Grass Evolution, the Kranz Syndrome, ¹³C/¹²C Ratios, and Continental Drift

Abstract

THE Kranz syndrome is a group of related anatomical, cytological and physiological characters reported one way or another in about 10 families of angiospermous plants. The syndrome centres around the CO₂ fixation aspect of photosynthesis, including synthesis of C₄-dicarboxylic acids as the initial products of CO₂ assimilation, utilization of most of the available CO₂ producing a high ¹³C/¹²C ratio, having maximum efficiency at high temperatures, and all related to a sheath around the bundles in the leaf. All present evidence indicates that detection of any one of the aspects of the syndrome denotes the whole syndrome¹. The syndrome is uncommon among plants including angiosperms and is therefore considered to be the evolved or derived condition. Although in Atriplex² it seems to have evolved recently within the genus, in the Gramineae the evolution of the Kranz syndrome seems to have occurred only two or three times early in grass evolution. This assumption, which is fundamental in this discussion, is based on the uniformity of tribes and subfamilies as Kranz or non-Kranz, and the proven predictability of the syndrome from single aspects of it (in this work from the ¹³C/¹²C ratio), from various other characters, or from classification³. The only exception reported, of Kranz and non-Kranz species within one grass genus, is within Panicum⁴ where the subgenus Dichanthelium is non-Kranz whereas the more typical species of the subgenus Eupanicum are characterized by the Kranz syndrome (Table 1). Recently we found, by determination of ¹³C/¹²C ratios of over 180 species, in 110 genera, of forty tribes of grasses, evidence of both types also among species of Eupanicum as well as among genera of the tribe Paniceae and among tribes of the subfamily Panicoideae (Table 1). It is this diversity of panicoid tribes (many of which are small and endemic), of the tribe Paniceae, and the genus Panicum that provides clues to the likely evolution of the grass family, superimposed, of course, on a great deal of other morphological, anatomical, cytological, and physiological evidence^3,5. Furthermore, now that geology has accepted Wegenerian drifting continents and the break-up of an ancient single world-continent (Pangaea) by rifting under the concept of plate tectonics, evolution of the Gramineae can be discussed in terms of that concept; that is, Upper Jurassic–Middle Cretaceous separation of East Gondwana and the Middle to Late Cretaceous separation of South America from Africa, with the tropical belt approximately in its present relationship to the continents since Early Jurassic time⁶.