Changes in O-J-I-P Fluorescence Rise Kinetics During Dark Chilling Provide Insight into Genotype-Specific Effects on Photosynthesis and N2 Fixation in Soybean

Dark chilling affects growth and yield of warm-climate crops such as soybean [Glycine max (L.) Merr.]. Several studies have investigated chilling stress effects on photosynthesis and otheraspects of metabolism, but none have compared effects of whole plant chilling (shoots and roots) with that of aboveground chilling in legumes. Thisis important since low root temperatures might induce additional constraints, such as inhibition of N2 fixation, thereby aggravating chilling stress symptoms. Effects of dark chilling on photosystem II, shoot growth, leaf ureide content and photosyntheticcapaCity were studied in two soybean genotypes, Highveld Top (chilling tolerant) and PAN809 (chilling sensitive), in experiments comparing effects of whole plant chilling (WPC) with that of shoot chilling (SC). Both treatments inhibited shoot growth in PAN809, but not Highveld Top. Also, WPC in PAN809 caused a decrease in leaf ureide content followed by severe chlorosis and alterations in O-J-I-P fluorescence rise kinetics, distinct from SC. A noteworthy difference was the appearance of a ?K-peak in the O-J-I-P fluorescence rise in response to WPC. These genotypic and treatment differences also reflected in thedegree of inhibition of CO2 assimilation rates. The appearance of a ΔK-peak, coupled with growth inhibition, reduced ureide content, chlorosis and lower CO2 assimilation rates, provide novel mechanistic information about how WPC might have aggravated chilling stress symptoms in PAN809.