Chloroplast Development and Photosynthesis in Echinochloa (Barnyard Grass) and Rice: O₂ and Flooding Stress

Abstract

Primary leaf plastid development and photosynthetic activity in Echinochloa (barnyard grass) were monitored during a 5 day time course of germination and growth in a nitrogen atmosphere. After 8 hours imbibition in the light but without oxygen, the proplastids were spheroidal and contained a few rudimentary stromal thylakoids. However, during the next 126 hours of anaerobiosis, plastid size and stromal thylakoid numbers increased markedly. Under anaerobiosis plastid morphology was also characterized by the development of prolamellar bodies, previously reported only in etiolated tissues, while changes in starch grains and plastoglobuli indicated carbohydrate and lipid metabolism, respectively. Upon exposure to air the arrested development of chloroplasts was reversed. Compared to etioplasts when exposed to light, however, there was a lag for the first 18 hours before the plastids developed rapidly into typical, C₄ mesophyll and bundle sheath chloroplasts.

Chlorophyll synthesis and development of photosynthetic capability began slowly when seedlings were transferred from N₂ to air. Whereas etiolated seedlings exhibited net CO₂ fixation after approximately 12 hours after exposure to light, anaerobically-grown seedlings did not show a positive CO₂ gas exchange until about 55 hours after transfer from N₂ (light) to air (light). The kinetics for CO₂ uptake more closely resemble those of RuBP carboxylase, the C₃ cycle enzyme, than they do PEP carboxylase. Upon transfer to air (light), “anaerobic” shoots had about 60% of their ultimate PEPcase, but less than 20% of their RuBPcase activity. The predominance of PEPcase over RuBPcase in shoots grown without oxygen also occurs in rice, a C₃ plant. Thus, when rice and oryzicola were transferred to air after germination in an N₂ (light) environment, the early-labeled ¹⁴CO₂ fixation products in both grasses were predominantly C₄ acids. Based on ¹⁴CO₂ labeled intermediates, however, oryzicola appeared capable of developing carbon cycle capability faster than rice. Such a head start may, in part, explain oryzicola’s ability to emerge and outcompete rice in field situations.