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Proteins and traces of polysaccharide are the only polymeric colloids consistently transported in the xylem sap of plants. The hypothesis that such proteins could have physical inhibitory effects on xylem water transport was investigated. Ovalbumin, with a molecular weight of 45 kDa and a molecular diameter of 5.4 nm, is an inert, water-soluble protein that is midway along the size range of endogenous xylem sap proteins. Solutions of ovalbumin conjugated to a fluorescent marker and supplied to transpiring shoot explants of tobacco (Nicotiana tabacum L.) and olive (Olea europaea L.) were shown by confocal laser scanning microscopy to accumulate specifically at wall-based pit membranes that connect neighbouring xylem conduits. In addition, pressure-induced perfusion of micro-filtered ovalbumin solutions, at concentrations similar to those of endogenous xylem sap proteins, through the xylem of tobacco stem or olive twig segments resulted in the retention of c. 40% of the ovalbumin and reductions in the axial hydraulic conductance of the xylem. Smaller molecules such as Texas Red 3000 (MW 3 kDa) and Alexafluor 488-cadaverin conjugates (MW 0.64 kDa) did not show similar characteristics. The partial reduction in xylem hydraulic conductance appeared to be related to the accumulation of ovalbumin at xylem pit membranes and the consequent fouling of trans-membrane water-conducting pores with smaller diameters than those of the ovalbumin molecules. Potential implications of these novel findings for whole-plant water relations are considered.

Several types of small, endogenous signal peptides are now known to induce a wide range of local and systemic responses in plants, but how such signal peptide activity is transported over long distances remains unclear. In particular, the possible occurrence and root-to-shoot transport of signal peptide activity in the xylem does not appear to have been previously investigated. Suspension-cultured cells of wild tomato Lycopersicon peruvanium L. were used in an established bioassay for detecting nanomolar concentrations of signal peptides via the induction of alkalinizing activity. Xylem sap naturally exuded from the cut and washed stem-surfaces of de-topped tomato plants (Lycopersicon esculentum L. cv. Castlemart) was collected, partially purified, concentrated, and shown by the bioassay consistently to contain significant alkalinizing activity. Plant salinity treatment induced further small increases in activity. Subsidiary experiments indicated that the alkalinizing activity found in the xylem-sap had properties similar to those of known plant signal peptides and was root derived. Thus, it was (i) detectable within minutes, (ii) eluted similarly during HPLC chromatography, (iii) destroyed by incubation with proteases and stable in the presence of protease inhibitor cocktail, and (iv) not found in bioassays of simulated xylem sap placed on the cut stem-surfaces of non-exuding roots in order to detect any significant release of wound peptides from the stem. Further investigations of the signal peptide activity in root xylem sap could provide new insights into its identity, genes, receptors, origins, and possible hormonal roles in regulating shoot growth and development.

We investigated the degree to which developing fruit compete directly with leaves for mineral nutrients, e.g. phosphate coming up from the roots. When soybean (Glycine max (L.) Merrill cv. Anoka) explants cut at mid-late podfill were given a 15-win pulse of 32Pi via the cut stem and then transferred to distilled water, 75% of the 32P accumulated hi the leaves and 21% in stem and petiole during the first hour. The amount of 32P entering the seeds was Low (1%) initially, but thereafter increased to 30% in 48 h. An accumulation of 32P in the seed coats preceded its entry into the embryos. Disruption (with hot steam) of the phblem between the leaf and the pods after - labelling indicated that more than 80% of the 32Pi pulse moved to the leaf before redistribution to the pods. Increasing "sink" size by adjusting the pod load from 1 to 2-3 did not increase the 32P accumulated by the pods proportionally. Conversely, excision of the seeds after pulse labelling did not prevent translocation of 32P out of the leaves. These results suggest that the rate of transport of phosphate to the pods at mid-late podfill is controlled primarily by factors in the leaves. The results are consistent with the observation that the relative size of the sink (pod load) does not regulate teat senescence. [ABSTRACT FROM AUTHOR]

Removal of the shoot above the primary node (detopping) of 3‐week‐old bean plants (Phaseolus vulgaris L. cv. Contender) altered the metabolism and development of the remaining leaves. An increase in levels of chlorophyll, protein, stomatal opening, photosynthesis and growth, i.e. rejuvenation of primary leaves, was established within 7 days of detopping. These levels were maintained while the primary leaves of equivalent intact plants senesced. The flux of xylem solution (mineral ions, cytokinins and water) into leaves is related to the leaf area to be supplied and root supply capacity; it has been suggested that detopping leads to an increased availability of root‐supplied solutes and hence rejuvenation of the remaining leaves. This assumes however that root output of solutes is not decreased by the defoliation treatment. We found that root output of ions (electrical conductivity of passive xylem exudate) in detopped plants was 30% lower than in intact plants after 24 h and 60% lower after 7 days. The output of Ca2+, Mg2+ and K+ were similarly reduced 7 and 14 days after detopping as were fresh and dry weights of roots. Furthermore, neither the calculated xylem flux of ions nor directly measured levels of Ca2+, Mg2+ and K+ were significantly increased in leaves of detopped plants during their rejuvenation. We therefore conclude that root output is tightly coupled to shoot demand and that the apparent rejuvenation of primary leaves caused by detopping bean plants is not a consequence of increased xylem flux of mineral ions into the leaves. [ABSTRACT FROM AUTHOR]