Susceptibility of Plants to Vascular Disruption by Macromolecules

Abstract

The xylem of alfalfa (Medicago sativa L.) was found to be susceptible to vascular obstruction by picomole quantities of dextrans. Not all parts of the xylem were equally susceptible to this plugging. The quantity ofdextran of 2 x 10' molecular weight required to stop vascular flow was 8 picomoles in petiole junctions and OA picomole in leaflet veins. Vascular Macromolecules in a plant's xylem are known to induce wilting (5). If large amounts of a big molecule enter the xylem, plugging will occur. Plant pathologists, however, have identified many macromolecules that induce wilt in plant cuttings within 2 h at concentrations as low as 50 ug/ml (6). It has been assumed that molecules in such low concentrations could not be merely plugging the vascular system and, as a result, they were classed as phytotoxins.Van Alfen and Turner showed that two of these large phytotoxins, produced respectively by Corynebacterium insidiosum (8) and Ceratocystis ulmi (9), act by interfering with water conduction in the plant's vascular system and not by any direct toxicity to plant cells. The decrease in vascular conductance through stems of the wilted cuttings caused by these macromolecules was not large.The flow of liquid through stems was reduced to about 20%o of its normal value (9). Such a reduction in liquid flow conductance is not sufficient to cause wilt (2). There was no question that the macromolecules had a significant effect on vascular conductance and it was assumed that their greatest effect on conductance was in locations other than in the stems.The work reported in this paper was initiated to determine if different components of a plant's vascular system differ in susceptibility to plugging by small amounts of macromolecules. MATERIALS AND METHODSNine-month-old DuPuits variety of alfalfa (Medicago sativa L.) grown in the greenhouse from seed was used in these studies.Vascular conductance of stems and petioles as well as the petiole junction was determined using previously described methods (9). To determine conductance of the vascular system to dextran solutions, the conductance of water through the system was first determined. This was followed by the dextran solution. As a 'Supported in part by National Science Foundation Grant PCM 77-08388.2 Joumal paper 2306, Utah Agricultural Experiment Station.control, the conductance of water followed by water rather than a dextran solution was determined. All dextran conductance data could then be expressed as a percentage of the conductance of water through the same section of the vascular system. The solutions used in these studies were prepared by dissolving dextrans obtained from Pharmacia Fine Chemicals in distilled H20. The water had previously been filtered through a 0.45-,um diameter filter (Millipore Corp.) and then degassed. The dextrans were normally used at a concentration of 800 ,ug/ml.Conductance of water through alfalfa leaflets could not be determined using a volumetric method so a gravimetric method was employed. Cuts were made about one-fourth of th...

Sequential Leaf Senescence and Correlatively Controlled Increases in Xylem Flow Resistance

“…The apparently physical effect of trace amounts of high mol wt dextrans on xylem flow in bean leaves coincide with the previous findings of Van Alfen et al with alfalfa plants (17,18 (2). Whether the correlative effects of aging and shoot growth are similarly mediated has yet to be determined.…”

“…Table II shows that during pressure infiltration of dextran solution, flow through the petiole to the leaf tip declined. Flow rates were reduced within 5 min of infiltration suggesting that physical plugging rather than a biochemical response was involved (17). Infiltration of 20 pmols of dextran reduced flow to 32% of the original rate with water; continued infiltration led to an almost complete cessation of flow.…”

“…Xylem vessel resistance to hydraulic flow could be increased by vapor embolisms, gel plugging, ingrowth of tylose cells and, possibly, progressive vessel constriction (17)(18)(19)21). The pressure infiltration assay used here would minimize the effects of air embolisms (15).…”

“…Moreover, light microscope examination of freehand sections of pulvinus-leaf blade junctions stained with either toluidine blue, to reveal the xylem (12), or with saturated ruthenium red, to reveal gel occlusions (19), did not show evidence of selective increases in the other types of blockage (PM Neumann, unpublished data). Xylem resistance in some species can also be rapidly increased by the introduction of trace amounts of high mol wt polysaccharides into the xylem fluid (17,18). An investigation of the possibility of polysaccharide-induced plugging of the xylem in bean leaves was therefore undertaken.…”

Role of Pit Membranes in Macromolecule-Induced Wilt of Plants

“…Neither of these plant responses can explain the instantaneous increase in resistance observed when macromolecules are introduced into plant xylem. Both gelation and tylose formation occur over periods of hours or days (5,15) rather than the seconds observed with macromolecule-induced xylem flow resistance (13).…”