Daily changes in carbon-dioxide exchange and photosynthate translocation of leaves of Vicia faba

Non-invasive 11C-Imaging Revealed the Spatiotemporal Variability in the Translocation of Photosynthates Into Strawberry Fruits in Response to Increasing Daylight Integrals at Leaf Surface

The efficiency of photosynthate translocation from leaves to fruits directly affects dry matter partitioning. Therefore, controlling photosynthate translocation dynamics is critical for high-yield and high-quality fruit production. Accordingly, photosynthate translocation changes must be characterized using data obtained at a higher spatiotemporal resolution than those provided by conventional methods. In this study, ¹¹C-photosynthate translocation into strawberry (Fragaria × ananassa Duch.) fruits in individual plants was visualized non-invasively and repeatedly using a positron emission tracer imaging system (PETIS) to assess the spatiotemporal variability in the translocation dynamics in response to increasing daylight integrals (i.e., 0.5-, 4.5-, and 9-h exposures to 400 μmol m^-2 s^-1 at the leaf surface). Serial images of photosynthate translocation into strawberry fruits obtained from the PETIS confirmed that ¹¹C-photosynthates were translocated heterogeneously into each fruit on the same inflorescence. The amount of translocated ¹¹C-photosynthates and the translocation rate into each fruit significantly increased as the integrated light intensity at the leaf surface increased. An analysis of the pedicel of each fruit also confirmed that the photosynthate translocation rate increased. The cumulated photosynthesis in leaves increased almost linearly during the light period, suggesting that an increase in the amount of photosynthates in leaves promotes the translocation of photosynthates from leaves, resulting in an increase in the photosynthate translocation rate in pedicels and enhanced photosynthate accumulation in fruits. Additionally, the distribution pattern of photosynthate translocated to fruits did not change during the light period, nor did the order of the sink activity (¹¹C radioactivity/fruit dry weight), which is the driving force for the prioritization of the ¹¹C-partitioning between competing organs, among fruits. Thus, this is the first study to use ¹¹C-radioisotopes to clarify the spatiotemporal variability in photosynthate translocation from source leaves to individual sink fruits in vivo in response to increasing daylight integrals at a high spatiotemporal resolution.

Gibberellic-acid-promoted transport of assimilates in stems of Phaseolus vulgaris L. : Localized versus remote site(s) of action

Gibberellic acid (GA3), applied as a dispersion in aqueous lanolin to the stumps of decapitated stems of P. vulgaris plants, was found to promote the transfer of (14)C-and (32)P-labelled assimilates to the site of hormone application. Measurements of the component transfer processes, operating between source and sink (site of hormone application), showed that GA3 was not acting to promote assimilate transfer by increasing the photosynthetic rate of, or the assimilate export rate from the source, nor by altering the mobilizing ability of the competing root sink. Here, it also was found that the time between GA3 application and detection of an enhanced transport flux was independent of the length of the transport pathway. Overall, the evidence obtained indicated that GA3 was not acting on any transfer process remote from its point of hormone application but was acting locally at this latter point.

Daily changes in nitrate uptake and metabolism in Capsicum annuum

The diurnal pattern of nitrate uptake by Capsicum annuum L. cv. California Wonder in a constant environment is described by a Fourier harmonic, with the maximum uptake in the middle of the photoperiod and the minimum in the middle of the dark period. Comparison of the uptake pattern with that of nitrate reductase (EC 1.6.6.1.) activity suggests against a direct control of one process by the other. This was confirmed by the observation that the pattern of nitrate reductase activity was not altered by restricting nitrate uptake to one hour per day. Translocation of (15)N from the roots is much greater in the lightperiod than in the dark period. Reduction of (15)N in the leaves occurs in the lightperiod but very little is reduced in the dark period. Amino acid levels showed marked daily fluctuations but in the roots neither amino acids, sucrose, fructose, glucose nor malate showed fluctuations. The amino acid composition of roots and leaves differed: glutamine+glutamate were relatively more important in leaves than in roots whereas alanine was a more important constituent of roots than of leaves.

