DEVELOPMENTAL AND GENOTYPIC DIFFERENCES IN THE RESPONSE OF PEA STEM SEGMENTS TO AUXIN

The objective of this investigation was to examine the response to exo genous auxin (indole-3-acetic acid; IAA)of stem segments at two develo pmental stages. The standard auxin response of excised stem segments a nd intact plants consists of an initial growth response and a prolonge d growth response. We found that this biphasic response does not occur in internodes at very early stages. Stem segments of light grown pea of various genotypes were cut when the fourth internode was at 6-13% o f full expansion (early-expansion) or at 18-25% of full expansion (mid -expansion). Length measurements of excised segments were made after 4 8 hours of incubation on buffer with or without auxin. An angular posi tion transducer linked to a computerized data collection system provid ed high-resolution measurement of growth of stacks of segments incubat ed in buffer over 20 hours. Early-expansion segments of all genotypes deviated from the standard auxin response, while mid-expansion segment s responded in a manner consistent with previous reports. Early-expans ion segments of tall, light-grown plants were unique in showing an aux in-induced inhibition of growth. The auxin-induced inhibition correlat ed with high endogenous auxin content, as determined by HPLC and GC/MS , across genotypes and between early-expansion and mid-expansion segme nts of tall plants. Measurement of ethylene evolved from stem segments in response to auxin, and treatment of segments with the ethylene act ion inhibitor, norbornadiene, showed the inhibition to be mediated in part by heightened ethylene sensitivity. Growth of early-expansion seg ments of dwarf and severe dwarf plants was stimulated by exogenous aux in, but the growth rate increase was delayed compared to that in mid-e xpansion segments. This is the first time that such a growth response, termed the delayed growth response has been demonstrated. It is concl uded that developmental stage and endogenous hormone content affect ti ssue response to exogenous auxin.