Dark chilling inhibition of photosynthesis and symbiotic nitrogen fixation in soybean during pod filling

The growth stage of a soybean [Glycine max (L.) Merrill] plant may influence its physiological response to dark chilling. Opposed to vegetative development, the intense nutrient and energy requirements of the developing seeds during pod filling could cause additional chilling damage and decreased recovery capacity. Previously, we investigated dark chilling tolerance during vegetative development in two soybean genotypes, 'Maple Arrow' and 'Fiskeby V' and consistently found that photosynthesis and symbiotic nitrogen fixation (SNF) was less affected by dark chilling in 'Maple Arrow'. In this study we describe the dark chilling response of the same genotypes during pod filling. Our aim was to establish whether the potential selection criteria for dark chilling tolerance, identified during vegetative development, was equally sensitive during pod filling. The results indicate that photosynthesis is less affected by dark chilling in 'Maple Arrow' than in 'Fiskeby V', not only during vegetative development, but also during the critical reproductive stage of pod filling. 'Fiskeby V' also lacks the ability to restore normal photosynthetic capacity during an extended recovery treatment. The decrease of nodule ureide content indicates that SNF was inhibited to a similar extent in both genotypes. Nodule ureide content was reduced more than stem ureide content, suggesting that the former is a more sensitive indicator of chilling stress effects on SNF. The results indicate that certain photosynthetic and fluorescence parameters are sensitive indicators of dark chilling tolerance throughout plant development and should prove valuable in future breeding programmes aimed at increasing the chilling tolerance of soybean.