Irradiance and spectral quality affect chlorosis of barley primary leaves during growth in elevated carbon dioxide

The development of chlorosis was studied in primary leaves of barley plants (Hordeum vulgare L. cv. Brant) grown at ambient and twice-ambient CO, partial pressures. Leaf yellowing was observed 17 d after sowing when plants were grown in controlled environment chambers equipped with high-intensify discharge lamps at an irradiance of 800 mu mol quanta m(-2) s(-1). The extent of leaf yellowing, measured as changes of total chlorophyll, increased when the CO, partial pressure was raised from 37 to 70 Pa. Chlorosis was increased further by increasing the irradiance from 800 to 1100 mu mol quanta m(-2) s(-1). Rates of photosynthetic O-2 evolution by primary leaves,measured 17 d after sowing, were 20% lower for elevated compared with ambient CO2-grown plants. This result agreed with the level of chlorosis. However soluble protein, Rubisco protein (ribulose, 1,5-bisphosphate carboxylase/oxygenase), and initial and total Rubisco activity 17 d after sowing were unaffected by CO2 enrichment and the extent of chlorosis. Leaf starch, sucrose, and glucose were increased by elevated CO2 treatment at almost every sampling. However,only glucose was correlated with leaf damage. Leaf yellowing also was observed on primary leaves of plants grown under microwave-powered sulfur lamps at 800 but not at 550 mu mol quanta m(-2) s(-1). The extent of leaf yellowing on plants grown under microwave-powered sulfur lamps was unaffected by CO2 enrichment. It was concluded that leaf yellowing was influenced by irradiance, photoquality, and CO2 enrichment. Photobleaching of antenna chlorophyll, rather than premature senescence, was the most Likely cause of visible leaf injury in barley.