Plants under Climatic Stress: V. Chilling and Light Effects on Radiocarbon Exchange between Photosynthetic Intermediates of Sorghum

Patterns of radiocarbon exchange between photosynthetic intermediates of the chilling sensitive Sorghum bicolor were modified by exposure to a combined environmental stress of low temperature (10 C) and moderate light levels (170 w·m⁻², visible). Pulse chase experiments with¹⁴CO₂ showed that this stress initially slowed the release of photosynthetically absorbed radiocarbon from malate. Further exposure caused an increased proportion of the radiocarbon to accumulate in aspartate. This trend continued, so that after 30 hours, some 80% of absorbed radiocarbon remained in aspartate after 1 minute of chasing and subsequent release of carbon into the C₃ cycle was very slow. In Sorghum, chilling combined with light seemed to cause a restriction in an early step of the C₄ pathway before ultrastructural changes could be detected in the mesophyll chloroplasts.