Several abiotic stresses destroy leaf chlorophyll and this is most prominent for drought stress. Leaf chlorophyll determination can be done by chemical analysis or by indirect in situ optical method. The optical sensing of leaf chlorophyll has become a popular method for scoring leaf greenness and even for indirectly estimating leaf nitrogen content.
Leaf chlorophyll cannot be used to assess the level of plant water deficit or water status. It can however be an estimate of the relative stress injury in terms of chlorophyll breakdown. This is a limited estimate of plant stress since it refers to stress only in terms of chlorophyll breakdown. Whether other plant functions are correlated with chlorophyll breakdown in a direct or indirect manner is another question.
Leaf chlorophyll determination is done spectrophotometrically in a solution of chlorophyll extracted from the leaf. Complete leaf chlorophyll extraction is performed by the slower and classical solvent, acetone or by the faster extraction with dimethyl sulfoxide (DMSO) or N,N-Dimethylformamide. Both solvents provide faster and simpler extraction than with acetone (see below).
Leaf chlorophyll estimation by the optical method is based on leaf reflectance as it related in a nonlinear function to chlorophyll content. This is a fast and non-evasive method that is well adapted to field conditions. The relations between leaf reflectance and chlorophyll content are affected by plant species, certain leaf properties and time of day in terms of available light. These offer no problem when the instrument is used as a relative comparison among genotypes in selection work as long as standard comparable leaves are used in a consistent manner. However, for accurate estimation of leaf chlorophyll content the instrument should be calibrated against the chemical analysis for the given plant materials. Two well performing instruments are available, the Minolta SPAD502 and Opi-Science CCM-200. This article does not make a judgment and users should consult the specifications of each at their respective web sites: at Minolta and Opti-Science.
ü Chlorophyll analysis by DMSO extraction as well as a typical test of the optical method is well described by Richardson et al., 2002 for Birch leaves.
ü The N,N-Dimethylformamide extraction method is described by Moran and Porath, 1980.
ü HPLC method is available for chlorophylls and carotenoids.
--- See also Spectroradiometric canopy signatures below.