Leaf Greenness Meter to Assess Ozone Injury to Tomato Leaves

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Abstract
One of the more subtle plant responses to ozone may be a reduction in chlorophyll content of leaf tissue (3). Existing chlorophyll may degrade (premature senescence) or chlorophyll synthesis may decline. This reduction of chlorophyll may impair photo-synthetic processes, leading to a reduction in carbon fixation. Reduced chlorophyll concentration may be reversible over time following ozone exposure. A nondestructive method of repeatedly measuring the greenness of a leaf can assess initial response and recovery of a leaf from ozone exposure. The unitless numbers generated by a portable chlorophyll meter (SPAD-501, Minolta Corp.) correlate with actual chlorophyll concentration determined by traditional extraction methods (1). The unitless readings may themselves constitute a useful response variable, without conversion using a standard curve. This instrument is potentially very useful for making rapid, repeated, in situ measurements of the same leaf over time, and is therefore suited to studying plant recovery from ozone exposure. The chlorophyll meter may also be useful for measuring leaf greenness prior to environmental stress. These values may be used as a covariate to adjust the post-treatment observations, potentially reducing the variability of the data and increasing the precision of data analysis (2). The SPAD-501 portable chlorophyll meter was used to determine greenness of intact leaves before and following exposure of Tiny Tim’ and ‘New Yorker’ tomato (Lycopersicon esculentum L.) plants to ozone. The usefulness of these SPAD-501 readings as a covariate in analysis of covariance (2) was evaluated.

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