Subcellular distribution of aluminum associated with differential cell ultra-structure, mineral uptake, and antioxidant enzymes in root of two different Al-resistance watermelon cultivars.

Crop plants, such as watermelon, suffer from severe Aluminum (Al)-toxicity in acidic soils with their primary root elongation being first arrested. However, the significance of apoplastic or symplastic Al-toxicity in watermelon root is scarcely reported. In this work, we identified a medium fruit type (ZJ) and a small fruit type (NBT) as Al-tolerant and sensitive based on their differential primary root elongation rate respectively, and used them to show the effects of symplastic besides apoplastic Al distribution in the watermelon's root. Although the Al content was higher in the root of NBT than ZJ, Al allocated in their apoplast, vacuole and plastid fractions were not significantly different between the two cultivars. Thus, only a few proportion of Al differentially distributed in the nucleus and mitochondria corresponded to interesting differential morphological and physiological disorders recorded in the root under Al-stress. The symplastic amount of Al substantially induced the energy efficient catalase pathway in ZJ, and the energy consuming ascorbate peroxidase pathway in NBT. These findings coincided with obvious starch granule visibility in the root ultra-structure of ZJ than NBT, suggesting a differential energy was used in supporting the root elongation and nutrient uptake for Al-tolerance in the two cultivars. This work provides clues that could be further investigated in the identification of genetic components and molecular mechanisms associated with Al-tolerance in watermelon.