Contrasting responses of crop legumes and cereals to nitrogen availability

In nonagricultural systems, the relationship between intrinsic water-use efficiency (WUE_i ) and leaf nitrogen (N_area ) is known to be stronger for legumes than for nonlegumes. We tested whether these relationships are retained for major agricultural legumes and nonlegumes. We compared the response to N nutrition of WUE_i (and its component parts, photosynthesis (A_sat ) and stomatal conductance (g_s )) for legumes Cicer arietinum, Glycine max, Lupinus alba and Vicia faba, nonlegume dicots Brassica napus and Helianthus annus, and nonlegume cereals Hordeum vulgare and Triticum aestivum. Surprisingly, and in contrast to studied cereals and nonlegume dicots, N_area was positively related to photosynthesis in the legumes, explaining nearly half of the variance in A_sat . WUE_i was tightly coupled to N_area for agricultural legumes and nonlegume dicots, but not for cereal crops. Our analysis suggests that breeding efforts to reduce g_s in legumes could increase WUE_i by 120-218% while maintaining A_sat at nonlegume values. Physiologically informed breeding of legumes can enhance sustainable agriculture by reducing requirements for water and N.