C4 photosynthesis in Cyperus longus L., a species occurring in temperate climates

Abstract. Cyperus longus L. , which has a widespread but disjunct distribution throughout Europe and extends northwards into Britain, was found to be a C₄ species based upon its Kranz leaf anatomy, low CO₂ compensation point and the labelling of malate as an early product of ¹⁴CO₂ fixation. The photosynthetic characteristics of C. longus are similar to many other C₄ species with a high maximum rate of photosynthesis (> 1.5 mg CO₂ m ⁻² s ⁻¹) and a relatively high temperature optimum (30–35°C), but unlike many C₄ species the rate of photosynthesis does not decline rapidly below the optimum temperature and a substantial rate (0.6 mgCO₂ m⁻²s⁻¹)occursat 15°C. Leaf extension is very slow at 15°C and shows a curvilinear response to temperatures between 15 and 25°C. Leaves extend at a rate of almost 4 cm d⁻¹ at 25°C.     

C4 photosynthesis at low temperatures

Abstract. C₄ plants grown in optimum conditions are, by comparison to C₃, capable of higher maximum dry-matter yields and greater efficiencies of water and nitrogen use, yet they are rare outside the subtropics. Both latitudinal and altitudinal limits of C₄ distributions correlate most closely with a mean minimum temperature of 8-10°C during the period of active growth. The possibility that the C₄ process is inherently incapable of functioning at low temperatures is examined. The reversible effects of chilling on the quantum efficiency of C₄ photosynthesis and the functioning of the individual steps in the C₄ cycle are examined. Chilling also produces an irreversible loss of capacity to assimilate CO₂ which is directly proportional to the light received during chilling. It is suggested that the reversible reduction in capacity to assimilate CO₂ and the lack of an alternative pathway for the utilization of lightgenerated reducing power may make C₄ species more prone to chilling-dependent photoinhibition. Laboratory studies and limited field observations suggest that this damage would be most likely to occur during photosynthetic induction at the temperatures and light levels encountered on clear, cool mornings during the spring and early summer in cool climates. Even those C₄ species occurring naturally in cool climates do not appear fully capable of tolerating these conditions; indeed their growth patterns suggest that they may be adapted by avoiding 'rather than enduring' such conditions.     

Comparative ecophysiology of C3 and C4 plants

Abstract. In this review we relate the physiological significance of C₄ photosynthesis to plant performance in nature. We begin with an examination of the physiological consequences of the C₄ pathway on photosynthesis, then discuss the ecophysiological performance of C₄ plants in contrasting environments. We then compare the performance of C₃ and C₄ plants when they occur together in similar habitats, and finally discuss the distribution of C₄ photosynthesis with respect to the physical environment, phylogeny, and life form.     

The origins and diversification of C4 grasses and savanna-adapted ungulates

C₄ grasses constitute the main component of savannas and are pervasive in other dry tropical ecosystems where they serve as the main diet for grazing animals. Among potential factors driving C₄ evolution of grasses, the interaction between grasses and grazers has not been investigated. To evaluate if increased grazing pressure may have selected for higher leaf silica production as the grasses diverged, we reconstructed the phylogeny of all 800 genera of the grass family with both molecular (combined multiplastid DNA regions) and morphological characters. Using molecular clocks, we also calculated the age and number of origins of C₄ clades and found that shifts from C₃ to C₄ photosynthesis occurred at least 12 times starting 30.9 million years ago and found evidence that the most severe drop in atmospheric carbon dioxide in the late Oligocene (between 33 and 30 million years ago) matches the first origin of C₄ photosynthesis in Chloridoideae. By combining fossil and phylogenetic data for ungulates and implementing a randomization procedure, our results showed that the appearance of C₄ grass clades and ungulate adaptations to C₄-dominated habitats match significantly in time. An increase of leaf epidermal density of silica bodies was found to correspond to postulated shifts in diversification rates in the late Miocene [24 significant shifts in diversification ( P <0.05) were detected between 23 and 3.7 million years ago]. For aristidoid and chloridoid grasses, increased grazing pressure may have selected for a higher leaf epidermal silica production in the late Miocene.     

