Root colonization vs. seedling growth, in two <i>Azospirillum</i> -inoculated wheat species

Data evaluating the growth promoting effects of <i>Azospirillum</i> on wheat seedlings according to the inoculum level/root colonization effectiveness (number of bacterial cells), is scarce. Uniform 1-cm size, 72-h old wheat seedlings grown in the dark at 22 °C were inoculated with: i) 10 ³ , 10 ⁵ , 10 ⁷ and 10 ⁸ <i>A. brasilense</i> cells per <i>T. aestivum</i> cv. ProINTA Federal seedling; ii) 10 ² , 10 ⁵ and 10 ⁸ <i>A. brasilense</i> cells per <i>T. durum</i> cv. Buck Topacio seedling; iii) 10 ⁶ heat killed bacteria (HKB) cells per cultivar seedling; iv) phosphate buffer pH 6.8 (NI) as control seedlings for both cultivars. Afterwards, seedling growth proceeded in water in the dark at 22 °C for another 48 h. Alive or dead <i>Azospirillum</i> cells were suspended in phosphate buffer pH 6.8. Root and shoot growth were determined measuring the length and projected area of their digitalized images. When treated with inocula concentrations ranging from 10 ² to 10 ⁵ cells per seedling, both <i>Triticum</i> species reached a maximum level of colonization harboring 10 ⁶ to 10 ⁷ cells per seedling. No differences could be detected between NI and HKB treated seedlings for both <i>Triticum</i> species. <i>Triticum aestivum</i> cv. ProINTA Federal seedlings reached the maximum growth promotion when roots were colonized with a number of bacterial cells ranging from 5 · 10 ⁶ to 1.5 · 10 ⁸ per seedling. <i>Triticum durum</i> cv. Buck Topacio seedlings showed maximum growth promotion when 3.3 · 10 ⁷ cells were present in their roots. Higher values of colonization showed no growth promoting effects with respect to the controls. It may be concluded that in these experimental conditions the optimum inoculum concentration is 5 · 10 ⁵ cells per seedling for both <i>T. aestivum</i> cv. ProINTA Federal and <i>T. durum</i> cv. Buck Topacio.