methods for cotton fiber analysis.