The Influence of Applied Nitrogen on Export and Partitioning of Current Assimilate by Field-grown Potato Plants

Field-grown potatoes were subjected to N deficiency (no appliedN) or received high levels of N (240 kg N ha^–1) at planting.The effects of these treatments were monitored at five stagesduring growth in terms of the allocation of photosynthate withinthe leaf, and the export and partitioning of carbon to differentsinks. N deficiency significantly raised the starch concentrationin all organs of the plants, particularly in leaves and stems,and as a consequence the total amount of starch in the canopyof the low N plants remained greater than that of the high Nplants until approx. 100 days after planting (DAP). The totalamounts of carbohydrates, protein and amino acids were calculatedfor each treatment and these values were used to derive a balancesheet for major reserves. Net losses of reserves occurred fromthe canopy in both treatments in the period 97–133 DAP,although these were shown to represent < 3 per cent of thetotal gain in tuber dry weight for the season. Partitioning of ¹⁴C assimilates was examined in whole plantsand also in single leaves. Reduced partitioning to the tubers,seen in high N plants throughout their growth, was shown tobe due to decreased percentage export by the leaf and accumulationof exported ¹⁴C by the stems. Partitioning to the tubers inlow N plants increased prior to senescence when 87 per centof the fixed ¹⁴C was exported within 24 h, 80 per cent of thisto the tubers. The equivalent values for the high N plants were77 and 60 per cent respectively. Increased percentage exportcoincided with decreased allocation to starch in the leaf, anda link between these processes is suggested. N also significantlyaltered the allocation of ¹⁴C within the leaf and may have influencedthe degradation of starch in the dark to a greater degree thanits synthesis in the light. The enzymes sucrose phosphate synthase (SPS), and starch synthasewere measured concurrently with partitioning. High N plantsshowed higher rates of activities of each of the enzymes althoughboth enzymes showed a similar pattern of development over theseason, irrespective of N treatment. The data are discussed in the light of conflicting reports concerningthe influence of N on translocation and partitioning. ¹⁴C assimilates, carbohydrates, nitrogen, potato (Solanum tuberosum L.), protein     

Sites of acid phosphatase in the differentiating root protophloem of Nymphoides peltata (S.G. Gmel.) O. Kuntze.

Abstract Sites of acid-phosphatase activity were found in the differentiating root protophloem of Nymphoides peltata by lead-salt and by azo-dye methods. Different substrates revealed different subcellular locations of the enzyme. The substrates β-glycerophosphate (β-GP) and naphthol ASBI phosphate revealed enzyme activity at similar sites within the sieve element. These sites included plasmodesmata, dictyosomes and small vacuoles in the cytoplasm. The substrate p-nitrophenylphosphate (p-NPP), however, revealed additional sites of acid-phosphatase activity which were not detectable by either naphthol ASBI phosphate or β-GP. For example, the inner region of the wall in mature sieve elements showed conspicuous acid-phosphatase activity only when p-NPP was used as substrate. The significance of the different locations of acid phosphatase within the sieve element is discussed. The convoluted ER, characteristic of immature sieve elements of N. peltata, failed to show acid-phosphatase activity whichever substate was used. By contrast, the stacked ER found in the parietal layer of mature sieve elements showed prominent acid-phosphatase activity regardless of the substrate used. The demonstration of acid-phosphatase activity in the stacked ER, and by both lead-salt and azo-dye methods, suggests that this organelle is a true site of acid-phosphatase activity. The onset of acid-phosphatase activity in the ER in later stages of sieve-element differentiation is compatible with the view that stacked ER plays a role in the final autolysis of the sieve-element protoplast.     

Long-term Metabolism of [14C]Sugars in Stored Potatoes

[¹⁴C]Sucrose, [¹⁴C]glucose and [¹⁴C]fructose were introducedinto potato tubers held at 10 °C and the redistributionof label chased over a 65 d period in storage. Respiratory losseswere identical in all treatments, as was the partitioning of¹⁴C between soluble and insoluble forms. Sucrose was the predominantlabelled sugar in the tubers after 20 h, regardless of the original[¹⁴C]sugar introduced, and was loaded and distributed throughoutthe tubers by the internal phloem system. After 20 h the proportionsof labelled sugars bore no relationship to those of the unlabelledendogenous sugars. However, with time the percentage of ¹⁴Cin sucrose fell while that in glucose increased and by 65 dthe proportions of the labelled sugars more closely resembledthe endogenous pools. Fructose represented a consistently lowproportion of both the labelled and unlabelled sugars. By 21d a considerable proportion of the soluble ¹⁴C had been convertedto starch (approx. 25% of the total tuber 14C), this value remainingrelatively constant for the remainder of the storage period.Sprouts which formed on the tubers contained up to 6% of thetotal tuber ¹⁴C but less than 0.2% of the tuber dry matter.It is suggested that the bulk of the translocated [¹⁴C]sucroseentered the symplast and exchanged slowly with the bulk of thesugars in the storage cell vacuoles. [¹⁴C]sugars, phloem loading, starch, potato tuber, Solunum tuberosum, cold storage     

The use of the fluorescent probe 8-anilino-1-naphthalene sulphonic acid (ANS) as a histochemical stain in plant tissues

Abstract. The fluorescent probe 8-anilino-l-naphthalene sulphonic acid (ANS) has been evaluated as a histochemical stain for plant tissues. The wide specificity of the compound for hydrophobic binding sites limits its analytical use, but renders it of considerable value as a general fluorescent stain for use in epi-illumination fluorescence microscopy. Used in this way it is analogous to the light-microscope stain toluidine blue. ANS has also been found to be a sensitive vascular tracer.     

