Replacing the nitrogen nutrition index by the chlorophyll meter to assess wheat N status

The performance of cultivars is strongly linked to the growing conditions that vary according to both controlled and uncontrolled experimental factors. Specifically, there is a need to control the efficiency of N use by wheat, Triticum aestivum L., to minimize nitrogen losses and deficiency. The nitrogen nutrition index (NNI) is a precise indicator of nitrogen status but it does not suit the users' practical constraints because it requires time-consuming measurements and destructive plant sampling at a precise growth stage. Here we tested the soil plant analysis development (SPAD) chlorophyll meter as an alternative to the nitrogen nutrition index (NNI). The chlorophyll meter is a more convenient, leaf clip-on device that determines the relative amount of chlorophyll present in plant leaves. We first identified which leaf should be used; we then compared SPAD and NNI data from various experiments. We also followed SPAD measurements around flowering time to determine a common time span of measurements for all the cultivars of a trial presenting a wide range of earliness. Our results show a non-cultivar-dependent, exponential relationship between the SPAD index and NNI at flowering, with a r(2) equal to 0.89. This result implies that the SPAD chlorophyll meter can be used as an alternative to NNI to measure N status in wheat. We also showed that SPAD measurements can be taken before flowering, e. g. during heading, to characterize nitrogen status at flowering. This result provides an organizational leeway to experimenters who can then follow more precisely the N status of their trials. Thus the SPAD index is a good substitute for NNI because it is convenient to use.