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.

l-Ascorbic acid (AsA) and its metabolic precursors give rise to oxalic acid (OxA) found in calcium oxalate crystals in specialized crystal idioblast cells in plants; however, it is not known if AsA and OxA are synthesized within the crystal idioblast cell or transported in from surrounding mesophyll cells. Isolated developing crystal idioblasts from Pistia stratiotes were used to study the pathway of OxA biosynthesis and to determine if idioblasts contain the entire path and are essentially independent in OxA synthesis. Idioblasts were supplied with various 14 C-labeled compounds and examined by microautoradiography for incorporation of 14 C into calcium oxalate crystals. [ 14 C]OxA gave heavy labeling of crystals, indicating the isolated idioblasts are functional in crystal formation. Incubation with [1-14 C]AsA also gave heavy labeling of crystals, whereas [6-14 C]AsA gave no labeling. Labeled precursors of AsA (l-[1-14 C]galactose; d-[1-14 C]mannose) also resulted in crystal labeling, as did the ascorbic acid analog, d-[1-14 C]erythorbic acid. Intensity of labeling of isolated idioblasts followed the pattern OxA Ͼ AsA (erythorbic acid) Ͼ l-galactose Ͼ d-mannose. Our results demonstrate that P. stratiotes crystal idioblasts synthesize the OxA used for crystal formation, the OxA is derived from the number 1 and 2 carbons of AsA, and the proposed pathway of ascorbic acid synthesis via d-mannose and l-galactose is operational in individual P. stratiotes crystal idioblasts. These results are discussed with respect to fine control of calcium oxalate precipitation and the concept of crystal idioblasts as independent physiological compartments.

No abstract

Flavonols are essential for pollen germination and tube growth in petunia and can be supplled by either the pollen or stigma at pollinatlon. HPLC analysis and a sensitive bloassay demonstrated that both pollinatlon and wounding induce flavonol accumulation, especlally kaempferol, in the outer cell layers and exudate of the stigma. Pollination and wounding induced nearly identical flavonol kinetics and patterns of accumulation in the same target tissue, suggesting that they sham elements of a common signal transductlon pathway. The wound response was systemlc, because kaempferol accumulated in the stlgma when dista1 tissues, such as the corolla, stamens, or sepals, were wounded. We have exploited the germination requirement for flavonols and the high leve1 of kaempferol that accumulates after wounding to enhance plant fecundity. Seed set was slgnificantly increased by mechanically wounding the corolla and stamens prior to the application of pollen to the stigma. A reproductive role for a plant secondary metabolite and the specific function of stigmatic kaempferol are discussed from an evolutionary perspective. INTRODUCTIONIn angiosperms, the stigma is the pollen-receptive surface of the female gametophyte-bearing pistil. In many species, the stigmatic surface is covered with a chemically complex exudate that provides the hospitable conditions and factors required for pollen adhesion, hydration, germination, and pollen tube penetration (Konar and Linskens, 1966;Knox, 1984; HeslopHarrison, 1987). In addition, discriminatory events, such as the rejection of self-pollen in sporophytic incompatible reactions, occur at the stigmatic surface (Nasrallah et al., 1991).The stigma also appears to offer a hostile environment to bacteria and fungi because growth of these organisms on the stigma is rare (Jung, 1956).Transcripts homologous to genes induced by wounding and stress have been detected in flowers from nonwounded plants (Gasser, 1991), and in some cases, defenserelated compounds have been found to accumulate specifically in the stigma. For example, high levels of proteinase inhibitor (PI) proteins (Atkinson et al., 1993) and chitinase (Leung, 1992) have been measured in stigmas of solanaceous plants, and the promoters of an hydroxyproline-rich glycoprotein gene and a phenylpropanoid biosynthetic gene, phenylalanine ammonia-lyase, direct transcription of a reporter gene in transgenic tobacco stigmas (Liang et al., 1989; Wycoff et al., 1990). Although thedefensive Current address: lnstitut für Pflanzenbiochemie, Halle, Germany. To whom correspondence should be addressed.role of these compounds in wounded tissue is well known, a reproductive function has not been established.Phenolic compounds function as stress indicators because they accumulate to high levels in many plant tissues in response to a wide range of biotic and abiotic signals, including wounding (Lawton and Lamb, 1987). Biosynthesis and accumulation of phenolics also occur as part of normal developmental programs in most higher plants (Wiermann, 1981). Ch...