seems like the toxins are present through all parts of the plant. different parts only contain different concentrations, otherwise its the same cocktail of alkaloids as i understood from wikipedia..
here' s a link with some info...maybe you already checked it, anyway here it is
http://en.wikipedia.org/.../Arundo_donax#Chemicals
the source is from erowid, you will find the link on the bottom of the page where all the references are written. there is some more information on erowid about extraction and separation of the alkaloids, but its not much. i think this plant still stands in the shadow and needs more experimentation.
edit :
some more info
If the content
NOTE! In principle, all samples demonstrated DMT comes from India . Samples from different states in the United States has not been shown to contain DMT.
Source 1
" DMT was reported by Ghosal and coworkers Using materials sourced in India. The first account of DMT Appeared in Dutta & Ghosal 1967 Which reported Recovering 20 mg of DMT from 200 grams of the dry whole plant. It was accompanied by 520 mg of Gramine, 128 mg of bufotenine and traces of 5-MeO-DMT , as well as unidentified Indoles ".
"This was investigated Further and published in Ghosal et al., 1969, Which Detailed the chemistry of the leaf and rhizome. Researchers Recovered 40 mg of DMT from 700 grams of rhizome (dried and with culm removed) It was accompanied by 180 mg of bufotenine and 16 mg of 5-MeO-MMT. Leaf chemistry was similar to the above but in much lower Amounts. Interestingly, 440 mg of dehydrobufotenine was Also Recovered from the rhizome (the Same 700 gm). This alkaloid is only known to OCCUR elsewhere as a toad venom component. It Seams to Be perceptibly active - albeit amidst what is a tryptamine soup - but is not Particularly pleasant or intense. This is conjectural based on bioassay results of Justin Case in 1992 or 1993, but I Suspect That it plays a role in the effects Experienced if a person is nuts enough to anxiety 500 or so grams of the fresh tender parts of the rhizome as an ayahuasca admixture. It is not likely be to Be Mistaken for DMT-even by a novice. "Source: (1)
Source 2
"The giant grass Arundo donax is a tall, perennial reed of a bamboo-like nature. A number of studies have found this plant to be a rich source tryptamine. Almost all the published reports Havering located These alkaloids in the flowers or in the roots of the seedling. The blossoms Have Both DMT and the 5-methoxylated N-demethylated analogue, 5-MeO-NMT. The roots are a rhizomal systems and not only They contain 'These tryptamines active, but bufotenine as well. The methylated quaternary salt of DMT is in the flowers, and bufotenidine And The cyclic dehydrobufotenidine are in the roots. Another report rendering 5-MeO-DMT in all the plant's extracts. "Source: Tihkal by Alexander Shulgin