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; Porro et al., 2001 ), grapevine ( Porro et al., 2001 ), and grapefruit ( Li et al., 1998 ). It is easy, rapid, and non-destructive. However, the correlation between the SPAD readings and N concentrations varied depending on growth stages of the tree

industry as an indicator for chlorophyll content and crop N status ( Loh et al., 2002 ) and has been used on various horticultural crops. Westerveld et al. (2003) found that SPAD readings in cabbage were correlated with total leaf N concentrations at the

; Penũelas et al., 1994 ; Wang et al., 2012a ; Xiong et al., 2007 ), but has limited reports for greenhouse settings. The objectives of this study were 1) to assess if sensor (SPAD, atLEAF, and NDVI) readings could be used to predict N status during

( Ajay et al., 1984 ). For some species, correlation between SPAD meter reading and leaf N concentration is low ( Martín et al., 2007 ), and for broad leaf species, multiple measurements per leaf are required to acquire a representative SPAD value for a

leaf chlorophyll concentration. Regression analysis can then be used to predict foliar N from SPAD readings ( Markwell et al., 1995 ). Originally developed for rice, SPAD meters have found use on a variety of field and tree crops for predicting N

-pot group (i.e., experimental unit) for all three NDVI readings and SPAD values at each measuring date. Analysis of variance using PROC GLM in SAS was used to calculate the effects of N and cultivar with six replicates on the parameters studied. Means of

by SPAD reading, plant height, and width (greatest width = width 1; width perpendicular to width 1 = width 2). The PGI was calculated as the average of plant height, width 1, and width 2. Three fully expanded new leaves were selected on a given plant

without foliar damage. Leaf greenness index was measured by the non-destructive handheld chlorophyll meters [measured as the optical density, SPAD-502 reading (Minolta Camera Co., Osaka, Japan)] at the end of the experiment on four or six leaves at similar

accumulation increasing leaf sensitivity to ethylene which enhanced chlorophyll loss ( Hsu et al., 2003 ). But, in some plants, NH 4 :NO 3 ratio had no effect on chlorophyll content or SPAD reading, such as in garlic mustard ( Alliaria petiolata ) ( Hewins and

weighed to determine shoot dry weight (grams). Plant SPAD values. The reflective greenness of plant leaves was measured for all plants by averaging readings from three recently matured (midsection) leaves per plant using a portable SPAD-502 m (Minolta Corp