Occurrence of distinctive cells and effects of irradiance on vascular formation in leaves of shade-tolerant C4 grass Paspalum conjugatum.

The denser leaf vasculature of C4 plants than of C3 plants may suit rapid export of assimilates associated with their higher photosynthetic rate. However, some C4 grasses have partially reduced leaf vasculature with vascular bundle (VB)-free bundle-sheath cells called distinctive cells (DCs). The shade-tolerant C4 grass Paspalum conjugatum has such a reduced leaf vascular system with DCs. We examined whether irradiance during growth affects vascular formation in leaves of P. conjugatum grown under 100%, 30%, or 14% sunlight for 1 month alongside the C4 grass maize. Under all conditions, P. conjugatum leaves had partially reduced vasculature: DCs and incomplete small VBs without phloem occurred between VBs with a normal structure consisting of both xylem and phloem. Shaded plants had less phloem in the small VBs than the full-sunlit plants. In maize, however, all VBs always had both xylem and phloem under all light conditions. The net photosynthetic rate of both grasses was reduced under shade; that of P. conjugatum was always lower than that of maize under all light conditions, but was reduced less by shade than that of maize. The light compensation point was lower in P. conjugatum than in maize, indicating that P. conjugatum acclimatizes better to low light. The reduction of phloem in VBs of P. conjugatum may be an acclimatization to shade, because dense vasculature may be expensive for C4 plants growing in environments where the higher photosynthetic rate is not realized.

Photosynthesis and leaf structure of F1 hybrids between Cymbidium ensifolium (C3) and C. bicolor subsp. pubescens (CAM).

BACKGROUND AND AIMS: The introduction of crassulacean acid metabolism (CAM) into C3 crops has been considered as a means of improving water-use efficiency. In this study, we investigated photosynthetic and leaf structural traits in F1 hybrids between Cymbidium ensifolium (female C3 parent) and C. bicolor subsp. pubescens (male CAM parent) of the Orchidaceae. METHODS: Seven F1 hybrids produced through artificial pollination and in vitro culture were grown in a greenhouse with the parent plants. Structural, biochemical, and physiological traits involved in CAM in their leaves were investigated. KEY RESULTS: Cymbidium ensifolium accumulated very low levels of malate without diel fluctuation, whereas C. bicolor subsp. pubescens showed nocturnal accumulation and diurnal consumption of malate. The F1s also accumulated malate at night, but much less than C. bicolor subsp. pubescens. This feature was consistent with low nocturnal fixation of atmospheric CO2 in the F1s. δ 13C values of the F1s were intermediate between those of the parents. The leaf thickness was thicker in C. bicolor subsp. pubescens than in C. ensifolium, and those of the F1s were more similar to that of C. ensifolium. This was due to the difference in mesophyll cell size. The chloroplast coverage of mesophyll cell perimeter adjacent to intercellular air spaces of C. bicolor subsp. pubescens was lower than that of C. ensifolium, and those of the F1s were intermediate between them. Interestingly, one F1 had structural and physiological traits more similar to those of C. bicolor subsp. pubescens than the other F1s. Nevertheless, all F1s contained intermediate levels of phosphoenolpyruvate carboxylase but as much pyruvate,Pi dikinase as C. bicolor subsp. pubescens. CONCLUSIONS: CAM traits were intricately inherited in the F1 hybrids, the level of CAM expression varied widely among F1 plants, and the CAM traits examined were not necessarily co-ordinately transmitted to the F1s.

Effects of growth temperature and nitrogen nutrition on expression of C3-C4 intermediate traits in Chenopodium album.

Proto-Kranz plants represent an initial phase in the evolution from C3 to C3-C4 intermediate to C4 plants. The ecological and adaptive aspects of C3-C4 plants would provide an important clue to understand the evolution of C3-C4 plants. We investigated whether growth temperature and nitrogen (N) nutrition influence the expression of C3-C4 traits in Chenopodium album (proto-Kranz) in comparison with Chenopodium quinoa (C3). Plants were grown during 5 weeks at 20 or 30 °C under standard or low N supply levels (referred to as 20SN, 20LN, 30SN, and 30LN). Net photosynthetic rate and leaf N content were higher in 20SN and 30SN plants than in 20LN and 30LN plants of C. album but did not differ among growth conditions in C. quinoa. The CO2 compensation point (Γ) of C. album was lowest in 30LN plants (36 µmol mol-1), highest in 20SN plants (51 µmol mol-1), and intermediate in 20LN and 30SN plants, whereas Γ of C. quinoa did not differ among the growth conditions (51-52 µmol mol-1). The anatomical structure of leaves was not considerably affected by growth conditions in either species. However, ultrastructural observations in C. album showed that the number of mitochondria per mesophyll or bundle sheath (BS) cell was lower in 20LN and 30LN plants than in 20SN and 30SN plants. Immunohistochemical observations revealed that lower accumulation level of P-protein of glycine decarboxylase (GDC-P) in mesophyll mitochondria than in BS mitochondria is the major factor causing the decrease in Γ values in C. album plants grown under low N supply and high temperature. These results suggest that high growth temperature and low N supply lead to the expression of C3-C4 traits (the reduction of Γ) in the proto-Kranz plants of C. album through the regulation of GDC-P expression.

