612 A. W. Galston and D. C. McCune 



In view of the fact that synergistic interactions between auxin 

 and gibberellin do occur in certain tissues, a second aim of this work 

 has been an attempt to elucidate some metabolic basis for the growth 

 interaction. In this search we have concentrated on the peroxidases, 

 which are known to inactivate auxins in vitro (5) and which have 

 been implicated in certain growth and developmental phenomena 

 (3). In corroboration of previous reports (10) we have found a marked 

 effect of gibberellins on the peroxidase activity of sensitive cells, es- 

 pecially in dwarf plants. We have further studied this effect in de- 

 tail by an electrophoretic separation of peroxidase into its various 

 component fractions, and by delineation of the particular fractions 

 which are affected by gibberellin. 



GROWTH EXPERIMENTS WITH PEA STEM SECTIONS 



Materials and Methods 



'Alaska' peas, purchased from Associated Seed Growers, New Ha- 

 ven, Connecticut, were soaked in tap water for 4 hrs. and sown in 

 polyethylene containers in water-saturated vermiculite. Etiolated 

 plants were grown for 7 days in a dark cabinet in a dark room main- 

 tained at ca. 27° C, and were exposed only briefly at harvest to a dim 

 green safelight, produced by wrapping a 15 watt Sylvania green flu- 

 orescent tube with three layers of green and three layers of amber du 

 Pont cellophane. Subapical 5 mm. sections were cut with a guillotine, 

 as previously described (6). Ten such sections w^ere permitted to grow 

 overnight in the dark in 5 ml. of growth medium in a 7.5 cm. petri 

 dish. This medium consisted of a 1 per cent sucrose solution con- 

 taining 0.02 M KH.PO^-Na.HPO^ buffer, pH 6.1, plus lO-^ M gib- 

 berellic acid (GA) and 10-c or lO-s M indole-3-acetic acid (lAA) where 

 indicated. The GA was obtained from Dr. P. W. Brian of Imperial 

 Chemicals Industries and the lAA from Nutritional Biochemicals Co. 

 Both were made up as 10=^ M stock solutions and stored in the dark 

 in the refrigerator for no longer than one month. Growth of the 

 sections was measured to the nearest 0.1 mm. under a dissecting micro- 

 scope after about 18 hrs. Group averages and standard error of the 

 mean were computed. In all experiments here reported, the standard 

 error was 5 per cent of the mean or below. Growth was also measured 

 by obtaining fresh weights of the groups of ten sections after gentle 

 blotting. 



Green pea plants were grown in three controlled-condition rooms 

 maintained at ca. 23° C, and at photoperiods of 8, 16.5, and 24 hrs. 

 The light intensity at the growing tables was ca. 1,500 foot candles, 

 coming from a bank of mixed fluorescent and incandescent lights. 

 Plants on the growing tables were automatically subirrigated twice 