Translocation from leaves to fruits of a legume, studied by a phloem bleeding technique: Diurnal changes and effects of continuous darkness

Diurnal changes in the carbohydrates of leaf laminae and fruits and in the bleeding of sugar and amino acids from fruit phloem were followed by successive sampling from a population of Lupinus albus L. plants. Phloem sap was collected for a standard 5 min period from cut distal tips of attached fruits. Daily fluctuations in leaf dry matter resulted largely from changes in starch and sugar. Leaf sugar rose to a maximum in the afternoon, starch to a maximum at, or shortly after, dusk. Leaves lost sugar and starch from dusk to dawn. Phloem bleeding rate varied little over a daily cycle but sucrose levels fluctuated from a noon maximum of 12-13% (w/v) to a dawn minimum of 9-10%. The rhythm of phloem sugar levels matched closely those of fruit and leaf. Phloem amino acid levels fluctuated in phase with that of sucrose: the relative composition of the amino fraction did not vary significantly over the daily cycle. Pulse feeding of source leaves with (14)CO2 at different times in the photoperiod allowed study of the pattern of release of labelled photosynthate to the fruit phloem and the build up and depletion of (14)C starch in leaves. Plants transferred to continuous darkness showed a rapid decline in output and concentration of phloem sap solutes, and translocated nitrogen to their fruits at only one quarter of the rate of control plants retained in natural daylight. The combined data from the experiments showed that the rate of output of sugar from cut phloem of a fruit was directly related to the current level of sugar in leaves. When leaf sugar levels were low (5-10 mg ml tissue water(-1)) sugar in phloem was 10-11 times more concentrated than in source leaves, but at high leaf sugar levels (25-30 mg ml(-1)) this concentration difference was only 3-4 fold.

Photosynthate translocation in Capsicum annuum

The leaves of Capsicum annuum L. cv. California Wonder function as efficient carbon exporting organs for a large part of their life span. Young and old leaves export the same percentage of fixed carbon (74% within 24 hours) although fixation rates per unit area of young leaves may be three times that of old leaves. A constant feature of translocate distribution is a supply to the roots and to the terminal fruit of the main axis from both the main axis leaves and branch leaves. Carbon supply to branch fruit is flexible in that main axis leaves supply carbon to the very small fruit, but the subtending branch leaves supply more as the fruit grows and the branch node becomes more self-sufficient. Fruit are capable of carbon fixation but translocation to terminal fruit from main axis leaves supplies about 7.5 times as much carbon as direct fixation.

The use of compartmental analysis in the study of the movement of carbon through leaves

The export of (14)C from leaves of Lycopersion esculentum (Mill.), Capsicum frutescens (L.) and Amaranthus caudatus (L.) was followed by in vivo counting after exposure of the leaf to a 5 min pulse of (14)CO2. In all instances the time course of export showed two or more exponential phases. There was an initial rapid period of export which was followed by a slower phase after about 2 h. About 12-14 h after exposure to (14)CO2 this second phase was superseded by an even slower phase of export which continued for more than 24 h. In tomatoes the initial phase was most rapid in plants bearing fruit which had been heated to 30°C instead of the standard 15-20°C; it was slowest when the fruit were removed. In Amaranthus the rate of the initial phase was shown to be positively correlated with photosynthesis and when the latter was prevented by either darkness or the absence of CO2 the rate of loss of (14)C was reduced. The data were used to test a model of carbon movement from a leaf which postulated the presence of two carbon pools which turned-over at different rates. The photosynthetic carbon entered the pool with the faster rate of turn-over-the 'labile' pool-and exchanged with the other, 'storage', pool. Export from the leaf was from the 'labile' pool. The results suggested that a third, longer term, storage pool should be included in the model and that the exchange between the pools should be non-linear.