© 2005 Heron Publishing—Victoria, Canada
Boron remobilization at low boron supply in olive (Olea europaea) in relation to leaf and phloem mannitol concentrations
Georgios Liakopoulos (1, 2), Sotiria Stavrianakou (1), Manolis Filippou (3), Costas Fasseas (3), Christos Tsadilas (4), Ioannis Drossopoulos (1) and George Karabourniotis (1)
1. Laboratory of Plant Physiology, Department of Agricultural Biotechnology, Agricultural University of Athens, Iera Odos 75,
Botanikos, Athens 118 55, Greece / 2. Corresponding author (g_liak@aua.gr) / 3. Laboratory of Electron Microscopy, Department of Agricultural Biotechnology, Agricultural University of Athens, Iera Odos
75, Botanikos, Athens 118 55, Greece / 4. Institute of Soil Classification and Mapping, National Agricultural Research Foundation, Larissa 413 35, Greece / Received May 18, 2004; accepted September 12, 2004; published online December 1, 2004
Summary
For plant species in which a considerable portion of the photoassimilates are translocated in the phloem as sugar alcohols,
boron is freely translocated from mature organs to growing tissues. However, the effects of decreased plant boron status on
boron remobilization are poorly understood. We conducted a growth chamber experiment (CE) and a field experiment (FE) to study
the effects of low boron supply on boron remobilization in olive (Olea europaea L.), a species that transports considerable amounts of mannitol in the phloem. For the CE, several physiological parameters
were compared between control (B+) and boron-deficient olive plants (B–) during the expansion of new leaves. Boron remobilization
was assessed by measuring boron content of selected leaves at the beginning and at the end of the CE. As expected, boron was
remobilized from mature leaves to young leaves of B+ plants; however, considerable boron remobilization was also observed
in B– plants, suggesting a mechanism whereby olive can sustain a minimum boron supply for growth of new tissues despite an
insufficient external boron supply. Boron deficiency caused inhibition of new growth but had no effect on photosynthetic capacity
per unit leaf surface area of young and mature leaves, thereby altering the carbon utilization pattern and resulting in carbon
allocation to structures within the source leaves and accumulation of soluble carbohydrates. Specifically, in mature B– leaves
in the CE and in B– leaves in the FE, mannitol concentration on a leaf water content basis increased by 48 and 27% respectively,
compared with controls. Carbon export ability (assessed by both phloem anatomy and phloem exudate composition of FE leaves)
was enhanced at low boron supply. We conclude that, at low boron supply, increased mannitol concentrations maintain boron
remobilization from source leaves to boron-demanding sink leaves.
Keywords:
boron deficiency, carbohydrates, phloem, photosynthesis, plant stress, plasmodesmata.
