In situ optical meters are widely used to estimate leaf chlorophyll concentration, but non-uniform chlorophyll distribution causes optical measurements to vary widely among species for the same chlorophyll concentration. Over 30 studies have sought to quantify the in situ/in vitro (optical/absolute) relationship, but neither chlorophyll extraction nor measurement techniques for in vitro analysis have been consistent among studies. Here we: (1) review standard procedures for measurement of chlorophyll; (2) estimate the error associated with non-standard procedures; and (3) implement the most accurate methods to provide equations for conversion of optical to absolute chlorophyll for 22 species grown in multiple environments. Tests of five Minolta (model SPAD-502) and 25 Opti-Sciences (model CCM-200) meters, manufactured from 1992 to 2013, indicate that differences among replicate models are less than 5%. We thus developed equations for converting between units from these meter types. There was no significant effect of environment on the optical/absolute chlorophyll relationship. We derive the theoretical relationship between optical transmission ratios and absolute chlorophyll concentration and show how non-uniform distribution among species causes a variable, non-linear response. These results link in situ optical measurements with in vitro chlorophyll concentration and provide insight to strategies for radiation capture among diverse species.

to ε and ε to v ) and direct calibrations (i.e., relation of measured property to v ) are used in estimating v from Transmission line-type electromagnetic (EM) methods for estimat-EM signal measurements (e.g., travel time, impedance, ing soil volumetric water content ( v ) have advanced significantly in capacitor charge time, oscillation frequency, frequency recent years, with many sensing systems now available. To estimate v , EM systems make use of the dependence of soil dielectric permittivityThe sensing systems considered are a TDR cable tester (1502B Metallic Cable Tester, Tektronix Inc., Beaverton, OR) connected to a custom three-rod probe with 0.15-m-long, 3.20-

cases salinity. However, it is sometimes the case that a new sensor is promoted and distributed only to be Performance differences in the growing number of electromagnetic disapproved of years later due to poor measurement (EM) sensors designed to estimate soil water content from a variety of indirect measurements (e.g., from measured travel time, capacitance, performance. The cost to users in unreliable experimenfrequency shift) suggests the need for a standardized sensor character-tal results or reduced productivity for growers ultimately ization methodology. We suggest that characterization and evaluation falls back on the company in the form of a damaged of EM sensors, which currently lack citable standards, be performed

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