Photosynthate Partitioning into Starch in Soybean Leaves

Starch accumulation and sucrose synthesis and export were measured in leaves of sugar beet (Beta vulgaris L.) during a period of prolonged irradiance in which illumination was extended beyond the usual 14-hour day period. During much of the 14-hour day period, approximately 50% of the newly fixed carbon was distributed to sucrose, about 40% to starch, and less than 10% to hexose. Beginning about 2 hours before the end of the usual light period, the portion of newly fixed carbon allocated to sucrose gradually increased, and correspondingly less carbon went to starch. By the time the transition ended, about 4 hours into the extension of the light period, nearly 90% of newly fixed carbon was incorporated into sucrose and little or none into starch. Most of the additional sucrose was exported. Gradual cessation of starch accumulation was not the result of a futile cycle of simultaneous starch synthesis and degradation. Neither was it the result of a decrease in the extractable activity of adenosine diphosphoglucose pyrophosphorylase or phosphoglucose isomerase, enzymes important in starch synthesis. Nor was there a notable change in control metabolites considered to be important in regulating starch synthesis. Starch accumulation appeared to decrease markedly because of an endogenous circadian shift in carbon allocation, which occurred in preparation for the usual night period and which diverted carbon from the chloroplast to the cytosol and sucrose synthesis.Carbon that is exported from leaves of sugar beet at night or during times of slow photosynthesis comes mainly from diumal starch reserves rather than from accumulated sucrose (8,9,11). In photosynthesizing sugar beet (Beta vulgaris L.) leaves, sucrose is exported essentially as fast as it is synthesized and therefore does not accumulate to a significant extent, even when plants are growing at irradiance levels that are half of full sunlight (9). Increasing photosynthesis by increasing CO2 or decreasing 02 concentrations can increase sucrose synthesis and export rate (10,26), whereas it is difficult to increase the proportion of currently assimilated carbon that is exported (6).

Section: Discussionmentioning

confidence: 99%

“…The relative amount of recently assimilated carbon that is allocated to transitory starch for subsequent export is inversely related to the duration of the daily photosynthetic period (3,19). The daily time course of starch accumulation in leaves increases and decreases with changes in the rate of photosynthesis throughout a natural day period (9).…”

mentioning

confidence: 99%

Evidence for Circadian Regulation of Starch and Sucrose Synthesis in Sugar Beet Leaves

Geiger²,

Shieh³

1992

“…Similar conclusions can be drawn from experiments involving changes in photosynthetic period. Transfer of plants from a long (14 h) to short (7 h) photosynthetic period resulted in increased starch accumulation (3)(4)(5) and decreased translocation (2,19). In soybean, transfer to the short photoperiod also was associated with decreased activity of SPS (13), but the activities of other enzymes involved in sucrose formation, such as UDP-glucose pyrophosphorylase and cytoplasmic fructose-1,6-bisphosphatase, were unchanged (Huber and Pharr, unpublished).…”

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confidence: 99%

Effect of Photoperiod on Photosynthate Partitioning and Diurnal Rhythms in Sucrose Phosphate Synthase Activity in Leaves of Soybean (Glycine max L. [Merr.]) and Tobacco (Nicotiana tabacum L.)

