canopy29. The upper leaves in a canopy may show elevated photo-protective responses. On the other hand, the partially shaded lower leaves may have different contents of photosynthetic components from the upper leaves. Consequently, lower leaves have a reduced overall photosynthetic capacity in normal light but equally efficient radiation use efficiency per unit of N at low light intensities. Such canopies would be more efficient. It has been argued that the majority of photosynthesis in field occurs at non saturating light; therefore, it would be more profitable to improve genetically photosynthetic efficiency at low light intensities30. Maintaining green leaf area longer, particularly after anthesis when there is usually a rapid decline in leaf area index, is another important means of increasing total crop photosynthesis and hence biomass production through increased and extended light interception. Indeed, a longer duration of leaf photosynthetic activity has contributed to increased yield in most major crops23. In maize, there has been an increase in the duration of photosynthetic activity by the leaves, manifested in their greater ‘stay green’31. A slower decline in the photosynthetic activity of canopies has been reported in soybean (Glycine max (L.) Merr.)32 and rice33. Improved agronomy and crop protection have made such changes possible34. Moreover, higher N availability also led to selection for increased chlorophyll and rubisco content in wheat. Genetic manipulation of the synthesis of cytokinins has resulted in a delay of leaf senescence and an increase in growth rate of tobacco35. In pulses, there is faster senescence of leaves after flowering due to the mobilization of leaf N and rubisco for development of protein-rich seeds36–39. Mobilization of leaf nitrogen decreases leaf photosynthesis, induces senescence and restricts the duration of seed-fill period40–42. In chickpea, however, leaf N mobilization was decreased by irrigation after flowering under north Indian conditions. This resulted in decreased HI, thus indicating the significance of such mobilization for seed yield in chickpea39,4