Coleataenia prionitis, a C4-like species in the Poaceae.

C4-like plants represent the penultimate stage of evolution from C3 to C4 plants. Although Coleataenia prionitis (formerly Panicum prionitis) has been described as a C4 plant, its leaf anatomy and gas exchange traits suggest that it may be a C4-like plant. Here, we reexamined the leaf structure and biochemical and physiological traits of photosynthesis in this grass. The large vascular bundles were surrounded by two layers of bundle sheath (BS): a colorless outer BS and a chloroplast-rich inner BS. Small vascular bundles, which generally had a single BS layer with various vascular structures, also occurred throughout the mesophyll together with BS cells not associated with vascular tissue. The mesophyll cells did not show a radial arrangement typical of Kranz anatomy. These features suggest that the leaf anatomy of C. prionitis is on the evolutionary pathway to a complete C4 Kranz type. Phosphoenolpyruvate carboxylase (PEPC) and pyruvate, Pi dikinase occurred in the mesophyll and outer BS. Glycine decarboxylase was confined to the inner BS. Ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco) accumulated in the mesophyll and both BSs. C. prionitis had biochemical traits of NADP-malic enzyme type, whereas its gas exchange traits were close to those of C4-like intermediate plants rather than C4 plants. A gas exchange study with a PEPC inhibitor suggested that Rubisco in the mesophyll could fix atmospheric CO2. These data demonstrate that C. prionitis is not a true C4 plant but should be considered as a C4-like plant.

Correction to: Transition from C3 to proto-Kranz to C3-C4 intermediate type in the genus Chenopodium (Chenopodiaceae).

The article Transition from C3 to Correspondence t.

Transition from C3 to proto-Kranz to C3-C4 intermediate type in the genus Chenopodium (Chenopodiaceae).

The Chenopodiaceae is one of the families including C4 species among eudicots. In this family, the genus Chenopodium is considered to include only C3 species. However, we report here a transition from C3 photosynthesis to proto-Kranz to C3-C4 intermediate type in Chenopodium. We investigated leaf anatomical and photosynthetic traits of 15 species, of which 8 species showed non-Kranz anatomy and a CO2 compensation point (Γ) typical of C3 plants. However, 5 species showed proto-Kranz anatomy and a C3-like Γ, whereas C. strictum showed leaf anatomy and a Γ typical of C3-C4 intermediates. Chenopodium album accessions examined included both proto-Kranz and C3-C4 intermediate types, depending on locality. Glycine decarboxylase, a key photorespiratory enzyme that is involved in the decarboxylation of glycine, was located predominantly in the mesophyll (M) cells of C3 species, in both M and bundle-sheath (BS) cells in proto-Kranz species, and exclusively in BS cells in C3-C4 intermediate species. The M/BS tissue area ratio, number of chloroplasts and mitochondria per BS cell, distribution of these organelles to the centripetal region of BS cells, the degree of inner positioning (vacuolar side of chloroplasts) of mitochondria in M cells, and the size of BS mitochondria also changed with the change in glycine decarboxylase localization. All Chenopodium species examined were C3-like regarding activities and amounts of C3 and C4 photosynthetic enzymes and δ13C values, suggesting that these species perform photosynthesis without contribution of the C4 cycle. This study demonstrates that Chenopodium is not a C3 genus and is valuable for studying evolution of C3-C4 intermediates.

Structural and photosynthetic re-acclimation to low light in C4 maize leaves that developed under high light.

BACKGROUND AND AIMS: C4 plants have higher photosynthetic capacity than C3 plants, but this advantage comes at an energetic cost that is problematic under low light. In the crop canopy, lower leaves first develop under high light but later experience low light because of mutual shading. To explore the re-acclimation of C4 leaves to low light, we investigated the structural and physiological changes of the leaves of maize plants grown in shaded pots. METHODS: Plants were first grown under high light, and then some of them were shaded (20 % of sunlight) for 3 weeks. Four types of leaves were examined: new leaves that developed under low light during shading (L), new leaves that developed under high light (H), mature leaves that developed under high light before shading and were then subjected to low light (H-L) and mature leaves that always experienced high light (H-H). KEY RESULTS: The leaf mass per area, nitrogen and chlorophyll contents per unit leaf area, chlorophyll a/b ratio and activities of C3 and C4 photosynthetic enzymes were lower in H-L than in H-H leaves and in L than in H leaves. Unlike L leaves, H-L leaves maintained the thickness and framework of the Kranz anatomy of H leaves, but chloroplast contents in H-L leaves were reduced. This reduction of chloroplast contents was achieved mainly by reducing the size of chloroplasts. Although grana of mesophyll chloroplasts were more developed in L leaves than in H leaves, there were no differences between H-L and H-H leaves. The light curves of photosynthesis in H-L and L leaves were very similar and showed traits of shade leaves. CONCLUSIONS: Mature maize leaves that developed under high light re-acclimate to low-light environments by adjusting their biochemical traits and chloroplast contents to resemble shade leaves but maintain the anatomical framework of sun leaves.