Rufty¹,

Kerr²

1984

ABSTRACIrStudies were conducted to identify the existence of diurnal rhythms in sucrose phosphate synthase (SPS) activity in leaves of three soybean (Glycine max L. IMerr.1) and two tobacco (Nicotiana tabacum L.) cultivars and the effect of photoperiod (15 versus 7 hours) on carbohydrate partitioning and the rhythm in enzyme activity. Acclimation of all the genotypes tested to a short day (7 hours) photoperiod resulted in increased rates ofstarch accumulation, whereas rates of translocation, foliar sucrose concentrations, and activities of SPS were decreased relative to plants acclimated to long days (15 hours). Under the long day photoperiod, two of the three soybean cultivars ('Ransom' and 'Jupiter') and one of the two tobarcco cultivars ('22NF') studied exhibited a significant diurnal rhythm in SPS activity. With the soybean cultivars, acclimation to short days reduced the activity of SPS (leaf fresh weight basis) and tended to dampen the amplitude of the rhythm. With the tobacco cultivars, photoperiod affected the shape of the SPS-activity rhythm. The mean values for SPS activity (calculated from observations made during the light period) were correlated positively with translocation rates and were correlated negatively with starch accumulation rates. Overall, the results support the postulate that SPS activity is closely associated with starch/ sucrose levels in leaves, and that acclimation to changes in photoperiod may be associated with changes in the activity of SPS.The principal end products of leaf photosynthesis are sucrose and starch. Sucrose is the primary transport form ofcarbohydrate in many higher plants, whereas starch accumulates in the chloroplast as a temporary storage form of carbohydrate. In the dark, starch reserves can be mobilized to support continued sucrose synthesis and export. One enzyme which may be involved in regulation of partitioning of carbon between starch and sucrose is SPS2. SPS is subject to metabolic regulation (1,7,10) Silvius et al. (20) established that soybean plants acclimated to high irradiance had higher rates of photosynthesis and translocation, and greater activities of SPS in leaf extracts, than did plants adapted to low irradiance. Importantly, transfer of low-irradiance plants to high irradiance resulted in increased rates of photosynthesis, but translocation rate was not increased. Rather, the rate of starch accumulation was increased. The authors noted that SPS activity in leaves was not increased (at least within 2 d) after transfer to high irradiance. Hence, SPS activity could have placed a biochemical constraint on translocation. Similar conclusions can be drawn from experiments involving changes in photosynthetic period. Transfer of plants from a long (14 h) to short (7 h) photosynthetic period resulted in increased starch accumulation (3-5) and decreased translocation (2, 19). In soybean, transfer to the short photoperiod also was associated with decreased activity of SPS (13), but the activities of other enzymes involved in sucrose formation, such ...

“…Although the initial study describing the daylength response was performed with plants from controlled environments designed to simulate greenhouse conditions during winter and spring (8), most subsequent investigations have exploited rapid shifts in assimilate partitioning that occur in response to sudden and large changes in daylength or nightlength (4,10). The relevance of results obtained with this approach has been questioned because drastic shifts in daylength may cause transient alterations in growth and sink demand that could influence assimilate partitioning in leaves ( 13).…”

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confidence: 99%

Regulation of Photosynthate Partitioning into Starch in Soybean Leaves

Britz

1990

Studies conducted in controlled environments indicate that daylength affects the proportion of photosynthate stored in leaves as starch or sucrose. To examine the response of partitioning to natural daylight, soybeans (Glycine max [L.] Merr. cv Williams)were grown at 12 different times between May and November in a constant temperature greenhouse without supplemental lighting. Plants were transferred from the greenhouse to a controlled environment chamber at the end of civil twilight at a set developmental stage (expanding seventh trifoliolate leaf, counting acropetally). Net photosynthesis and the accumulation of starch and sugar in fully expanded fourth trifoliolate leaves were determined the following day under standard conditions in the chamber (lights-on synchronized with sunrise). Photosynthesis on a leaf area basis decreased about 10% between midsummer and early autumn. Leaf soluble sugar accumulation was low at all harvests. However, a twofold increase in photosynthate partitioning into starch occurred over the same time period, resulting in an 80% increase in absolute starch accumulation rate. Starch was responsible for about 78% of the increase in leaf dry matter during the light at all harvests, indicating that starch accumulation as affected by prior daylight conditions will alter export of photosynthate during the light period. Photosynthate partitioning into starch was linearly correlated with daylength at harvest, prior average peak solar irradiance, and other parameters that correlated with daylength and solar radiation such as harvested top dry matter. The relation between growth and seasonal changes in daylight (including daylength, irradiance, and light integral) are discussed in relation to photosynthate partitioning under field conditions.In more than a dozen species studied to date, the proportion of photosynthate stored in leaves depends on daylength. Rel