Remotely Measuring Chlorophyll Content in Corn Leaves with Differing Nitrogen Levels and Relative Water Content

A plant’s photosynthetic potential is directly proportional to the quantity of chlorophyll present in the leaf Measuring a crop’s physiological parameters provides information tissue. Rapid estimates of photosynthetic potential can for interpreting its response to the environment. Remote sensing is quickly becoming recognized as a valuable tool that has the potential be important for studies of gas exchange and the C to assess a wide variety of physiological properties over a large area cycle. Carter (1998) showed that normalized difference in a short amount of time. This study examined the relationship of vegetation index calculated with reflectance at 701 2 corn (Zea mays L.) leaf spectral response to its chlorophyll content nm and 820 2 nm or the simple ratio of the two is and relative water content. The effects of N stress and water stress strongly associated with photosynthesis and stomatal were examined on each of these physiological parameters. Over the conductance of a loblolly pine (Pinus taeda L.) canopy. years, several spectral indices have been introduced for a wide variety Verma et al. (1993) examined the simple ratio of near of applications. A number of these indices were evaluated for their infrared reflectance to red reflectance over unstressed ability to predict the parameters in question. The normalized differprairie vegetation. The conclusion was that a nearly ence between the first derivatives at 525 and 570 nm, as well as the linear relationship exists between simple ratio and the wavelength location of the red edge, showed a strong association with chlorophyll content (r2 0.81 and 0.80, respectively). Even stronger derivatives of stomatal conductance and photosynthesis, relationships to chlorophyll content were observed with the ratios of with respect to photosynthetically active radiation, for 600/680 nm (r2 0.83) and 630/680 nm (r2 0.83). Thus, our results unstressed canopies. suggest that spectral reflectance measurements hold promise for the The objective of this study was to determine how assessment of some physiological parameters at the leaf level. Further corn leaf spectral reflectance is related to leaf tissue investigation is needed to evaluate the effectiveness of such techniques chlorophyll content and relative water content (RWC) at the canopy level. under N and water-limiting conditions. An additional objective was to determine if the two different stress conditions would confound the estimation of either P nutrients and water are the most important, chlorophyll content or RWC. and readily manageable, variables for producing a profitable crop (Stevenson, 1982; Kozlowski, 1972). NiMATERIALS AND METHODS trogen, because of its high demand in the plant and This experiment was conducted during the winter months variability within the soil, is the most intensively manof 1998–1999 under greenhouse conditions. Supplemental inaged plant nutrient in crop production. Stresses that candescent lighting was provided to extend the daylength to involve deficiencies of N and water will adversely affect 14 h. A day/night temperature of 29/18 C was maintained for the amount of chlorophyll plants produce as well as the duration of the study. Seeds of the corn hybrid Pioneer cell turgidity. brand ‘3489’ were planted on 11 Dec. 1998 in pots with a A measurement commonly made by plant scientists volume of 0.013 m containing a mixture of one-half sand and is leaf chlorophyll content. This parameter is fundamenone-half potting soil. This was done to create a soil deficient tal to understanding a plant’s response to the environin N while still allowing the soil to provide the plants with the required micronutrients. A solution of macronutrients, ment in which it resides. Remotely sensed measurewithout N, was added to each pot at appropriate levels to ments of this parameter could provide information avoid any deficiencies of these nutrients. Since light environabout the status of the plant without the use of destrucments may differ throughout the greenhouse bay, the pots tive tissue sampling. Instruments have been developed were moved weekly to minimize any influence location within that examine the reaction of light energy with leaf tissue the greenhouse might have on plant growth. Treatments confor the purpose of providing rapid estimates of chlorosisted of a factorial combination of three N levels (56, 112, phyll content. Markwell et al. (1995) reported a very and 168 kg N ha 1 rate) and two water management strategies. strong relationship between the Minolta SPAD-502 The water management strategies consisted of two levels: adechlorophyll meter readings and direct measurements of quate water level, which replenished daily use, and a water chlorophyll content in corn and soybean [Glycine max deficit level, which had water withheld 3 d before data collection. Four replications were established for this study. (L.) Merr.] leaves. Since chlorophyll content is usually Leaf physiological and optical properties were assessed on strongly related to N concentration, these meters can be the plants at the V6 to V7 growth stage (Ritchie et al., 1997). used as indicators of need for agricultural N application The Minolta SPAD-502 chlorophyll meter was used, as per (Schepers et al., 1992; Blackmer and Schepers, 1995). Markwell et al. (1995), to acquire a rapid estimate of leaf chlorophyll content. The measurements were taken on the upperUSDA-ARS, 119 Keim Hall, Univ. of Nebraska, Lincoln, NE 685830915. Received 11 May 2004. *Corresponding author (mschlemmer1@ unl.edu). 1 Mention of trade names or commercial products in this article is solely for the purpose of providing specific information and does not Published in Agron. J. 97:106–112 (2005). imply recommendation or endorsement by the USDA. © American Society of Agronomy 677 S. Segoe Rd., Madison, WI 53711 USA Abbreviations: RWC, relative water content.