Development of the C4Photosynthetic Apparatus in Cotyledons and Leaves ofSalsola richteri(Chenopodiaceae)

Abstract

Development of characteristics of the C4 syndrome was studied in cotyledons and leaves of NADP‐ME type C4Salsola richteri, a desert shrub. This species has seeds in which the cotyledons contain chloroplasts, storage proteins, and lipid bodies but no starch. Following imbibition (day 0), tissue types are already apparent in the cotyledons, and the chloroplasts have extensive grana stacking, but Kranz type anatomy and C4 photosynthesis have not developed. At day 0, there is high Rubisco and low phosphoenolpyruvate carboxylase (PEPC) content, distributed throughout all tissue. After 15 d of development in the dark, the cotyledons were in a C3‐like default condition with three distinct layers of chlorenchyma (hypodermis, mesophyll, and bundle sheath [BS]); all contained Rubisco in their chloroplasts. Light was required for development of the C4 syndrome, including differentiation of chloroplasts in mesophyll and BS cells, development of thick BS cell walls, selective compartmentation of Rubisco in BS cells and PEPC in mesophyll cells, and development of intercellular air spaces around hypodermal and mesophyll cells. During cotyledon development in the light, Rubisco content decreased and PEPC content increased, and starch synthesis was associated with tissue‐specific compartmentation of Rubisco. There was also a C3 to C4 transition during leaf development. After 3 d of leaf development in the light, cell division was still occurring, the tissue was not fully differentiated, and there was no cell‐specific compartmentation of Rubisco or PEPC. There was the beginning of tissue and organelle differentiation together with Rubisco and PEPC compartmentation in 5‐d leaves. In mature leaves, Kranz anatomy was fully developed with selective compartmentation of PEPC in mesophyll.