Plants adjust leaf senescence in response to light heterogeneity across branches

The senescence of productive leaves is expected to decrease canopy photosynthesis, but it may allow plants to use resorbed nitrogen for new growth. If plants use leaf senescence as part of a competitive strategy, we expect that leaves with low competitive value would be sacrificed to supply nitrogen to leaves with high competitive value. However, when plants have multiple branches, it is unclear if plants can target leaves in shaded conditions for senescence and whether they prioritize the growth of their shaded or unshaded branches. We measured changes in leaf nitrogen in sunflowers (Helianthus annuus) grown with two symmetrical branches that were then subjected to either shaded or unshaded light conditions. We complemented the experiment with a mathematical model of leaf senescence to test whether our proposed physiological mechanisms could explain the results. When both branches were grown in low light, leaf senescence was inhibited relative to plants with both branches grown in high light. For plants with one branch in high light and one branch in low light, the senescence of older leaves was inhibited in the high-light branch, but accelerated in the low-light branch. This was associated with an increase in the growth rate of the youngest leaves in the low-light branch. Our mathematical model reproduced these results, suggesting that leaf senescence may be mediated by the carbon status of the plant. The pattern of leaf senescence and growth in this experiment was consistent with a strategy that prioritizes competing with neighbors for light rather than avoiding competition.