Production efficiency in sunflower: The role of water and nitrogen stress

We monitored the impact of water stress, nitrogen stress, and the combination of both on the fractional interception of photosynthetically active radiation (fPAR) and growth per unit of intercepted radiation in sunflower canopies over a growing season. The efficiency with which intercepted radiation was converted into biomass (ε) was reduced by approximately 35% in the nitrogen-limited canopy and approximately 30% in the water-limited canopy. The relative effect of the stress treatments on ε was sensitive to assumptions concerning solar geometry. The impact of stress on fPAR depended more on whether the canopy was nitrogen- and/or water-limited. In the second half of the season, nitrogen stress reduced fPAR by approximately 25%, whereas water stress reduced fPAR by about 70%. Reduction in leaf area index, 45% and 85% for the nitrogen-stressed and water-stressed canopies, respectively, appeared to be a major factor in the fPAR reductions. In the water-stressed canopy, wilting also contributed to the reduced fPAR. Neither of the stress treatments affected the strong correlation between fPAR and simple ratio (SR) or the normalized difference vegetation index (NDVI). We used three approaches to calculate fPAR, with two based on a single sun position and one based on the sun position updated at hourly intervals. In general, ε was notably higher in the calculations with higher solar elevation angles.