Radiation capture and radiation use efficiency in response to N supply for crop species with contrasting canopies

Nitrogen (N) is a key determinant of plant productivity. A shortage of N can give three responses: (1) reduce leaf area (light capture) and maintain leaf N content (photosynthetic capacity); (2) maintain leaf area and reduce leaf N content; or (3) a combination of these responses. We examined these responses in contrasting crop canopies. Forage rape (C3 dicotyledons), wheat (C3 monocotyledons) and forage sorghum (C4 monocotyledons) were grown with fertiliser N supplies of 0, 50, 100 or 200kgN/ha. Crop biomass, leaf area index (LAI) and photosynthetically active radiation (PAR) interception were measured regularly, while accumulated intercepted PAR and radiation use efficiency (RUE) were calculated. Each crop exhibited distinct functional responses of biomass production to N shortage. For all crops specific leaf N (SLN), RUE, LAI and PAR capture were reduced in response to N shortage. However, the extent of the response differed between species. The SLN and RUE were reduced to a greater extent in forage rape than in wheat. In contrast, the LAI and PAR capture were reduced to a greater extent in wheat than in forage rape. Sorghum was intermediate between these two crops. Leaf inclination could account for these distinct responses – with forage rape's more horizontally inclined leaves, a change in LAI has a much smaller effect on PAR interception compared with wheat and sorghum. Under N shortage the amount of PAR intercepted per unit leaf N increased in forage rape but not in the other two crops. Alternatively leaf structure could account for these distinct responses – forage rape has larger petioles and midribs than wheat. Petioles and midribs have a large N content but only a small area, therefore to achieve given RUE, forage rape requires an extra 1.2gN/m2 leaf than wheat. Therefore the functional response that maximised growth under limited N supply was to reduce SLN and RUE to a greater extent than LAI and PAR interception for forage rape; reduce LAI and PAR interception to a greater extent than SLN and RUE for wheat; and reduce both RUE and LAI for sorghum to a similar extent. These were the observed responses for each species.