Optimal leaf area indices in C3 and C4 mono- and dicotyledonous species at low and high nitrogen availability

From an analytical model it was shown that for a given total amount of nitrogen in the canopy, there exists an optimal leaf area index (LAI), and therefore an optimal average leaf introgen content, at which canopy photosynthesis is maximal. If the LAI is increased above this optimum, increased light interception will not compensate for reduction in photosynthetic capacity of the canopy resulting from reduced leaf nitrogen contents. It was further derived from the model that the value of the optimal LAI increases with the photosynthetic nitrogen use efficiency (PNUE) and decreases with the canopy extinction coefficient for light (K_L) and incident photon flux density (PFD) at the top of the canopy. These hypotheses were tested on dense stands of species with different photosynthetic modes and different architectures. A garden experiment was carried out with the C₄ monocot sorghum ( Sorghum bicolor [L.] Moensch cv. Pioneer), the C₃ monocot rice ( Oryza sativa L. cv. Araure 4), the C₄ dicot amaranth ( Amaranthus cruentus L. cv. K113) and the C₃ dicot soybean ( Glycine max [L.] Merr. cv. Williams) at two levels of nitrogen availability.
The C₄ species had higher PNUEs than the C₃ species while the dicots formed stands with higher extinction coefficients for light and had lower PNUEs than the monocots. The C₄ and monocot species were found to have formed more leaf area per unit leaf nitrogen (i.e., had lower leaf nitrogen contents) than the C₃ and dicot species, respectively. These results indicate that the PNUE and the extinction coefficient for light are important factors determining the amount of leaf area produced per unit nitrogen as was predicted by the model.     

Zeaxanthin and non‐photochemical quenching in sun and shade leaves of C3 and C4 plants

The relationships between non‐radiative energy dissipation and the carotenoid content, especially the xanthophyll cycle components, were studied in sun and shade leaves of several plants possessing C₃ ( Hedera helix and Laurus nobilis ) or C₄ ( Zea mays and Sorghum bicolor ) photosynthetic pathways. Sun‐shade acclimation caused marked changes in the organisation and function of photosynthetic apparatus, including significant variation in carotenoid content and composition. The contents of zanthophyll cycle pigments were higher in sun than in shade leaves in all species, but this difference was considerably greater in C₃ than in C₄ plants. The proportion of photoconvertible violaxanthin, that is the amount of violaxanthin (V) which can actually be de‐epoxidised to zeaxanthin, was much greater in sun than in shade leaves. The amount of photoconvertible V was always linearly dependent on the chlorophyll a/b ratio, although the slope of the relationship varied especially between C₃ and C₄ species. The leaf zeaxanthin and antheraxanthin contents were correlated with non‐radiative energy dissipation in all species under different light environments. These relationships were curvilinear and variable between sun and shade leaves and between C₃ and C₄ species. Hence, the dissipation of excess energy does not appear to be univocally dependent on zeaxanthin content and other photoprotective mechanisms may be involved under high irradiance stress. Such mechanisms appear largely variable between C₃ and C₄ species according to their photosynthetic characteristics.     

Chromosome numbers of C3 and C4 variants within the species Alloteropsis sernialata (R.Br.) Hitchc. (Poaceae)

FREAN, M. L. & MARKS, E., 1988. Chromosome numbers of C₃ and C₄ variants within the species Alloteropais semialata (R.Br.) Hitch. (Poaceae). In a study of mid-lamina leaf sections, different variants of A. semialata were found to have C₃ or C₄ anatomy. The C₄ leaf showed a specialized photosynthetic vascular bundle sheath not present in the C₃ form. Chromosome counts made from pollen mother cell squashes showed that the C₃ form of A. semialata is a fertile diploid 2n = 2x = 18 and the C₄ form, a fertile allohexaploid 2n = 6x = 54. The cytological evidence suggests that the two forms should be considered as separate species.     

