Root Colonization vs. Seedling Growth, in Two Azospirillum-Inoculated Wheat Species
Data evaluating the growth promoting effects of Azospirillum 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) 103, 105, 107 and 108A. brasilense cells per T. aestivum cv. ProINTA Federal seedling; ii) 102, 105 and 108A. brasilense cells per T. durum cv. Buck Topacio seedling; iii) 106 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 Azospirillum 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 102 to 105 cells per seedling, both Triticum species reached a maximum level of colonization harboring 106 to 107 cells per seedling. No differences could be detected between NI and HKB treated seedlings for both Triticum species. Triticum aestivum cv. ProINTA Federal seedlings reached the maximum growth promotion when roots were colonized with a number of bacterial cells ranging from 5 · 106 to 1.5 · 108 per seedling. Triticum durum cv. Buck Topacio seedlings showed maximum growth promotion when 3.3 · 107 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 · 105 cells per seedling for both T. aestivum cv. ProINTA Federal and T. durum cv. Buck Topacio.
KeywordsAzospirillum colonization wheat seedling growth growth promotion
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