Structural, biochemical and geographical relations in Australian C4 grasses (Poaceae)

Abstract

I have studied aspects of the biology of Australian grasses (Poaceae) in relation to photosynthetic pathway. In a cytological investigation of interspecific relationships in the photosynthetically variable tribe Neurachneae (subfamily Panicoideae), the chromosomes of all species (C₃, C₃-C₄ , C₄) were found to be small (x= 2 µm) and either metacentric or submetacentric. The base number (x= 9) is typical for panicoid grasses; tetraploidy is the most common of the three ploidy levels (2x, 4x, 6x). Using the Bioclimate Prediction System, climatic profiles were compiled for each species anc: actual lnd predicted geographical distributions were mapped. In conjunction Jith selective biochemical sampling, a survey of c. 370 C₄ species was then undertaken to document them for leaf blade structural ueed as predictors of the three C₄ acid decarboxylat~on types Jf the C₄ pathway. In the largest Australian C₄ grass genus, Eragrostis (Chloridoideae), some species have features previously associated with the PEP carboxykinase type (PCK), but their main decarbohylation enzyme is NAD-malic enzyme (NAD-ME) as in species with typical ('classical') NAD-ME anatomy. An intermediate structure is also described. Other NAD-ME species with broadly 'PCK-like' anatomy were found in Enneapogon, Triodia, Triraphis (Chloridoideae) and Panicwn (Panicoideae). ALLoteropsis semialata (Panicoideae) is the first recorded PCK species with NADP-malic enzyme-like (NADP-ME) anatomy, and Eriachne and Pheidoehloa (Arundinoideae) contain the first known NADP-ME species with anatomy like that of 'classical' NAD-ME or PCK species. A schematic summary of all currently known structural/biochemical associations in grasses is presented, and possible physiological implications with regard to the maintenance of a high [C0₂] in PCR (Photosynthetic Carbon Reduction, or Kranz) tissue are discussed. Chlorophyll a:b ratios from all biochemical experiments were statistically analysed. Means (and standard errors of the mean) for each C₄ type are: NADP-ME, 4.45 (±0.04); NAD-ME, 4.01 (±0.03); PCK, 3.45 (±0.02). The differences between the means are statistically significant, even in comparisons across major taxonomic and/or anatomical groups. Cynoahloris macivorii and C. reynoldsensis (Chloridoideae) are unusual examples of spontaneous intergeneric hybrids between parental species of different C₄ types and leaf blade structures. Both were found to be structurally and biochemically intermediate between the NADME parent Cynodon daatylon and the PCK Chloris parent( s). Controlled, reciprocal crossing experiments between Cynodon and Chloris could explore genetic relationships between C₄ types. Estimates were made of the total number of native Australian C₄ grass species of each C₄ type: NADP-ME, 348 species; NAD-ME, 195 species; PCK, 65 species. All three types are most numerous in the megatherm seasonal bioclimate of northern Queensland. NADP-ME species dominate, species number-wise, in 48 out of 73 State and Territory subdivisions, with NAD-ME species dominant in the remainder (and codominant in two). NAD-ME species are proportionally at their most numerous in the megatherm/mesotherm arid bioclimate. The extent of the megatherm seasonal (rainfall) bioclimate is paralleled by the distribution of most PCK species, by Eragrostis species with centrifugal/peripheral PCR cell chloroplasts, and by a relatively high proportion of species of all C₄ types with a suberized lamella in their PCR cell walls. The physiological reasons for these correlations are unknown. Taxonomic, ecological and historical factors in relation to C₄ type distribution are discussed.