Light environment, nitrogen content, and carbon balance of basal leaves ofsunflower canopies

A good association between leaf N and photosynthetic photon flux density (P PFD) canopy distribution profiles has often been found. However, it is not clear whether N export from basal leaves anticipates the reduction in basal leaf PPFD that occurs as the canopy develops; our first objective was to e xamine this issue. We documented the dynamics of the light environment and specific leaf nitrogen (Nf) for target leaves at a single level of insertio n under sunflower (Helianthus annuus L.) canopies grown at 0.47, 1.18, and 4.76 plants m(-2) and contrasted observed Nf with the estimated value of Nf required to maximize net carbon exchange (NCE) for the actual PPFD impingi ng on target leaves. The second objective of this work was to define the re lationships between Nf and PPFD and between Nf and the spectral composition of Light impinging on basal leaves. Paired (red and far-red) photodiode se nsors placed on the adaxial leaf surface mere used to document light qualit y dynamics of basal leaves. Measured Nf was greater than or similar to that required to maximize NCE for 79% of the observations made between achievem ent of maximum target leaf area and senescence of these leaves. Nf shelved a strong association with both red/far-red ratio of light impinging on targ et leaves and PPFD, but the association tvas stronger for the former variab le. We conclude that N export from basal leaves does not precede PPFD reduc tion and that the red/far-red ratio is a better predictor of Nf gradients w ithin the canopy than PPFD,