A number of useful photosynthetic parameters are commonly derived from saturation pulse-induced fluorescence analysis. We show, that qP, an estimate of the fraction of open centers, is based on a pure 'puddle' antenna model, where each Photosystem (PS) II center possesses its own independent antenna system. This parameter is incompatible with more realistic models of the photosynthetic unit, where reaction centers are connected by shared antenna, that is, the so-called 'lake' or 'connected units' models. We thus introduce a new parameter, qL, based on a Stern-Volmer approach using a lake model, which estimates the fraction of open PS II centers. We suggest that qL should be a useful parameter for terrestrial plants consistent with a high connectivity of PS II units, whereas some marine species with distinct antenna architecture, may require the use of more complex parameters based on intermediate models of the photosynthetic unit. Another useful parameter calculated from fluorescence analysis is ΦII, the yield of PS II. In contrast to qL, we show that the ΦII parameter can be derived from either a pure 'lake' or pure 'puddle' model, and is thus likely to be a robust parameter. The energy absorbed by PS II is divided between the fraction used in photochemistry, ΦII, and that lost non-photochemically. We introduce two additional parameters that can be used to estimate the flux of excitation energy into competing non-photochemical pathways, the yield induced by downregulatory processes, ΦNPQ, and the yield for other energy losses, ΦNO.
An important adaptation to CO2-limited photosynthesis in cyanobacteria, algae and some plants was development of CO2-concentrating mechanisms (CCM). Evolution of a CCM occurred many times in flowering plants, beginning at least 15-20 million years ago, in response to atmospheric CO2 reduction, climate change, geological trends, and evolutionary diversification of species. In plants, this is achieved through a biochemical inorganic carbon pump called C4 photosynthesis, discovered 35 years ago. C4 photosynthesis is advantageous when limitations on carbon acquisition are imposed by high temperature, drought and saline conditions. It has been thought that a specialized leaf anatomy, composed of two, distinctive photosynthetic cell types (Kranz anatomy), is required for C4 photosynthesis. We provide evidence that C4 photosynthesis can function within a single photosynthetic cell in terrestrial plants. Borszczowia aralocaspica (Chenopodiaceae) has the photosynthetic features of C4 plants, yet lacks Kranz anatomy. This species accomplishes C4 photosynthesis through spatial compartmentation of photosynthetic enzymes, and by separation of two types of chloroplasts and other organelles in distinct positions within the chlorenchyma cell cytoplasm.
SummaryKranz anatomy, with its separation of elements of the C 4 pathway between two cells, has been an accepted criterion for function of C 4 photosynthesis in terrestrial plants. However, Bienertia cycloptera (Chenopodiaceae), which grows in salty depressions of Central Asian semi-deserts, has unusual chlorenchyma, lacks Kranz anatomy, but has photosynthetic features of C 4 plants. Its photosynthetic response to varying CO 2 and O 2 is typical of C 4 plants having Kranz anatomy. Lack of night-time CO 2 ®xation indicates it is not acquiring carbon by Crassulacean acid metabolism. This species exhibits an independent, novel solution to function of the C 4 mechanism through spatial compartmentation of dimorphic chloroplasts, other organelles and photosynthetic enzymes in distinct positions within a single chlorenchyma cell. The chlorenchyma cells have a large, spherical central cytoplasmic compartment interconnected by cytoplasmic channels through the vacuole to the peripheral cytoplasm. This compartment is ®lled with mitochondria and granal chloroplasts, while the peripheral cytoplasm apparently lacks mitochondria and has grana-de®cient chloroplasts. Immunolocalization studies show enzymes compartmentalized selectively in the CC compartment, including Rubisco in chloroplasts, and NAD-malic enzyme and glycine decarboxylase in mitochondria, whereas pyruvate, Pi dikinase of the C 4 cycle is localized selectively in peripheral chloroplasts. Phosphoenolpyruvate carboxylase, a cytosolic C 4 cycle enzyme, is enriched in the peripheral cytoplasm. Our results show Bienertia utilizes strict compartmentation of organelles and enzymes within a single cell to effectively mimic the spatial separation of Kranz anatomy, allowing it to function as a C 4 plant having suppressed photorespiration; this raises interesting questions about evolution of C 4 mechanisms.
