Application of digital images for nitrogen status diagnosis in three Vigna unguiculata cultivars

Abstract

The SPAD chlorophyll meter is widely used to indirectly monitor the nitrogen (N) status of various crops, which is important to ensure proper management of N fertilizers. Here, we propose that the freely available digital foliar analysis software AFSoft may be used to monitor N status with the same effectiveness as the SPAD meter. To verify the usefulness of AFSoft, we employed both SPAD meter readings and AFSoft note to estimate relative chlorophyll content in cowpea beans exposed to different N fertilization rates. The experiment followed a completely randomized design comprised of four replicates carried out in a factorial scheme, with five N fertilization rates (0, 50, 100, 200 and 300 mg pot-1) applied to three cowpea bean cultivars (BRS Itaim, BRS Guariba and BRS Novaera). At 45 days after emergence (DAE) of the plants, SPAD readings were obtained on the second trifoliate leaf. The whole leaf was subsequently digitally photographed and dried for the determination of total N concentration by applying acolour scale using AFSoft® software. There was no significant interaction observed between N fertilization rate and cultivar for the evaluated variables. For all cultivars, increasing the rate of N fertilization increased the total observed N concentration as determined by both SPAD index and AFSoft note. The AFSoft note value was positively correlated with the total N concentration (r=0.95, P≤0.05; r=0.80, P≤0.01; r=0.86, P≤0.01) and the SPAD readings (r=0.99, P≤0.01; r=0.97, P≤0.05; r=0.97, P≤0.05) for the Itaim, Guariba and Novaera cultivars. These results suggest that N concentration in the leaves of cowpea bean cultivars can be estimated using AFSoft, which may serve as an alternative to SPAD meter readings.