Nondestructive diagnostic test for nitrogen nutrition of grapevine (Vitis vinifera L.) based on dualex leaf-clip measurements in the field.

Crop nitrogen status is a major issue for crop yield and quality. It is usually assessed by destructive leaf or petiole tissue analysis. A quantitative nondestructive optical estimation of N sufficiency would be a great leap forward toward precision crop management. We therefore calibrated three optical indices against leaf nitrogen content: chlorophyll (Chl), epidermal flavonols, and the nitrogen balance index (NBI), which is the ratio of the former two indices. NBI was the best estimator of leaf N content measured by the Dumas or Kjeldahl method with a root-mean-square error smaller than 2 mg of N g(-1) dry weight, followed by Chl (3 mg g(-1)) and flavonols (4 mg g(-1)). This allowed us to propose the threshold values for the Dualex optical indices that characterize nitrogen supply to grapevines: the first is the threshold below which N supply to the vine can be considered deficient, and the second is the threshold above which N supply is excessive. For a putative optimal N content of 30 mg g(-1) < x < 40 mg g(-1), these thresholds are 30 µg cm(-2) < x < 40 µg cm(-2) for Chl and 11 < x < 18 for NBI at flowering. At bunch closure, for N thresholds of 22 < x < 32, Chl is 29 < x < 37 and NBI is 8 < x < 11, in respective units. These values should be verified and refined in the future for various growth regions and cultivars using the specified protocol. The sample size should be 36-60 leaves from a fixed node position, preferably node no. 5 from the tip of the shoot. An alternative to the use of the NBI would be to discard leaves that are not light exposed by checking their flavonol content and to deduce the N sufficiency directly from the Chl values.

A new optical leaf-clip meter for simultaneous non-destructive assessment of leaf chlorophyll and epidermal flavonoids.

We have characterized a new commercial chlorophyll (Chl) and flavonoid (Flav) meter called Dualex 4 Scientific (Dx4). We compared this device to two other Chl meters, the SPAD-502 and the CCM-200. In addition, Dx4 was compared to the leaf-clip Dualex 3 that measures only epidermal Flav. Dx4 is factory-calibrated to provide a linear response to increasing leaf Chl content in units of µg cm(-2), as opposed to both SPAD-502 and CCM-200 that have a non-linear response to leaf Chl content. Our comparative calibration by Chl extraction confirmed these responses. It seems that the linear response of Dx4 derives from the use of 710 nm as the sampling wavelength for transmittance. The major advantage of Dx4 is its simultaneous assessment of Chl and Flav on the same leaf spot. This allows the generation of the nitrogen balance index (NBI) used for crop surveys and nitrogen nutrition management. The Dx4 leaf clip, that incorporates a GPS receiver, can be useful for non-destructive estimation of leaf Chl and Flav contents for ecophysiological research and ground truthing of remote sensing of vegetation. In this work, we also propose a consensus equation for the transformation of SPAD units into leaf Chl content, for general use.