ALBERT

Notes: We investigated the phloem loading pathway in barley, by determining plasmodesmatal frequencies at the electron microscope level for both intermediate and small blade bundles of mature barley leaves. Lucifer yellow was injected intercellularly into bundle sheath, vascular parenchyma, and thin-walled sieve tubes. Passage of this symplastically transported dye was monitored with an epifluorescence microscope under blue light. Low plasmodesmatal frequencies endarch to the bundle sheath cells are relatively low for most interfaces terminating at the thin- and thick-walled sieve tubes within this C3 species. Lack of connections between vascular parenchyma and sieve tubes, and low frequencies (0.5% plasmodesmata per μm cell wall interface) of connections between vascular parenchyma and companion cells, as well as the very low frequency of pore-plasmodesmatal connections between companion cells and sieve tubes in small bundles (0.2% plasmodesmata per μm cell wall interface), suggest that the companion cell-sieve tube complex is symplastically isolated from other vascular parenchyma cells in small bundles.The degree of cellular connectivity and the potential isolation of the companion cell-sieve tube complex was determined electrophysiologically, using an electrometer coupled to microcapillary electrodes. The less negative cell potential (average –52 mV) from mesophyll to the vascular parenchyma cells contrasted sharply with the more negative potential (–122.5 mV) recorded for the companion cell-thin-walled sieve tube complex. Although intercellular injection of lucifer yellow clearly demonstrated rapid (0.75 μm s-1) longitudinal and radial transport in the bundle sheath-vascular parenchyma complex, as well as from the bundle sheath through transverse veins to adjacent longitudinal veins, we were neither able to detect nor present unequivocal evidence in support of the symplastic connectivity of the sieve tubes to the vascular parenchyma. Injection of the companion cell-sieve tube complex, did not demonstrate backward connectivity to the bundle sheath.We conclude that the low plasmodesmatal frequencies, coupled with a two-domain electropotential zonation configuration, and the negative transport experiments using lucifer yellow, precludes symplastic phloem loading in barley leaves.

Notes: The pattern and sequence of cellular degradation during the course of leaf senescence remains obscure and the nature of the trigger that induces cell senescence is unknown. In order to probe the pre-mortem phase of senescence temporal changes in cell ultrastucture were studied in aging leaves of light-grown achlorophyllous Hordeum vulgare L. cv. Dyan mutant seedlings. Electron microscope examination of the ultrastructure of mesophyll cell plastids revealed the absence of ribosomes and a highly disorganized prolamellar body. Both the number and size of plastoglobuli increased with aging and this change coincided with depletion of starch grains and dilation of lamellar membranes. Aging of mesophyll cells occurred coincident with a decline in ribosome content of the cytoplasm and loss of matrix granularity. Loss of ribosomes associated with the outer nuclear envelope membrane and a reduction in chromatin were also apparent. Only after 10 days was there evidence of loss of internal membrane integrity and swelling of mitochondrial cristae. Compartmentation was thus maintained during the aging process with membrane dissolution occurring late in senescence. These results suggest that an inability to produce chlorophyll and carotenoids and form thylakoid stacks due to the absence of plastid ribosomes, contributes to the rapid onset of senescence in light-grown achlorophyllous seedlings. Furthermore, disruption of chloroplast ribosome synthesis/assembly may constitute part of the plastid signal involved in triggering cell senescence.

Notes: Pollination greatly accelerates petal senescence. The first observed event after pollination is an increase in the flower's sensitivity to ethylene, followed by an increase in ethylene biosynthesis. Our objectives were to study the mode of action of the increase in ethylene sensitivity and the possible involvement of short-chain saturated fatty acids (SCSFAs) in this process. Application of SCSFAs, ranging in chain length from 7 to 10 carbons onto stigmas of Phalaenopsis (Phalaenopsis hybrid, cv. Herbert Hager) flowers increased their sensitivity to ethylene in the same way as pollination. Following pollination, there was a significant increase in the endogenous content of these fatty acids in the flower's column and perianth, with octanoic acid (C8) being the main SCSFA observed. The increase in SCSFA content was observed as early as 6 h after pollination and began to decline 6 h later. Incorporation of octanoic acid into liposomes or microsomal membranes isolated from Phalaenopsis petals resulted in a decrease in lipid order that was detected by fluorescence polarization of dansyl pyrrolidine (DNSP) but not of 1,6-diphenyl-1,3,5-hexatriene (DPH). At peak ethylene sensitivity, 10 h after pollination, there was a significant decrease in the lipid order of microsomal membranes isolated from Phalaenopsis columns and perianths, again as detected by DNSP but not by DPH. Stigmatic application of octanoic acid mimicked the effect of pollination on membrane lipid order. We suggest that SCSFAs may be the ethylene ‘sensitivity factors’ produced following pollination, and that their mode of action involves a decrease in the order of specific regions in the membrane lipid bilayer, consequently altering ethylene action.

Notes: The response of Phaseolus vulgaris L. cv. Contender grown under controlled environment at either ambient or elevated (360 and 700 μmol mol-1, respectively) CO2 concentrations ([CO2]), was monitored from 10 days after germination (DAG) until the onset of senescence. Elevated CO2 had a pronounced effect on total plant height (TPH), leaf area (LA), leaf dry weight (LD), total plant biomass (TB) accumulation and specific leaf area (SLA). All of these were significantly increased under elevated carbon dioxide with the exception of SLA which was significantly reduced. Other than high initial growth rates in CO2-enriched plants, relative growth rates remained relatively unchanged throughout the growth period. While the trends in growth parameters were clearly different between [CO2], some physiological processes were largely transient, in particular, net assimilation rate (NAR) and foliar nutrient concentrations of N, Mg and Cu. CO2 enrichment significantly increased NAR, but from 20 DAG, a steady decline to almost similar levels to those measured in plants grown under ambient CO2 occurred. A similar trend was observed for leaf N content where the loss of leaf nitrogen in CO2-enriched plants after 20 DAG, was significantly greater than that observed for ambient-CO2 plants. Under enhanced CO2, the foliar concentrations of K and Mn were increased significantly whilst P, Ca, Fe and Zn were reduced significantly. Changes in Mg and Cu concentrations were insignificant. In addition. high CO2 grown plants exhibited a pronounced leaf discoloration or chlorosis, coupled with a significant reduction in leaf longevity.