The indoleacetic acid (IAA) oxidase activity of root tips of boronsufficient, -deficient, recovering, and IAA-treated boron-sufficient squash plants (Cucurbita pepo L.) was determined. Apical and subapical root sections displayed an increase in IAA oxase activity between 6 and 9 hours after boron was withheld, and after 24 hours the activity of the apical sections showed a 20-fold increase over +B controls. Root elongation of -B plants was inhibited before an increase in oxidase activity could be detected. Roots One of the more recurrent proposals for a role of boron in higher plants is that it may play a part in IAA metabolism and thereby influence growth (5). Eaton (7) obtained evidence that led him to conclude that boron-deficient plants were also IAAdeficient. Attempts to confirm Eaton's work, however, have been unsuccessful (10, 12). More recently, Neales (14) proposed that instead of boron-deficient plants lacking sufficient IAA, they may actually contain supraoptimal levels which impair normal growth. Shkol'nik et al. (19) reported a decrease in the level of free acid auxins and an increase in the bound form in deficient corn and sunflower plants. Other investigators have reported higher levels of IAA in the apical regions of borondeficient plants and have speculated that the increase in IAA might be due to an inactivation or change in the activity of IAA oxidase (4, 9).Oxidative enzyme activity appears to be altered by boron deficiency. Higher polyphenol oxidase activity and an increased 02 consumption by tissue homogenates have been reported for boron-deficient plants (11,16). Peroxidase activity of tissue homogenates (13) and of root tips (15) has been found to increase, and to the contrary, decrease in stem callus and root cultures (6). IAA oxidase activity has been reported to decrease in plants subjected to several days of boron deficiency (19).Due to the conflicting nature of these reports and because of its role in IAA metabolism, it was decided to investigate the relationship between boron deficiency and IAA oxidase activity Determination of IAA Oxidase Activity. The in vivo IAA oxidase activity of squash root sections was determined using a modified technique of Galston and Dalbert (8). Fifty each of 5-mm apical and 5-mm subapical root sections were excised and quickly placed in separate incubation flasks containing 10 ml of 2,4-dichlorophenol (10-5 M), IAA (40 ,tg/ml), and citric acid phosphate buffer (pH 5.6) solution. The flasks were placed in a water bath shaker (30 C + 1 at 130 rpm) for 1 hr. Where results of preliminary experiments indicated considerable enzymic activity, the number of root sections/flask was reduced to 25 and the incubation time to 0.5 hr.When incubation was complete, the flasks were removed from the water bath and the solution in the flask tested for residual IAA. Aliquots of the solution were placed in test tubes with Salkowski reagent (50 ml 35% perchloric acid-I ml 0.5 M Fe2CL3) and allowed to develop for 1 hr. Absorbance was measured on a Gilford spectropho...

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