Update 2011 Chlorophyll Meter Trial

Chlorophyll Meter Trail

A common question of interest to growers and plant scientists concerned the accuracy of a handheld leaf chlorophyll meter and it’s ability to predict leaf N content. To address this interest, leaf samples were collected in almond and pistachio in 2009 to test the performance of a chlorophyll meter. In the almond sampling, trees with chlorotic leaves due to zinc deficiency were sampled for chlorophyll content.

The optical absorbance spectra of the leaf samples collected for chlorophyll assay were measured. Discs cut from these leaves were extracted with solvent, shaken and centrifuged, resulting in a clear solutions, which were pipeted into glass curvettes for spectral measurement. The absorption spectra were recorded for each sample using a spectrophotometer. Concentrations of chlorophyll a and b were determined for each leaf samples on a surface area basis (ug/cm2).

Results

In some annual crops (e.g., corn) SPAD measurements are well correlated with leaf N content. However, in our trials the ability of SPAD measurements to predict N content was poor, (Fig.1). The main effects of species and leaf sampling date were both significant, indicating that the SPAD measurements were affected by the phenological age of the leaves and that a species-specific calibration would be needed.

It was possible, however, to use the SPAD meter to predict leaf chlorophyll content (r2 = 0.75 in almond for example, Fig. 2). In the Zn deficiency trial in almond, the SPAD measurements were able to distinguish between almond leaves with two different levels of chlorosis and agreed well with subjective assessments of Zn stress.