Salsola arbusculiformis, a C3-C4 intermediate in Salsoleae (Chenopodiaceae)

Salsola arbusculiformis is identified as a C-3-C-4 intermediate species based on anatomical, biochemical and physiological characteristics. This is the first report of a naturally occurring intermediate species in the Chenopodiaceae, the family with the largest number of C-4 species amongst the dicots. In the genus Salsola, most species have Salsoloid anatomy with Kranz type bundle sheath cells and C-4 photosynthesis, while a few species have Sympegmoid anatomy and were found to have non-Kranz type bundle sheath cells and C-3 photosynthesis. In the cylindrical leaves of C-4 Salsola with Salsoloid type anatomy, there is a continuous layer of distinct, chlorenchymatous Kranz type bundle sheath cells surrounded by a single layer of mesophyll cells; whereas species with Sympegmoid type anatomy have an indistinct bundle sheath with few chloroplasts and multiple layers of chlorenchymatous mesophyll cells. However, S. arbusculiformis has intermediate anatomical features. While it has two-to-three layers of mesophyll cells, characteristic of Sympegmoid anatomy, it has distinctive, Kranz-like bundle sheath cells with numerous chloroplasts and mitochondria. Measurements of its CO2 compensation point and CO2 response of photosynthesis show S. arbusculiformis functions as an intermediate species with reduced levels of photorespiration. The primary means of reducing photo respiration is suggested to be by refixing photorespired CO2 in bundle sheath cells, since analysis of photosynthetic enzymes (activity and immunolocalization) and (CO2)-C-14 labelling of initial fixation products suggests minimal operation of a C-4 cycle. (C) 2001 Annals of Botany Company.