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The natural geographical occurrence, carbon assimilation, and structural and biochemical diversity of species with C photosynthesis in the vegetation of Mongolia was studied. The Mongolian flora was screened for C plants by using C/C isotope fractionation, determining the early products of CO fixation, microscopy of leaf mesophyll cell anatomy, and from reported literature data. Eighty C species were found among eight families: Amaranthaceae, Chenopodiaceae, Euphorbiaceae, Molluginaceae, Poaceae, Polygonaceae, Portulacaceae and Zygophyllaceae. Most of the C4 species were in three families: Chenopodiceae (41 species), Poaceae (25 species) and Polygonaceae, genus Calligonum (6 species). Some new C species in Chenopodiaceae, Poaceae and Polygonaceae were detected. C Chenopodiaceae species make up 45% of the total chenopods and are very important ecologically in saline areas and in cold arid deserts. C grasses make up about 10% of the total Poaceae species and these species naturally concentrate in steppe zones. Naturalized grasses with Kranz anatomy,of genera such as Setaria, Echinochloa, Eragrostis, Panicum and Chloris, were found in almost all the botanical-geographical regions of Mongolia, where they commonly occur in annually disturbed areas and desert oases. We analyzed the relationships between the occurrence of C plants in 16 natural botanical-geographical regions of Mongolia and their major climatic influences. The proportion of C species increases with decreasing geographical latitude and along the north-to-south temperature gradient; however grasses and chenopods differ in their responses to climate. The abundance of Chenopodiaceae species was closely correlated with aridity, but the distribution of the C grasses was more dependent on temperature. Also, we found a unique distribution of different C Chenopodiaceae structural and biochemical subtypes along the aridity gradient. NADP-malic enzyme (NADP-ME) tree-like species with a salsoloid type of Kranz anatomy, such as Haloxylon ammodendron and Iljinia regelii, plus shrubby Salsola and Anabasis species, were the plants most resistant to ecological stress and conditions in highly arid Gobian deserts with less than 100 mm of annual precipitation. Most of the annual C chenopod species were halophytes, succulent, and occurred in saline and arid environments in steppe and desert regions. The relative abundance of C succulent chenopod species also increased along the aridity gradient. Native C grasses were mainly annual and perennial species from the Cynodonteae tribe with NAD-ME and PEP-carboxykinase (PEP-CK) photosynthetic types. They occurred across much of Mongolia, but were most common in steppe zones where they are often dominant in grazing ecosystems.

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ABSTRACTpropose that they should be more correctly termed sucrolysis and sucroneogenesis. Before recent work it was customary to assume that sucrose synthase action resulted in the formation of UDP-glucose and then other nucleotide sugars leading into sugar polymer synthesis, such as plant cell walls. However, a substrate level pool of PPi was measured in plants (2,7,27), and we successfully tested the pyrophosphorolysis of UDPglucose feeding glucose 1-P directly into plant metabolism (32). These steps are an integral part of the recently proposed sucrose synthase pathway (1,15,30). Then, of course, from glycolysis carbon can be directed into essentially every metabolic activity of a cell. Therefore, the ability of a tissue or organ to metabolize sucrose must be one determinant of sink strength. Here we have tested the feasibility of biochemically measuring sucrose sink strength by assaying sucrose cleavage activities, i.e. sucrolysis via either the invertases or by the sucrose synthase pathway. The reasons we developed this UDP and PPi-dependent assay for sucrose synthase activity were (a) to measure activity in the sucrose breakdown direction whereas many other workers measured the opposite, namely sucrose synthesis (3,4,22,26), and (b) others who measured sucrose breakdown often assayed for UDP-glucose accumulation as a precursor of the synthesis of cell walls or other nucleotide sugars (6, 22). But in our assay we couple through the PPi-dependent UDP-glucopyrophosphorylase, which is very active in plants, to the formation of glucose 1-P which feeds carbon directly into glycolysis and possibly on to starch formation (31,32

Plant cells have two cytoplasmic pathways of glycolysis and gluconeogenesis for the reversible interconversion of fructose 6‐phosphate (F‐6‐P) and fructose 1,6‐bisphosphate (F‐1,6‐P2). One pathway is described as a maintenance pathway that is catalyzed by a nucleotide triphosphate‐dependent phosphofructokinase (EC 2.7.1.11; ATP‐PFK) glycolytically and a F‐1,6 bisphosphatase (EC 3.1.3.11) gluconeogenically. These are non‐equilibrium reactions that are energy consuming. The second pathway, described as an adaptive pathway, is catalyzed by a readily reversible pyrophosphate‐dependent phosphofructokinase (EC 2.7.1.90; PP‐PFK) in an equilibrium reaction that conserves energy through the utilization and the synthesis of pyrophosphate. A constitutive regulator cycle is also present for the synthesis and hydrolysis of fructose 2,6‐bisphosphate (F‐2,6‐P2) via a 2‐kinase and a 2‐phosphatase, respectively. The pathway catalyzed by the ATP‐PFK and F‐1,6‐bisphosphatase, the maintenance pathway, is fairly constant in maximum activity in various plant tissues and shows less regulation by F‐2,6‐P2. Plants use F‐2,6‐P2 initially to regulate the adaptive pathway at the reversible PPi‐PFK step. The adaptive pathway, catalyzed by PPi‐PFK, varies in maximum activity with a variety of phenomena such as plant development or changing biological and physical environments. Plants can change F‐2,6‐P2 levels rapidly, in less than 1 min when subjected to rapid environmental change, or change levels slowly over periods of hours and days as tissues develop. Both types of change enable plants to cope with the environmental and developmental changes that occur during their lifetimes. The two pathways of sugar metabolism can be efficiently linked by the cycling of uridylates and pyrophosphate required for sucrose breakdown via a proposed sucrose synthase pathway. The breakdown of sucrose via the sucrose synthase pathway requires half the net energy of breakdown via the invertase pathway. Pyrophosphate occurs in plant tissues as a substrate pool for biosynthetic reactions such as the PPi‐PFK or uridine diphosphate glucose pyrophosphorylase (EC 2.7.7.9; UDPG pyrophosphorylase) that function in the breakdown of imported sucrose. Also, pyrophosphate links the two glycolytic/gluco‐neogenic pathways; and in a reciprocal manner pyrophosphate is produced as an energy source during gluconeogenic carbon flow from F‐1,6‐P2 toward sucrose synthesis.