Re-evaluation of proposed C4 photosynthetic characteristics in the genus Larix

It has been suggested previously that Japanese larch ( Larix kaempferi ) exhibits characteristics of C₄ photosynthesis. To further evaluate this suggestion, stable carbon isotope ratios were determined for leaf and bark tissue of Larix gmelini, L. kaempferi, L. laricina, L. Iyallii, L. occidentalis , and L. sibirica. All δ¹³C values were more negative than –22‰. Short-term labeling with ¹⁴CO₂ showed that phosphoglyceric acid and other phosphorylated compounds were the first products of photosynthesis in L. sibirica. Both of these results strongly suggest that the initial fixation of atmospheric CO₂ in these six Larix species is accomplished solely via the C₃ photosynthetic pathway.     

Relationship between C2H2 reduction, H2 evolution and 15N2 fixation in root nodules of pea (Pisum sativum)

The quantitative relationship between C₂H₂ reduction, H₂ evolution and ¹⁵N₂ fixation was investigated in excised root nodules from pea plants ( Pisum sativum L. cv. Bodil) grown under controlled conditions. The C₂H₂/N₂ conversion factor varied from 3.31 to 5.12 between the 32nd and the 67th day after planting. After correction for H₂ evolution in air, the factor (C₂H₂-H₂)/N₂ decreased to values near the theoretical value 3, or in one case to a value significantly ( P < 0.05) below 3. The proportion of the total electron flow through nitrogenase, which is not wasted in H₂ production but used for N₂ reduction, is often stated as the relative efficiency (1-H₂/C₂H₂). This factor varied significantly ( P < 0.05) during the growth period. The actual allocation of electrons to H₂ and N₂, expressed as the H₂/N₂ ratio, was independent of plant age, however. This discrepancy and the observation that the (C₂H₂-H₂)/N₂ conversion factor tended to be lower than 3, suggests that the C₂H₂reduction assay underestimates the total electron flow through nitrogenase.     

C2H2 and Ar induce rapid declines in nitrogenase activity and CO2 evolution in nodules of Datisca glomerata

Preliminary studies have indicated that after addition of C₂H₂ there is a rapid decline in nitrogenase activity in the nodules of Datisca glomerata . The present work was undertaken to determine whether (1) there is also a decline in respiration and (2) the decline is associated with the cessation of ammonia production. The rates of C₂H₄ and CO₂ evolution by nodulated root systems of Datisca were measured as a function of time after exposure to C₂H₂. The peak rate of C₂H₄ evolution occurred at 30 s after C₂H₂ exposure, while the rate of CO₂ evolution started to decline at 60 s after exposure to C₂H₂. Incubation of nodules in a gas mixture containing Ar also caused a decline in CO₂ evolution. Further, pretreatment with Ar eliminated most of the C₂H₂-induced decline in nitrogenase activity and CO₂ evolution. These C₂H₂- and Ar-induced declines in Datisca nodules are more rapid than those reported in any other nodules. They are evidence that continued ammonia formation is essential for maintenance of normal nitrogenase activity in Datisca nodules.     

Leaf blade structure and C4 acid decarboxylation enzymes in xCynochloris spp. (Poaceae), intergeneric hybrids between species of different C4 type

