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).

Partitioning of assimilated carbon among sink organs is a critical factor that controls the rate and pattern of plant growth. Time-course measurements of plant and organ growth rates are useful for determining how regulation of carbon partitioning controls plant growth. Measuring growth rates over a 24 h period reveals the current pattern of carbon partitioning that can be used to predict growth rates of specific sinks. Comparison of growth rates among sinks under defined conditions can point out key factors that regulate partitioning of recently assimilated carbon among sinks. Internal control of carbon partitioning by developmental programmes regulates the timing and site of carbon distribution among developing parts, thereby establishing the adaptive traits of a species, cultivar or transgenic construct. Regulation of partitioning in response to environmental factors establishes or restores allometric growth among plant parts and functional balance between the supply and use of carbon. Environmental stress often restricts resource availability while successful acclimation sets in motion processes that restore the supply. A key mechanism contributing to regulation of carbon partitioning is an expression of genes that control activity of the enzymes which initiate sucrose metabolism at specific sites and stages of ontogeny.

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Abstract. Partitioning of recently‐fixed carbon among plant organs and subsequent distribution of reserve carbon were studied by supplying whole shoots of bean plants (Phaseolus vulgaris L.) with 14C‐labelled CO2 of constant specific radioactivity throughout a photoperiod. The gain of tracer carbon in each part revealed net accumulation of recently‐fixed carbon from direct fixation, import or both. Growth rate coefficients describing the present pattern of plant growth were calculated from ratios of tracer carbon to total carbon present in plant organs and were used to project future plant form. The period 10–20 d after the start of flowering was marked by a major increase in partitioning of recently‐fixed carbon to reproductive growth. Growth rates for the plant and its parts during this period were projected on the basis of growth rate coefficients and were found to be similar to rates measured by gravimetric growth analysis. Changes in tracer carbon recovered in individual organs after chase periods of various lengths revealed net decreases for leaves and stems. About 9% of the carbon distributed to fruits came from reserves even in the absence of obvious stress. Respiratory loss during the chase period was determined from the progressive drop in recovery of the original tracer carbon. The methods are being applied to measure current net accumulation rates in studies of sink organ physiology, and to compare partitioning of recently‐fixed and of stored carbon in several plant species under defined growth conditions.

Development of vegetative and floral buds was found to be a key factor in establishing the way carbon is distributed among growing leaves and fruits in Phaseolus vulgaris L. plants. Leaves emerged principally during a period 14 to 32 days after planting while flowers were produced during a 10-to 12-day period near the end of leaf emergence. Timing of anthesis established the sigmoidal time course for dry weight accumulated by the composite of all fruits on the plant. During the first 12 days following anthesis, fruit growth mainly consisted of elongation and dry weight accumulation by the pod wall. Thereafter, seed dry weight increased for about 1 week, decreased markedly for several days, and then increased again over the next 2 weeks. Accumulation of imported carbon in individual seeds, measured by steady-state labeling, confirmed the time course for dry weight accumulation observed during seed development. Seed respiration rate initially increased rapidly along with dry weight and then remained nearly steady until seed maturation. A number of developmental events described in the literature coincided with the different phases of diauxic growth. The results demonstrated the feasibility of relating current rates of carbon import in individual seeds measured with tracer 14C to the rates of conversion of imported sucrose and use of the products for specific developmental processes. The resulting data are useful for evaluating the roles of conversion and utilization of imported sucrose in regulating import by developing seeds.