Location: Crop Production Systems Research Unit
Title: Spectral reflectance changes with the effect of sample storage on corn leaves excised from field Authors
Submitted to: Journal of Agricultural Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: June 11, 2014
Publication Date: N/A
Interpretive Summary: It is a common practice for researchers to collect leaf samples in field, transport to laboratories, and measure spectral reflectance of the leaves using a hyperspectral sensor. It is assumed that the reflectance measured on sampled leaves characterizes the plant canopies when viewed from above with airborne or space borne hyperspectral sensor. The leaf samples begin to wilt when they are clipped or excised from the plant, however there have been very few studies that examined the effect of leaf wilting on the spectral reflectance or how to preserve the leaf samples of common agricultural plants. The scientists in USDA-ARS Crop Production Systems Research Unit, Stoneville, MS, and Geosystems Research Institute, Mississippi State University have collaboratively conducted a study on corn leaves to model the quantitative effects of the elapsed time between leaf sample collection and spectral reflectance measurement, and to determine if it is useful to refrigerate leaf samples in a cooler during elapsed time. The research shows that corn leaves respond differently from cotton and soybean leaves, and proposes that a different equation be used to model the reflectance spectra of wilting corn leaves. The model was then used to determine how much time should elapse (after the leaves are collected) before the spectral reflectance of the leaves no longer characterizes the plant canopy. This study shows that the wilting effects are more pronounced than commonly believed and largely unaffected by refrigeration in a cooler. Scientists should consider the effects of the elapsed time when designing their field studies.
Technical Abstract: Excising leaves from plants in field studies and transporting them to a laboratory for hyperspectral measurement is a common practice. It is assumed that the leaf spectra characterize the plant canopies when viewed from above with an airborne or spaceborne sensor. However, few studies have explored the effect of elapsed time between plant excision and reflectance measurement. This paper compares the effect of the elapsed time when the excised corn leaves are stored in paper bags kept in a cooler at 17.2 degrees Celsius and outside a cooler at 22.9 degrees Celsius. Reflectance measurements were obtained 15 minutes after plant excision, then again at 1, 2, 3, 4, 5, 6, and 24 hours after excision. Each hyperspectral band was modeled independently using a piecewise function consisting of a linear part for the first hour after plant excision, and an exponential function after the first hour. The results show that for the first hour, the storage technique does not matter, but after the first hour, the leaves stored outside the cooler showed less change than the leaves stored in the cooler. Furthermore, the leaves required approximately 30 minutes after excision for a large percentage of hyperspectral bands to drift beyond the level of significance (as determined by the mean plus or minus two standard deviations). Thus, the hyperspectral measurements should be taken before 30 minutes elapses. These facts should be considered when designing a field study or collecting ground truth data for a remote sensing campaign.