4 to 10 %
||Geigenberger P. Response of plant metabolism to too little oxygen. Curr Opin Plant Biol. 2003 Jun6(3):247-56. p.248 left column 2nd paragraphPubMed ID12753974
||Eric S. Ober and Robert E. Sharp, A microsensor for direct measurement of O2 partial pressure within plant tissues, J. Exp. Bot. (1996) 47 (3): 447-454. doi: 10.1093/jxb/47.3.447
studies have shown that oxygen falls to low concentrations
within root meristems (4–10% [primary source]) and in phloem
tissue (5–6% )."