A Response Surface Analysis of the Effects of Cyclohexanecarboxylic Acid and 2,4-Dichlorophenoxy Acetic Acid on Nitrogen Metabolism in Phaseolus vulgaris L.

Abstract

Primary leaves of 2-week-old bush bean plants (Phaseolus vulgaris cv ‘Top Crop’) were treated with one of 36 combinations of cyclohexanecarboxylic acid (CHCA) and 2,4-dichlorophenoxy acetic acid (2,4-D), each at six equally spaced concentrations Five, 7, 9 and 11 days after treatment, one plant from each combination was harvested, and primary leaves were monitored for nitrate reductase activity (NRA), nitrate nitrogen (N) concentration, organic N concentration, and d wt NRA was assayed by an in vivo procedure Data from the resulting 6 × 6 × 4 factorial configuration for each of the four responses were analysed using the method of orthogonal polynomials, which is described in detail This method, based on equally spaced levels, produced independent polynomial expressions in CHCA concentration, 2,4-D concentration, and/or time-to-harvest after treatment, which were then fitted to the data on each response by simple linear regression analyses. A graphical procedure was used to compare predicted responses from the best fit equations with the actual data. Examination of computer-generated plots of the three-dimensional response surfaces revealed that NRA, organic N concentration, and d wt decreased with increasing 2,4-D concentration, whereas nitrate N content increased Increasing CHCA concentration generally had the opposite effect on NRA and nitrate N content, although interaction between the two chemicals was evident in the latter response Organic N content remained unchanged as CHCA concentration increased, while d wt first increased then decreased Possible explanations for these results are discussed, as are the advantages of using factorial designs with equally-spaced levels in plant growth studies.