INTERACTION OF SUGARS AND AUXINS IN PEA EPICOTYL SECTION GROWTH

PURVES, W. K., and A. W. GALSTON. (Yale U., New Haven, Conn.) Interaction of sugars and auxins in pea epicotyl section growth. Amer. Jour. Bot. 47(8): 665-669. Illus. 1963. The nature and magnitude of the response of "S," etiolated pea-epicotyl sections to auxin are determined by the concentration of sugar in the growth medium. For example, the concentration of IAA inducing maximum elongation shifts through at least 3 orders of magnitude in response to varying sucrose concentrations, from ca. 10-4 M, with no sucrose, to 10-7 M, with 2% sucrose. Similarly, the inhibitory action of high levels of IAA on elongation occurs only in the presence of sucrose. By contrast, although sucrose also promotes water uptake, it affects the IAA optimum for this process only slightly. The action of IAA on growth can be detected immediately, but the growth response to sucrose occurs only after a 6-8 hr. lag. If tissues are supplied with sucrose, then 1-hr. exposures to IAA can be as effective on grow:h as continuous 20-hr. exposures, depending on the time at which such exposures are given. Thus, 10-4 M IAA applied in the presence of 2% sucrose is markedly inhibitory to elongation in hours 1-3, relatively inactive in hours 4-6, and strongly promotive after hour 7. The change from inhibitory to promotive action thus coincides in time with the length of the lag period for sucrose action. SUCROSE is known to interact synergistically with auxin (Schneider, 1938), with cobalt (Miller, 1954) and with gibberellin (Purves and Hillman, 1959). Although the mechanisms underlying these interactions are largely unexplored, auxin is known to have marked effects on various aspects of sugar metabolism (Galston and Purves, 1960). A detailed analysis of auxin-sugar interactions in the control of growth was therefore considered appropriate. Kinetic studies have sometimes revealed new details in the actions of growth substances. For instance, Busse (1959) has found that cobalt-induced growth promotion in Avena coleoptile sections occurs only after a lag of some hours and that this promotion may result from an inhibition of deposition of cell wall material. Using this same approach, we have found that the effect of sucrose on the growth of etiolated pea epicotyl sections is manifested only after a considerable lag period. It is interesting that, under appropriate conditions, brief exposures to auxin (Galston and Hand, 1949) or to gibberellin (Purves, 1959) at the beginning of the growth period promote elongation as effectively as do constant exposures to these substances. Our results indicate that this effectiveness of brief exposures to auxin is dependent upon the presence of sucrose in the growth medium. These results emphasize the importance of sugar in auxincontrolled growth processes. MATERIALS AND METHODS. Seeds of Pisum sativum L., var. 'Alaska' (obtained from Associated Seed Growers, Inc. of New Haven Conn.) were soaked in tap water for 3-4 hr. and then sown in 1 Received for publication January 8, 1960. Much of this material was included in a dissertation (Purves, 1959) submitted to the Graduate School of Yale University in partial fulfillment of the requirements for the degree of Doctor of Philosophy. 2 National Science Foundation Predoctoral Fellow 19571959. thoroughly washed vermiculite (Mica-Gro Type B, supplied by Platt Seed Co., Branford, Conn.). The seedlings were allowed to develop in a curtained cabinet in a dark room maintained at 27 ? 10 C. They were exposed sparingly to dim green light at the time of harvest and, briefly, when the curtains were opened during normal activity in the dark room. The light source was a 15-w. Sylvania green fluorescent tube wrapped with at least 3 layers of dark green and 3 layers of amber DuPont cellophane, and located about 30 cm. from the working area. All sections were taken from plants having recurved apical crooks and third internodes batw2en 15 and 50 mm. in length, a stage of development reached 7 or 8 days after planting. Sections 5-mm. in length taken 1 mm. below the apical crook were used; these are called S, sections (Purves and Hillman, 1958). The sections were randomized in the basal medium, which was 0.02 M KH2PO4Na2HPO4 buffer, pH 6.1, and lots of 10 sections were floated on 5 ml. of various media in 5-cm. Petri dishes. After approximately 20 hr. growth in darkness, sections were measured to the nearest 0 1 mm. by means of a dissecting microscope with an ocular micrometer. Sections were weighed in lots of 10 on a Roller-Smith torsion balance. All experiments were replicated at least 3 times. RESULTS. Basic interaction of sugar and a!txin. -The growth of etiolated pea epicotyl sections is promoted by auxins (Thimann and Schneider, 1939) and by sucrose (Galston and Hand, 1949). When sucrose and indoleacetic acid (IAA) are supplied simultaneously to Si sections, a complex interaction results (fig. 1). In the absence of sucrose, 10-7 M IAA is without effect, and increasing concentrations of IAA give a regular increase in promotion of clongation. In the presence of 2% sucrose, on the other hand, 10-7 M IAA is optimal, and increasing concentrations give a regular decrease in elonga-