PRENDERGAST, H. D. V., STONE, N. E. & HATTERSLEY, P. W., 1988. Leaf blade structure and C₄ acid decarboxylation enzymes in x Cynochloris spp. (Poaceae), intergeneric hybrids between species of different C₄ type. x Cynochloris macivorii Clifford and Everist and x C. reynoldensis B. K. Simon (Poaceae) are intergeneric C₄ hybrids between Cynodon dactylon (L.) Pers., and NAD-malic enzyme (NAD-ME) species, and two different PEP carboxykinase (PCK) species of Chloris . Parental species of each hybrid species have the 'classical' leaf blade structure of their respective C₄ acid decarboxylation types. The outline of the photosynthetic carbon reduction (PCR or Kranz) bundle sheath and the position of the PCR cell chloroplasts in x Cynochloris are intermediate between those of the parental species, C. macivorii being more like Cynodon dactylon and x C. reynoldensis more like Chloris spp. The PCR chloroplast shape in x C. macivorii and x C. reynoldensis is like that of Cynodon dactylon and Chloris spp., respectively. Differences between the hybrids in their enzyme activities complement these structural differences: x C. macivorii has more NAD—ME and less PCK activity than x C. reynoldensis , although in both species PCK activity is the greater. Both hybrids, however, have a suberized lamella in PCR cell walls as do Chloris spp. The close taxonomic relationship between Cynodon and Chloris make these genera especially suitable for reciprocal crossing experiments aimed at increasing understanding of the genetic relationships between subtypes of the C₄ photosynthetic pathway.     

Comparison of the specific activity of ribulose-1,5-bis-phosphate carboxylase-oxygenase from some C3 and C4 plants

The specific activity of ribulose-1,5-bisphosphate carboxylase-oxygenase (Rubisco, EC 4.1.1.39) was measured from the crude extracts of five C₃ plants consisting of wheat ( Triticum aestivum L. cv. Maris Mink), spinach ( Spinacia oleracea L.), pea ( Pisum sativum L. cv. Greenfeast), pumpkin ( Cucurbita pepo L. cv. Jättiläismeloni) and Ceratodon purpureus (Hedw.) Brid., and two C₄ plants, maize ( Zea mays L. ETA F₁) and sugar sorghum [ Sorghum saccharatum (L. emend, L.) Moench]. The amount of Rubisco in the crude extracts was estimated by polyacrylamide gel electro-phoresis with the Coomassie Brilliant Blue staining procedure. The amounts of the dye bound to the purified Rubisco of different higher plants were similar. The method gave a linear response for both purified enzyme and crude extracts, and the results agreed with those observed by immunochemical methods. The addition of positive effectors such as inorganic phosphate was necessary to obtain maximal activity in the crude extracts of all the studied plants except in that of maize. No significant differences in the specific carboxylase activity at 25°C were found between the C₃ and C₄ plants.     

Metabolism of C19- and C20-gibberellins by cell-free preparations from immature Phaseolus coccineus seed

Gibberellin biosynthesis pathways were investigated using isotopically-labelled C₁₉- and C₂₀-gibberellins and cell-free preparations from immature seed of Phaseous coccineus cv. Prizewinner. The initial steps in an early 13-hydroxylation pathway involved the conversion gibberellin A₁₂-aldehyde (GA₁₂-aldehyde) to GA₁₂ which was 13-hydroxylated to yield GA₅₃, Metabolism of GA₅₃ yielded GA₄₄. In contrast to other cell-free systems, GA₄₄ was not further converted, either as a δ-lactone or an open-lactone structure, to the C-20 aldehyde GA₁₉. GA₁₉ was, however, metabolised to GA₂₀, GA₅ and GA₁. GA₂₀ represented a branch point in the pathway as it was converted both to GA₁, which was an end product, and GA₅ which was further converted to GA₆. Like GA₁, GA₆ was also an end-product of the early 13-hydroxylation pathway.
A non-13-hydroxylation pathway involving GA₄, GA₁₅, GA₂₄ GA₃₇ and GA₃₆ also originated from GA₁₂. The terminal product of this pathway was the 3β-hydroxy C₁₉-gibberellin, GA₄.     

