Effects on both the roots and shoots of soybean during dark chilling determine the nature and extent of photosynthesis inhibition

The relative contribution of low soil and air temperatures towards the overall inhibition of photosynthesis in soybean is still unclear. The mechanisms involved in the dark chilling-induced inhibition of photosynthesis were explored further in a chilling tolerant (Highveld Top) and sensitive (PAN809) soybean genotype in experiments where low soil temperatures were present (whole plant chilling, WPC) or absent (shoot-localised chilling, SC). Initially (after three nights of chilling) both the WPC and SC treatments induced the same symptoms in PAN809. These symptoms could thus be ascribed to chilling stress effects on the shoots. Typical symptoms included reduced CO2 assimilation capacity, inhibition of photosystem II function and lower chloroplast fructose-1,6-bisphosphatase (cFBPase) and sucrose-phosphate-synthase (SPS) activity. When the nodulated root systems of PAN809 were also exposed to low temperatures (WPC treatment), additional constraints gradually developed, which were not observed in Highveld Top. Novel evidence is provided showing that the response in PAN809 is influenced by whether whole-plant or shoot-localised dark chilling occurs and that cFBPase is specifically targeted resulting in severe inhibition of CO2 assimilation capacity.

• Shoot and whole-plant chilling initially induced similar photosynthetic inhibition in soybean. • These symptoms included inhibition of PSII, chloroplast FBPase and SPS activity. • However, when nodulated root systems are also chilled, additional constraints gradually developed. • Chloroplast FBPase is specifically targeted resulting in severe inhibition of CO2 assimilation capacity.