5.5.4  Regulation of phloem loading

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Figure 5.26 Time-course of photoassimilate export from source leaves of tomato plants. Control plants, in which fruits were a major sink for photoassimilates, were maintained at 20°C. Treatments involved (1) removing fruit or (2) exposing plants with fruits to 30°C. The proportion of 14C label remaining in source leaves after a radioactive pulse was monitored through time to show that (1) presence of major sinks or (2) more rapid metabolism accelerated 14C export from source leaves (Based on Moorby and Jarman 1975)

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Figure 5.27 Effect of varying net photosynthesis on rates of photoassimilate translocation in sugar beet plants. Photosynthetic rate was varied by exposing leaves to three different light levels and ambient CO2 concentrations (hence different symbols). After a 14CO2 pulse, simultaneous estimates of export (translocation) were made by determining (1) arrival rate of 14C label in sink tissues and (2) export of 14C from source leaves (Based on Servaites and Geiger 1974; reproduced with permission of the American Society of Plant Physiologists)

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Figure 5.28 A strong relationship exists between photoassimilate export rate and activity of sucrose phosphate synthase (SPS) in fully expanded leaves of several species. Export rates from leaves were measured over 4 h during the middle of the light period. Leaves were then harvested for in vitro analysis of the activity of SPS. F, field-grown plants; GH, glasshouse-grown plants; phytotron, plants grown under controlled climates (Based on Huber et al. 1985)

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Figure 5.29 Relationship between cotton leaf starch content at the end of a light period and subsequent export rates in the dark. Starch was measured at the end of the light period. To measure export, leaves were exposed to atmospheres containing 14CO2 throughout the light period. Therefore, when the dark period began, all photoassimilate pools were labelled to a known specific activity so carbon export would relate closely to 14C export. Discs were simply cut from leaf margins and 14C measured to estimate carbon export at night (Based on Hendrix and Grange 1991; reproduced with permission of the American Society of Plant Physiologists)

Phloem loading forms an interface between photoassimilate production by leaves and photoassimilate use by importing sink regions. Rates of phloem loading are likely to be regulated by both source and sink processes to ensure efficient matching of photoassimilate production with demand. Understanding control of phloem loading requires information on location of source photoassimilate pools within leaves.

(a) Cellular locations of photoassimilate destined for export

Reduced carbon from photosynthesis is not necessarily immediately available for export from source leaves. Photoassimilate flow through carbohydrate pools can be followed by exposing a source leaf to a two-minute pulse of 14CO2 and monitoring 14C activity remaining in the leaf over time (Figure 5.26). Typically, the initial rapid loss of 14C gives way to a slower phase of loss after about two hours. Taking into account 14C losses through leaf respiration and differences in photoassimilate pool sizes (Wardlaw 1990), kinetics of 14C loss have been used to interpret turnover of photoassimilate pools in source leaves. In general, rapid loss of 14C is considered to reflect loading of the phloem from a labile transport pool. Loss from the more slowly exchanging pool (after two hours) is interpreted as remobilisation of 14C photoassimilates from starch within the chloroplasts or from sucrose stored in vacuoles of the mesophyll cells.

As irradiance decreases towards sunset and limits photosynthesis, current photoassimilate production becomes inadequate to maintain translocation. Under these conditions, photoassimilate stored in starch and/or as vacuolar sucrose can be mobilised and exported (Figure 5.26). The rate of export at night is generally much slower than during daylight hours.

(b)  Source regulation

Under conditions of source limitation, increases in net photosynthetic rate can lead to proportionate changes in photoassimilate export from leaves (Figure 5.27). Increased flow of triose phosphates from chloroplasts induces feed-forward upregulation of sucrose biosynthesis with a consequent amplification of the sucrose transport pool. The relationship breaks down when net photosynthesis is low and reserves are mobilised or when net photosynthesis suddenly exceeds the levels to which the leaf is acclimated. An upper limit to photoassimilate export appears to be set by activity of sucrose phosphate synthetase (SPS), an enzyme which determines the size of the sucrose transport pool (Figure 5.28). The activity of SPS has been shown to be under sink (Section 5.5.5) as well as environmental control. For instance, daylength has a profound effect on partitioning of photoassimilate between sucrose and starch. Under short-day conditions starch accumulation increases up to five-fold and SPS activity declines. Diversion into starch during the day enables maintenance of photoassimilate export during the long night period. Consistent with this remobilisation, the rate of export is a function of starch levels in leaves at the end of day (Figure 5.29).

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