The Foliar Uptake and Translocation of Caesium

A study has been made of the foliar uptake and translocationof caesium-137 by pea plants which have been grown for a periodin either the light or the dark. Keeping plants in the darkreduces both processes, and also reduces the proportion of mobilecaesium which is transported downwards. Other experiments involved darkening only parts of the plantand also the use of various metabolic inhibitors. These showedthat the reduction in foliar uptake and translocation in thedark was connected with changes in the carbohydrate metabolismof the leaf to which the caesium was applied. They further suggestedthat a metabolically controlled process was involved in thetransfer of caesium from the mesophyll into the phloem of theleaf veins. Changes in the distribution of caesium were alsogoverned in some way by changes in the carbohydrate metabolismof the ‘source’ leaf.     

Investigations of Carbon Transport in Plants: I. THE USE OF CARBON-11 TO ESTIMATE VARIOUS PARAMETERS OF THE TRANSLOCATION PROCESS

A technique is described which enables several components ofthe translocation system to be measured routinely, in vivo andsimultaneously. Plants are exposed to a 1-min pulse of ¹¹CO₂and the movement of the ¹¹C pulse through the plant followedusing an array of scintillation counters. It is possible toestimate the time taken for the ¹¹C to enter the sieve tubes.In addition, the speed of translocation can be obtained frommeasurements of the arrival of the ¹¹C pulse at different positionsalong the translocation pathway. The increasing half-width ofthe pulse as it enters and moves through the sieve tubes givessome indication of the extent of the delays in the differentparts of the translocation system. The short half-life of the¹¹C allows the measurements to be repeated on the same plantseveral times per day. A technique is discussed whereby dataon the movement of ¹¹C could be combined with simultaneous measurementsof the rate of uptake of carbon dioxide to provide an estimateof the rate of movement of carbon.     

The Influence of Carbohydrate and Mineral Nutrient Supply on the Growth of Potato Tubers

The uptake and distribution of nitrogen, phosphorus, and potassiumhas been studied throughout the life of potato plants. Thereappears to be a net loss of all three elements from the plantduring emergence growth even though uptake occurs. When daughtertubers are formed they very quickly become the dominant sinkfor mineral nutrients, the concentrations of N, P, and K remainingsteady for a long period. These concentrations are maintainedin spite of decreasing rates of uptake, indicating the transferenceof mobile ions from the haulm to the growing tubers. ¹⁴C tracer experiments have shown that after tuberization thereis a greater export of recently incorporated photosynthate fromthe leaves than takes place before tuberization. There is nogood correlation between the size of individual tubers and theamount of photosynthate transported into them. This is thoughtto be because the largest tubers are not necessarily growingfaster than the smaller tubers. The most active sinks are alsomost active in converting the mobile ¹⁴C into storage compounds.The mother tuber continues to import ¹⁴C until it is detachedfrom the plant, but over much of this period there is no changein the tuber dry-weight, indicating that there is an equivalentexport from the tuber. The similarities between these distribution patterns and thosefound in tubers showing second-growth are described and theimplications with respect to the control of tuber growth discussed.     

The Transport of Sugar, Water, and Ions into Developing Potato Tubers

Diurnal variations in the pattern of movement of sugars, water,and ions into developing tubers of the potato (Solanum tuberosumL.) were investigated. It was demonstrated using a recordingbalance that large increases in the fresh weight of tubers occurduring a dark period of reduced transpiration. Movement of assimilated¹⁴C did not reflect similar large changes and much of the weightchange observed is considered to be fluctuations in tuber watercontent. This water was shown to be moving predominantly throughthe xylem of the stolon by introducing labelled ions, ³²P and⁸⁹Sr into the plants. ³²P, which moves in both xylem and phloem,was transported to the tuber at a constant rate whereas ⁸⁹Sr,which behaves like calcium and is relatively immobile in thephloem, only moved into the tuber during the dark period. As well as the over-all long-term diurnal fluctuations severalsmaller rapid changes were recorded in the rate of water movement.Switching from darkness to light caused a transient increasefollowed by a rapid decrease in tuber weight. Switching fromlight to darkness caused a rapid increase in tuber weight. Insome experiments small oscillations in tuber weight were recorded.The possibility of these oscillations being directly relatedto cyclic changes in transpiration is considered. The resultsare discussed in relation to solute movement within plants.     

Variation in Translocation during the Photoperiod: Experiments Feeding 11CO2 to Sunflower

An array of 14 miniature Geiger counters arranged along petioleand stem has been used to follow the translocation kineticsfor ¹¹C assimilate in sunflower. Experiments over a 2 d periodindicate that (a) speed is insensitive to time of day and (b)the loading delay in the morning is comparable to that in thelate afternoon but smaller than that at mid-day. There is evidencefor substantial accumulation and retention of label along thepath on all occasions.     