Photosynthesis in Salsola Species (Chenopodiaceae) from Southern Africa Relative to their C4 Syndrome Origin and their African-Asian Arid Zone Migration Pathways

Abstract: Over 60 Salsola species of Chenopodiaceae from South Africa were studied for their photosynthesis type, using δ¹³C analysis and light microscopy of leaf anatomy. These species cover about 70 % of the total list of Southern African Salsola species and grow naturally in South and Southwest African desert regions. All species are shrubby forms and belong to the single subsection Caroxylon. Only C₄ photosynthesis was found in the Salsola species determined with ¹³C/¹²C carbon isotope discrimination values that ranged from - 11.04 to - 14.03 % (PDB), plus the presence of a Kranz type assimilation tissue anatomy. The apparent absence of C₃ in Salsola in South and Southwest Africa and the known presence of C₃ and C₃ - C₄ intermediate photosynthesis in Caroxylon, Salsola species in Asia strongly indicate that the genus Salsola originated in Asia and later migrated to South Africa.     

Effect of nitrogen nutrition on photosynthesis and growth in C4Panicum species

Abstract. The growth and photosynthetic responses to high and low N nutrition were measured in 2 NADP-malic enzyme and 4 NAD-malic enzyme C₄ subtype Panicum species to evaluate whether differences in C₄ photosynthetic biochemistry result in differences in the N requirement for growth. All species had lower biomass production, photosynthesis rates, and shoot N concentrations at low N, and no consistent differences between the C₄ subtypes were apparent. The assimilation rates (biomass accumulated over the period of growth) for the NADP-malic enzyme species were higher than the NAD-malic enzyme species at high N but not at low N. When assimilation rates were evaluated on a shoot N basis a higher N-use-efficiency was found for the NADP-malic enzyme species at high N. Thus the NADP-malic enzyme Panicum species had a greater amount of growth for a given shoot N concentration, but only above a certain level of shoot N concentrations.     

The evolutionary relationship between stomatal mechanism, crassulacean acid metabolism and C4 photosynthesis

Abstract. All of the features of crassulacean acid metabolism (CAM) and most characteristics of C₄ photosynthesis are exhibited by stomatal guard cells. It is proposed that CAM and possibly also C₄ photosynthesis result from the expression in photosynthetic cells of genetic information which is expressed only in guard cells of C₃ plants.     

Effect of H2 evolution on 15N2 fixation, C2H2 reduction and relative efficiency of leguminous symbionts

The (C₂H₄+ H_2(C2H₂))/¹⁵N₂ ratios of 15 clover- Rhizobium symbionts. soybean, and black medick symbionts were measured. Relative efficiency based on the C2H4 production and on ¹⁵N₂ incorporation were compared, and in most symbionts there was little difference between the two measures of relative efficiency. Total measurable electron flux through nitrogenase during acetylene reduction and ¹⁵N₂ incorporation were nearly equal for most symbionts studied. The relative efficiency and the (C₂H₄+ H_2(C2H₂))/¹⁵N₂ ratio showed an inverse correlation. Use of this ratio appears preferable to use of the ratio of C2H2 reduction/N₂ reduction. Some evolution of H₂ was observed in the presence of C₂H₂.     

N2(C2H2)-fixation in early stages of a primary succession on a reclaimed salt marsh

Nitrogen fixation activity was determined for Lotus tenuis. Medicago lupulina and Trifolium pratense . The three species grew in clones in grassland in an area reclaimed from brackish water in the 1940s. The N₂[C₂H₂]-fixation was measured in soil cores throughout 1974 and 1975. From cores taken in dense and uniform stands of the species, the yearly N₂[C₂H₂]-fixation at maximum cover was estimated. L. tenuis fixed about 4 g N m⁻² yr⁻¹ (area with max. cover 130%), i.e. 30–56% of its requirement. Both M. lupulina and T. pratense fixed about 7 g N m⁻² yr⁻¹ (maximum cover 37% and 80%) i.e. 67% of their N-requirement. Average N₂[C₂H₂]-fixation for the whole area was 0.4 g N m⁻² yr⁻¹, considerably less than the N-addition through rainfall.