A mutant of the NAD-malic enzyme-type C(4) plant, Amaranthus edulis, which lacks phosphoenolpyruvate carboxylase (PEPC) in the mesophyll cells was studied. Analysis of CO(2) response curves of photosynthesis of the mutant, which has normal Kranz anatomy but lacks a functional C(4) cycle, provided a direct means of determining the liquid phase-diffusive resistance of atmospheric CO(2) to sites of ribulose 1,5-bisphosphate carboxylation inside bundle sheath (BS) chloroplasts (r(bs)) within intact plants. Comparisons were made with excised shoots of wild-type plants fed 3,3-dichloro-2-(dihydroxyphosphinoyl-methyl)-propenoate, an inhibitor of PEPC. Values of r(bs) in A. edulis were 70 to 180 m(2) s(-1) mol(-1), increasing as the leaf matured. This is about 70-fold higher than the liquid phase resistance for diffusion of CO(2) to Rubisco in mesophyll cells of C(3) plants. The values of r(bs) in A. edulis are sufficient for C(4) photosynthesis to elevate CO(2) in BS cells and to minimize photorespiration. The calculated CO(2) concentration in BS cells, which is dependent on input of r(bs), was about 2,000 microbar under maximum rates of CO(2) fixation, which is about six times the ambient level of CO(2). High re-assimilation of photorespired CO(2) was demonstrated in both mutant and wild-type plants at limiting CO(2) concentrations, which can be explained by high r(bs). Increasing O(2) from near zero up to ambient levels under low CO(2), resulted in an increase in the gross rate of O(2) evolution measured by chlorophyll fluorescence analysis in the PEPC mutant; this increase was simulated from a Rubisco kinetic model, which indicates effective refixation of photorespired CO(2) in BS cells.
Wild-type (wt) Arabidopsis plants, the starch-deficient mutant TL46, and the near-starchless mutant TL25 were grown in hydroponics under two levels of nitrate, 0.2 versus 6 mm, and two levels of CO 2 , 35 versus 100 Pa. Growth (fresh weight and leaf area basis) was highest in wt plants, lower in TL46, and much lower in TL25 plants under a given treatment. It is surprising that the inability to synthesize starch restricted leaf area development under both low N (N L ) and high N (N H ). For each genotype, the order of greatest growth among the four treatments was high CO 2 /N H Ͼ low CO 2 /N H , Ͼ high CO 2 /N L , which was similar to low CO 2 /N L . Under high CO 2 /N L , wt and TL46 plants retained considerable starch in leaves at the end of the night period, and TL25 accumulated large amounts of soluble sugars, indicative of N-limited restraints on utilization of photosynthates. The lowest ribulose-1,5-bisphosphate carboxylase/oxygenase per leaf area was in plants grown under high CO 2 /N L . When N supply is limited, the increase in soluble sugars, particularly in the starch mutants, apparently accentuates the feedback and down-regulation of ribulose-1,5-bisphosphate carboxylase/oxygenase, resulting in greater reduction of growth. With an adequate supply of N, growth is limited in the starch mutants due to insufficient carbohydrate reserves during the dark period. A combination of limited N and a limited capacity to synthesize starch, which restrict the capacity to use photosynthate, and high CO 2 , which increases the potential to produce photosynthate, provides conditions for strong down-regulation of photosynthesis.In plants grown under elevated CO 2 there is often acclimation, reducing the capacity of photosynthesis. A variety of factors have been proposed to contribute to the down-regulation of photosynthesis under prolonged elevated CO 2 , including limited sink capacity, N limitation, end-product limitation, excess accumulation of starch, a decrease in photosynthetic enzymes such as Rubisco, and accelerated senescence.Rubisco small subunit (rbcS) transcripts often decrease in elevated CO 2 , for example, as reported in wheat (Triticum aestivum; Nie et al., 1995), Arabidopsis (Cheng et al., 1998), pea (Pisum sativum;Majeau and Coleman, 1996), and tomato (Lycopersicon esculentum; Van Oosten and Besford, 1995). This correlates with a decrease in Rubisco activity (Van Oosten and Besford, 1995;Majeau and Coleman, 1996) and content (Cheng et al., 1998;Moore et al., 1998). The decrease in Rubisco has been associated with an increase in the levels of soluble sugars in some studies (Van Oosten and Besford, 1995;Majeau and Coleman, 1996;Cheng et al., 1998), implicating sugarmediated repression of photosynthetic gene expression of which hexokinase is one of the likely sensors in the signaling pathway (Jang and Sheen, 1994;Pego et al., 2000;. The decrease in gene transcripts is not always correlated with absolute levels of soluble sugars (Nie et al., 1995;Moore et al., 1998), and sugar repression of photosynthesis...
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