ABSTRACT
Many plant species accumulate calcium oxalate crystals in specialised cells called crystal idioblasts. In one species of crystal-forming plants (Pistia stratiotes L.; forming raphide crystals), it has been shown that ascorbic acid is the primary precursor of oxalic acid. The question remains if this is true of other calcium oxalate crystal-forming plants. One way of answering the above question is by examining ascorbic acid as the oxalic acid precursor in diverse species with a variety of crystal types. In this study we tested ascorbic acid as the primary precursor of oxalic acid in four different species, each forming one of the four, thus far, unexamined crystal types: water hyacinth, styloid (and raphide); tomato, crystal sand; winged-bean, prismatic; water lily, astrosclereids with surface prismatic crystals. Pulse-chase feeding of 1-[14C]-ascorbic acid followed by resin embedding, microautoradiography and light microscopy were employed to examine incorporation of label into calcium oxalate crystals. For the species and crystal types studied, ascorbic acid is the primary precursor of oxalic acid and further, oxalic acid is added to crystals in patterns that correlate with the age and type of crystal involved.
ABSTRACT
L-Ascorbic acid (AsA) was found to be loaded into phloem of source leaves and transported to sink tissues. When L-[(14)C]AsA was applied to leaves of intact plants of three different species, autoradiographs and HPLC analysis demonstrated that AsA was accumulated into phloem and transported to root tips, shoots, and floral organs, but not to mature leaves. AsA was also directly detected in Arabidopsis sieve tube sap collected from an English green aphid (Sitobion avenae) stylet. Feeding a single leaf of intact Arabidopsis or Medicago sativa with 10 or 20 mM L-galactono-1,4-lactone (GAL-L), the immediate precursor of AsA, lead to a 7- to 8-fold increase in AsA in the treated leaf and a 2- to 3-fold increase of AsA in untreated sink tissues of the same plant. The amount of AsA produced in treated leaves and accumulated in sink tissues was proportional to the amount of GAL-L applied. Studies of the ability of organs to produce AsA from GAL-L showed mature leaves have a 3- to 10-fold higher biosynthetic capacity and much lower AsA turnover rate than sink tissues. The results indicate AsA transporters reside in the phloem, and that AsA translocation is likely required to meet AsA demands of rapidly growing non-photosynthetic tissues. This study also demonstrates that source leaf AsA biosynthesis is limited by substrate availability rather than biosynthetic capacity, and sink AsA levels may be limited to some extent by source production. Phloem translocation of AsA may be one factor regulating sink development because AsA is critical to cell division/growth.
