Quantitative relationships between boron and mannitol concentrations in phloem exudates of Olea europaea leaves under contrasting boron supply conditions
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Boron remobilization (BR) occurs in plants that form and export phloem–mobile borate–polyol complexes. Previous studies demonstrate that a quantitative relationship exists between polyol translocation and BR. Here we investigate if mannitol translocation is quantitatively related to BR in olive plants thus allowing acclimation to boron limiting conditions. Plants were cultivated under different boron supply ranging from adequate (23 μM) to insufficient or zero supply (0.5 μM or 0 μM). Measurement of boron in the leaf phloem sap exudates (BP) of olive leaves of low or zero boron supply treatments showed that, whereas boron was remobilized, its absolute amounts in the phloem were lower compared to the control. However, BR from source leaves at 0.5 μM or 0 μM was maintained at relatively high levels in regard to the amounts of boron available in the cells of these leaves, indicating a strategy of the source leaves to remobilize boron by depleting their cell sap boron pool. Concurrently, in the above treatments, leaf phloem mannitol (MP) was up to two-fold higher, resulting in a up to five-fold higher ratio of mannitol to boron in the phloem (MP/BP), compared to the control. Furthermore, both MP and MP/BP were negatively correlated with cell sap boron concentration of the leaves indicating the trend for BR under boron limitation. It is concluded that, in this plant species, mannitol concentrations in the leaf phloem may be involved in the promotion of BR under inadequate external supply of boron.
KeywordsBorate–polyol complexes Boron remobilization Cellular boron Mannitol translocation Olea europaea L. Phloem sap exudate
Mannitol concentration in phloem sap exudates
Boron concentration in leaf phloem sap exudates
Boron concentration in cell sap (plus apoplastic fluid)
23 μM/0.5 μM/0 μM boron nutrient solution treatments
Authors thank Dr. Mariangela N. Fotelli for useful suggestions during manuscript preparation and Mr. G. Kostelenos (Kostelenos Olive Nurseries) for the supply of plant material.
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