Enhanced root growth and nitric oxide content in ‘Mandarin’ lime under drought stress: experimental evidence

Our aim was to test the hypothesis that root growth of ‘Mandarin’ lime—a drought tolerant citrus rootstock—under water deficit is associated with higher nitric oxide (NO) content in roots. As a reference, ‘Swingle’ citrumelo was compared with ‘Mandarin’ lime, two well-known citrus rootstocks. ‘Valencia’ sweet orange plants grafted on ‘Mandarin’ lime or ‘Swingle’ citrumelo were kept well-hydrated or grown under water withholding. After 14 days of water deficit, root morphology was evaluated, and leaf and root samples were collected for NO analysis. Under low water availability, ‘Mandarin’ lime exhibited higher root NO content and root growth than ‘Swingle’ citrumelo. Indeed, NO seems to modulate not only root biomass accumulation but also root morphology as higher NO was associated with large increase in root length and area. Such findings suggest that root NO content is a relevant marker for early screening of drought-tolerant citrus genotypes. As a novelty, we show that ‘Mandarin’ lime drought tolerance is significantly associated with root NO content.