Abstract: Availability of nitrogen almost always limits plant growth. Therefore, efficient use of nitrogen is essential for the plants. In upright plants, especially when they form dense plant stands, old, lower leaves are shaded by young, upper leaves. Nitrogenous compounds in such shaded leaves are degraded and re-allocated to the developing young, upper leaves. These processes raise efficiency of nitrogen use in photosynthetic production of the plant. For this to occur in the most effective way, leaves would need to sense their photosynthetic status in a plant and increase, maintain or decrease their photosynthetic capacity accordingly. Hypotheses that explain how a leaf can sense its photosynthetic status in the plant are reviewed. They include systems involving phytochrome, sugar-sensing, or cytokinin. Our experimental results with Helianthus annuus and Phaseolus vulgaris plants, which were subject to various shading treatments, are examined in the light of these hypotheses. Our experimental results favoured the sugar-sensing hypothesis: A leaf can sense demand of other plant parts for photosynthates produced by it and nitrogen abundance or deficiency by monitoring its sugar concentration. Problems that are to be challenged in the near future are also pointed out.