Coiled-sprout in the potato: the effect of various storage and planting conditions

The incidence of coiled-sprout was determined in Scotch and local-grown Arran Pilot and Duke of York seed tubers which had been stored at 10°, 4° and 15° C. and in a farm store with no temperature control. All four tuber types were planted in the field and, in addition, the two types of Duke of York were planted in Perlite at 7°, 10° and 15° C. In the field, and when maintained at 7° and 10° C, the percentage of sprouts coiling and the intensity of coiling was greater in tubers stored at 10° and 15° C. than at 4° C. There was no coiling when the Duke of York tubers were planted at 15° C. In a further experiment tubers were stored at 20° C. in the light and dark and samples were planted monthly for 3 months at temperatures of 7°, 10° and 15° C. During the following 3-month period only light-stored were planted because of the excessive amount of tip-death in the tubers stored in the dark. There was very little coiling in the dark-stored tubers. In the first two plantings of the light-stored tubers there was virtually no coiling of those planted at 15° C. There was some, however, at 7° and 10° C. In subsequent plantings there was more coiling and no effect of planting temperature. Attempts to isolate Verticiculum nubilum from sprouts were successful in only a small percentage of attempts and it was not possible to demonstrate any difference between its distribution on coiled and normal sprouts. It was not possible to induce coiling by infection of sprouts with spores of V. nubilum. Over a wide range of sprout sizes the amount of coiling was a function of sprout size at planting. However, the parts which coiled were those in the apical bud of the sprout at the time of planting and hence contributed only a small amount to the total sprout size. It is likely, therefore, that the correlation between coiling and sprout size reflects the changing metabolism of the elongating regions of sprouts with their increase in length, these regions developing in such a way as to produce a greater tendency to coiling. The internal reactions concerned in these changes, however, are not known.     

Investigations of Carbon Transport in Plants: II. THE EFFECTS OF LIGHT AND DARKNESS AND SINK ACTIVITY ON TRANSLOCATION

The use of ¹¹C as a tracer has allowed repetitive measurementsof the speed of assimilate translocation to be made on singlemaize plants throughout prolonged periods of light and darkness.The speed appeared to double when the light was switched on.The time required to achieve a maximum speed, usually about3·5 cm min^–1, depended on the duration of the previousdark period. When the plant was transferred to darkness thespeed immediately decreased by about 20 per cent and continuedto decrease over the next 20 h to values of 0·5 to 0·9cm min^–1. The mean speed of translocation in tomato in the light, andother C3 plants, was usually about 1 cm min^–1. It wasreduced by 15–30 per cent when the fruit was removed orcooled from 26 to 10°C.     

Ageing in Woody Plants

The shoot systems of trees undergo characteristic changes withtime, as they increase in size and complexity. Certain of thesechanges, such as reduced annual growth increment, loss of apicaldominance, and changes in the geotropic responses, are easilyreversible, by pruning or by grafting shoots from older treeson to young stocks. These changes are referred to as ‘ageing’. This paper describes the changes occurring in Pinus sylvestrisand Larix leptolepis during ageing and reports an investigationinto the possible mechanisms controlling these changes. Pruningexperiments suggest that ageing is due to increased competitionfor nutrients between the various constituent shoots as thebranch systems increase in complexity, but the distributionof available nutrients is modified by apical dominance. Theseconclusions were supported by the results of experiments involvingthe injection of radioactive phosphorus (²²P), in associationwith the pruning treatments.     

The Production, Storage, and Translocation of Carbohydrates in Developing Potato Plants

¹⁴C-tracer experiments showed that the export of assimilateslabelled with a short pulse of ¹⁴CO₂ continued for 5 weeks.There appeared to be an approximately exponential loss of ethanol-insoluble¹⁴C from the haulm with a half-time of about 3.5 weeks. Initiallythere was a poor correlation between the fresh weight of thetubers and their ¹⁴C content but the correlation became goodafter 5 weeks. Field data showed that, in many instances, the rate of tubergrowth was greater than the rate of growth of the whole plant.This suggests the transfer of large amounts of dry matter fromthe haulm to the tubers. Dry-matter loss from the haulm couldaccount for the discrepancy and the amount transferred in thisway was about 10 per cent of the final dry weight of the tuber. There was an increase in net assimilation rate when the leafarea of the plants started to decline. It is suggested thatthis increase was due to an increase in the rate of photosynthesisbrought about by the influence of the rapidly growing tubers.     

A Model for the Translocation of Photosynthate in the Soybean

The transport of photosynthetically incorporated carbon downthe soybean stem is discussed in terms of flow from a reservoirin the upper part of the plant. A comparison of this model withthe experimental data from the preceding paper leads to an estimateof translocation velocity of about 60 cm. per hr. and to leakageof material from the sieve tubes of about o.8 per cent. percm. of stem traversed. The effects of velocity distributionsand delays in the translocation system are considered.