Cellular boron allocation and pectin composition in two citrus rootstock seedlings differing in boron-deficiency response

Aims

Variation in boron (B) efficiency in citrus in different rootstock genotypes is expressed as large differences in the occurrence of leaf symptoms and dry mass production under low B conditions, but the mechanisms responsible for such differences are unknown. This paper aims to determine whether differences in B uptake, cellular B allocation, and pectin content can explain genotype differences in B efficiency between B-efficient citrange (Citrus sinensis (L.) Osb. × Poncirus trifoliata (L.) Raf.) and B-inefficient trifoliate orange (Poncirus trifoliata (L.) Raf.) citrus rootstock.

Methods

Plants were grown hydroponically in a nutrient solution supplemented with 5 μM B for 14 days and then transferred to a B-free medium (0 μM B) or control medium (5 μM B) for 35 days. Boron uptake and allocation and cell wall pectin contents were examined.

Results

After 35 days under B deprivation, shoot dry mass in trifoliate orange decreased by 28 %, but shoot dry mass of citrange was not significantly affected. Root growth of both types of rootstock seedlings was inhibited, but the trifoliate orange was affected more than the citrange. In comparison with B concentrations in plants prior to the commencement of B treatments, B deprivation for 35 days decreased B concentration in various parts of citrange plants, and the reduction was much greater in trifoliate orange plants. Trifoliate orange seedlings contained higher B concentration and total B in cell wall on a dry leaf basis than citrange subject to 5 μM B treatment. However, the proportion of leaf B allocated in cell wall was higher in citrange than trifoliate orange when B supply was deficient in the nutrient. The changes in pectin composition in cell wall due to B deprivation differed between citrange and trifoliate orange. The decreased uronic acid (UA) content in the Na₂CO₃-soluble pectin was observed in both rootstock, but the increased UA content in CDTA-soluble pectin was observed only in citrange.

Conclusions

These results demonstrated that a combination of greater B uptake ability, greater B accumulation in cell walls, as well as the increased CDTA-soluble pectin, under limited external B supply, contribute to the integrity of cell walls in citrange and therefore increased tolerance to B deficiency.