Photorespiration is a problem for plants in hot, dry conditions. When it is hot and dry, plants close their stomata to conserve water. As a result, oxygen from the light-dependent reactions accumulates in leaf tissues. Oxygen competes with carbon dioxide for the binding site on rubisco. Photorespiration occurs when oxygen levels in the leaf are high relative to carbon dioxide levels. Some groups of plants have adapted to these conditions by using alternative pathways to fix carbon. C₄ and CAM plants avoid photorespiration in different ways. C₄ plants use spatial separation. CAM plants use temporal separation.