Changes in Partitioning of Current Assimilate During Tuber Bulking in Potato (Solarium tuberosum L.) cv Maris Piper

Potato plants were labelled with ¹⁴CO₂ at six stages duringtuber bulking and changes in current assimilate partitioning(20 h after ¹⁴CO₂ assimilation) were examined in relation toincreasing tuber size on a plant. There was no relationshipbetween the node of origin of a tuber and the amount of ¹⁴Cwhich entered it, and those tubers on a plant which importedthe most ¹⁴C did not all belong to the same stem. However, competitionwithin a node was evident as many tubers borne on second-orderstolons were smaller and contained significantly lower concentrationsof ¹⁴C than those on primary stolons. Two weeks after tuberinitiation there was an almost linear relationship between thefresh weights of the tubers and their ¹⁴C content but the correlationbecame less good at subsequent harvests. Within the tubers theratio of ¹⁴C starch: soluble fell over the course of the experimentand was reflected in an increase in the percentage of ¹⁴C sucroseappearing in the tubers at each harvest. At any one harvest,however, the ¹⁴C starch: soluble ratios were similar but notrelated to the rates of ¹⁴C import of the tubers. The data areconsistent with the view that sucrose is compartmentalized inpotato tubers. Assimilate, ¹⁴Carbon, potato, sugars, tuber     

Acid phosphatase activity in intercellular spaces in roots of Nymphoides peltata (S. G. Gmel.) O. Kuntze

Abstract. Acid phosphatase was found cytochemically in intercellular spaces in the root of Nymphoides peltata. Different methods, using lead salts and azo-dyes, gave similar results. Reaction product appeared on material, possibly cytoplasmic, within the intercellular spaces and also against the outer walls of cells which formed the intercellular spaces. Possible functions of acid phosphatase in intercellular spaces are discussed.     

Movement of fluorescein into isolated caryopses of wheat and barley

Abstract. The movement of fluorescein, a symplastic fluorescent tracer, into isolated caryopses of wheat and barley is described. The dye followed the pathway to the endosperm which has been proposed previously from anatomical studies, namely a movement from the phloem, through cells of the pigment strand and nucellar projection, followed by a radial spread of the dye from the endosperm cavity into the starchy endosperm. By contrast, the fluorochromes calcofluor white M2R and ANS remained confined to the apoplast and failed to cross the 'xylem discontinuity' at the base of the caryopses.     

Nitrate Reduction in Solanum tuberosum L.: Development of Nitrate Reductase Activity in Field-grown Plants

Nitrate reductase activity (in vivo method, substrate non-limiting)in unshaded leaves from the top of the canopy has been determinedfor field-grown potato plants over the course of the growingseason. The pattern of change was almost identical for plantsreceiving no added fertilizer and those receiving 24 g N m^–2.Activity increased to a peak at about 90 days after plantingand declined thereafter. On a fresh weight basis activity wasalways higher in fertilized plants. Nitrate reductase activitywas positively and significantly correlated with leaf proteincontent in high N plants (r² = 0.71; P = 0.05), but poorly correlatedwith both the nitrate content of the leaf lamina and the nitrateconcentration in petiole sap. Up until 90 days after planting(mid-July) there appeared to be a positive relationship betweenincreased activity of nitrate reductase and solar radiation.However, results obtained over two seasons showed that the declinein activity after this time was not consistently linked witha fall in the level of solar radiation. Remobilization of reduced-Nand stored nitrate from leaves and stems accompanied this declinein nitrate reductase activity and in the latter part of theseason appeared to account for all of the N gained by growingtubers. In unfertilized plants nitrate-N accounted for 5 per cent orless of total plant N. Fertilized plants contained up to 25per cent nitrate-N. While nitrate availability limited growthin unfertilized plants, sub-optimal rates of nitrate assimilationin fertilized plants, particularly during the early stages ofpost-emergence growth, may contribute to inefficient use ofacquired nitrate. The carbohydrate status of leaf lamina and petiole sap weremodified by N supply. The soluble sugar and starch contentsof low N leaves were higher than in their high N counterparts.By contrast, the concentration of soluble sugars in petiolesap increased to a higher value in high N samples. Althoughsap sugar levels declined in both treatments towards the endof the season, N application delayed this decline for severalweeks. Solanum tuberosum, nitrate reductase, nitrate assimilation, senescence     

Behaviour of plasma membrane, cortical ER and plasmodesmata during plasmolysis of onion epidermal cells

During plasmolysis of onion epidermal cells, the contracting protoplast remains connected to the cell wall by an intricate, branched system of plasma membrane (PM) ‘Hechtian strands’ which stain strongly with the fluorescent probe DiOC₆. In addition, extensive regions of the cortical endoplasmic reticulum (ER) network remain anchored to the cell wall during plasmolysis and do not become incorporated into the contracting protoplast with the other cell organelles. These ER profiles become tightly encased by the PM as the latter contracts towards the centre of the cell. Thus, although the cortical ER is left outside the main protoplast body, it is nonetheless still bound by the PM of the cell. As well as being anchored to the wall, the cortical ER remains intimately linked with plasmodesmata and retains continuity between cells via the central desmotubules which become distended during plasmolysis. The PM also remains in close contact with the plasmodesmatal pore following plasmolysis. It is suggested that plasmodesmata, although sealed, may not be broken during plasmolysis, their substructure being preserved by continuity of both ER and PM through the plasmodesmatal pore. A structural model is presented which links the behaviour of PM, ER and plasmodesmata during plasmolysis.