Plant growth promotion by Azospirillum sp. in sugarcane is influenced by genotype and drought stress

Azospirillum influences growth and development of several crops by producing phytohormones such as auxins which have a major impact on root development. An improved root system leads to better water and nutrient uptake that in turn may influence yield positively. In this study, two agronomically contrasting sugarcane cvs R 570 and M 1176/77 adapted to different agroclimatic zones were inoculated with Azospirillum sp., with and without drought stress, to gauge how far they could benefit from this bacterial association. As early as 103 days after planting, cv M 1176/77 responded positively to inoculation with 15% improved growth (shoot height) and 75% more root dry mass when subjected to drought stress, whereas cv R 570 responded negatively particularly in the absence of drought stress. The significant interaction of cultivar x water regime x Azospirillum inoculation suggests a complex interplay of these factors, possibly involving the indigenous plant auxin pool. Therefore, plant genotype needs to be taken into account when recommending bacterial inoculation for direct plant growth promotion. Furthermore, enhanced growth under sub-optimal water conditions shows clearly the benefits that could be obtained in semi-arid conditions where water deficits frequently occur.