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 addition 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 recovered in the shoot (avg. = 7.2%) with little accumulation in the root (avg. = 0.43%). A portion of the assimilated IAA-label in the plant was identified by co-chromatography and UV spectral confirmation as IAA-glycine and IAA-aspartic acid conjugates. Little of the assimilated IAA label was found as free IAA in the wheat plants. These same assimilation patterns were observed when 2'-C-14-IAA was added to wheat plants grown in sterile and nonsterile soil. In contrast, the wheat varieties assimilated considerably less (avg. = 1.3%) of the added microbial 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 and increased yields when L-TRP (10(-5) and 10(-7) M) was added to the root zone. It is surmised that this positive response is a result of microbial auxin production within the rhizosphere upon the addition of the precursor, L-TRP. The amino acid composition of the root exudates plays a critical role in microbial production of auxins in the rhizosphere. 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.