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ISHS Acta Horticulturae 1217: VIII International Symposium on Mineral Nutrition of Fruit Crops

Effects of soil pH on the root growth of passion fruit and the mechanism of mineral uptake

Authors:   S. Niwayama, H. Higuchi
Keywords:   acidic soil, aluminum toxicity, rhizosphere development, Passiflora, vegetative growth
DOI:   10.17660/ActaHortic.2018.1217.14
Acidic soil has recently been reported to be rather preferable for passion fruit growth, although the mechanisms underlying these observations are not known. For these reasons, in the present study the effects of soil pH on the root growth and mineral uptake were examined. The vines and roots of passion fruit plants were pruned in half before transplanting them to 10-L pots filled with soils adjusted to four different pH levels (3.5, 4.5, 5.5, and 6.5). Soil exchangeable cations, leaf and root mineral contents, root length density, vegetative growth, photosynthetic rate, stomatal conductance, and leaf water potential were measured. Soil exchangeable aluminium (Al) as well as Al saturation were higher at the lower pH (pH 3.5, 4.5) than at higher pH (pH 5.5, 6.5), whereas negative effects of neither root elongation nor mineral uptake were observed. On the contrary, the root growth was even the highest of all the treatments at pH 3.5. The root Al content at pH 3.5 was similar to the other treatments, and the leaf Al content was lower than the average concentrations detected in other herbaceous plants. Accordingly, the passion fruit root system seems to use an Al exclusion mechanism, thus resulting tolerant to Al toxicity. Vegetative growth, photosynthetic rate, stomatal conductance, and leaf water potential were lowered at higher pH values of the soils. Insufficient root growth at higher pH caused water deficit in leaves, and the water stress might lead to stomatal closure, which might cause depression of photosynthetic rate. At acidic soil such as pH 3.5, on the contrary, a larger root volume is considered to be advantageous to keep higher leaf water potential and resulted in higher photosynthetic rate with strong vegetative growth.

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