ASSIMILATION OF EXOGENOUS 2'-C-14-INDOLE-3-ACETIC ACID AND 3'-C-14-TRYPTOPHAN EXPOSED TO THE ROOTS OF 3 WHEAT-VARIETIES

This study was conducted to determine if plants can assimilate indole- 3-acetic acid (IAA) from rooting media and if exogenous L-tryptophan ( L-TRP) can be assimilated and converted by plants into auxins. The add ition of 2'-C-14-IAA (3.7 kBq plant(-1)) to wheat (Triticum aestivum L .) seedlings of three varieties grown in nutrient solution resulted in the uptake (avg. = 7.6%) of labelled IAA. Most of the label IAA was r ecovered in the shoot (avg. = 7.2%) with little accumulation in the ro ot (avg. = 0.43%). A portion of the assimilated IAA-label in the plant was identified by co-chromatography and UV spectral confirmation as I AA-glycine and IAA-aspartic acid conjugates. Little of the assimilated IAA label was found as free IAA in the wheat plants. These same assim ilation patterns were observed when 2'-C-14-IAA was added to wheat pla nts grown in sterile and nonsterile soil. In contrast, the wheat varie ties assimilated considerably less (avg. = 1.3%) of the added microbia l IAA precursor, 3'-C-14-L-TRP (3.7 kBq plant(-1)) and thus much lower amounts of IAA conjugates were detected. Glasshouse soil experiments revealed that 2 out of 3 wheat varieties had increased growth rates an d increased yields when L-TRP (10(-5) and 10(-7) M) was added to the r oot zone. It is surmised that this positive response is a result of mi crobial auxin production within the rhizosphere upon the addition of t he precursor, L-TRP. The amino acid composition of the root exudates p lays a critical role in microbial production of auxins in the rhizosph ere. This study showed that wheat roots can assimilate IAA from their rooting media, which will supplement the endogenous IAA levels in the shoot tissue and may positively influence plant growth and subsequent yield.