Photosynthesis, respiration and conservative carbon use efficiency of four field grown crops

The present work aims at testing the hypothesis that carbon use efficiency (CUE) of sunflower, grain sorghum, wheat and chickpea crops, having different photosynthetic pathways (C3, C4) and yield composition (carbohydrates, proteins, lipids), will hold constant over the natural thermal regime occurring during the entire crop cycle in the open field. All crops were well watered. Sunflower and sorghum had two treatments of nitrogen application, while wheat had only one suitable level of nitrogen, and chickpea had no nitrogen at all. Canopy temperature, day-time net photosynthesis (P), and night-time respiration (R) were monitored by closed-system canopy chambers, properly automated for continuous measurements. Night-time respiration response to temperature and noon-time photosynthesis were measured at leaf scale, as well. Results showed a strictly linear relationship (i.e. constant CUE) between cumulative P (∑P) and cumulative R (∑R) over the entire cycle of sorghum (slope=2.28) and wheat (3.35), and up to anthesis of sunflower (2.08) and chickpea (2.83), irrespective of the thermal regimes evolution and nitrogen nutritional levels. The same linearity was maintained when relationships were observed in terms of biomass versus ∑R. In sunflower, significant deviation from linearity is observed after anthesis, with a difference between the two nitrogen treatments. No conclusions could be drawn for post-anthesis chickpea due to the interruption of the experiment caused by an intense thunderstorm. Leaf-scale respiration responses to temperature were insufficient to explain the corresponding behaviour at canopy-level.