Endogenous Rhythms in Photosynthesis, Sucrose Phosphate Synthase Activity, and Stomatal Resistance in Leaves of Soybean (Glycine max [L.] Merr.)

Abstract

Experiments were conducted with soybean (Glycine max [L.] Merr. cv 'Ransom') plants to determine if diurnal rhythms in net carbon dioxide exchange rate (CER), stomatal resistance, and sucrose-phosphate synthase (SPS) activity persisted in constant environmental conditions (constant light, LL constant dark DD) and to assess the importance of these rhythms to the production of nonstructural carbohydrates (starch, sucrose, and hexose). Rhythms in CER, stomatal resistance, and SPS activity were observed in constant environmental conditions but the rhythms differed in period length, amplitude, and phase. The results indicated that these photosynthetic parameters are not controlled in a coordinated manner. The activity of UDPG pyrophosphorylase, another enzyme involved in sucrose formation, did not fluctuate rhythmically in constant conditions but increased with time in plants in LL. In LL, the rhythm in CER was correlated positively with fluctuations in total chlorophyll (r = 0.810) and chlorophyll a (r = 0.791) concentrations which suggested that changes in pigment concentration were associated with, but not necessarily the underlying mechanism of, the rhythm in photosynthetic rate. Assimilate export rate, net starch accumulation rate, and leaf sucrose concentration also fluctuated in constant light. No single photosynthetic parameter was closely correlated with fluctuations in assimilate export during LL thus, assimilate export may have been controlled by interactions among the endogenous rhythms in CER, SPS activity, or other metabolic factors which were not measured in the present study.Circadian rhythms of certain physiological and biochemical processes have been observed in many unicellular and multicellular plant species (3,6,8,24). Rhythmic processes include phtosynthetic parameters, such as 02 evolution (16-18, 23, 26), net C02 fixation (2,19,20), Chl a concentration (2), whole chain electron flow (16, 18), and electron flow through PSII (26). Although rhythms in C3 photosynthesis apparently are not caused by changes in the activity of enzymes of the Calvin cycle (7,17), rhythms in CAM photosynthesis are, in part, attributed to the rhythm in PEP carboxylase activity (24). Thus, it is evident that enzymes of carbon metabolism, as well as components of Recently, the activity of SPS,2 a key regulatory enzyme of sucrose formation (9, 10, 12), was reported to change diurnally in soybean leaves (25). The plants from which these observations were made had not been cultured nor measured in constant environmental conditions, so it is not known if the rhythm is endogenously controlled or attributable to some exogenous stimulus (i.e. light or temperature). Although the metabolic implications of rhythms in SPS activity and net CO2 fixation are not entirely clear at present, a synchronization (or lack thereof) between these rhythms could conceivably affect carbohydrate formation.To determine if rhythms in SPS activity and net CO2 fixation are endogenously controlled and to assess the potential significance o...

Assimilate partitioning in leaves of the raffinose-storing herb Lamium album L.: photosynthesis and carbon partitioning throughout the photoperiod

“…Flowers were the strongest sinks, since radioactivity in the soluble as well as in the insoluble fractions increased over time. These results were in accordance with data from soybean, showing that diurnal variation in storage was greatest in the leaf and decreased in the order leaf > petiole > stems > roots (Kerr et al 1985).…”

Chilling Delays Circadian Pattern of Sucrose Phosphate Synthase and Nitrate Reductase Activity in Tomato1

“…Our 1997 study showed that SPS activity exhibits diurnal and circadian rhythms in tomato, which are the result of corresponding oscillations in SPS protein phosphorylation state (Jones and Ort, 1997). To our knowledge, this was the first report of a circadian rhythm in SPS activity, although an endogenous ultradian rhythm with a period of about 12 h had been reported in soybean (Kerr et al, 1985). Our evidence indicates that the circadian rhythm in tomato SPS phosphorylation state is the result of circadian-regulated transcription of a protein phosphatase, possibly the one that dephosphorylates and thereby activates SPS.…”

“…In fact, few proteins have been shown to have circadian oscillations in enzyme activity; those that have include NR (Lillo, 1984), SPS (Jones and Ort, 1997), and PEP carboxylase (Carter et al, 1991). Because circadian fluctuations are exhibited in many overt higher-plant processes, including leaf movement (Darwin, 1880;Satter and Galston, 1981), stomatal resistance (Martin and Meidner, 1971;Kerr et al, 1985;Holmes and Klein, 1986), and net photosynthesis (Chia-Looi and Cumming, 1972;Hennessey and Field, 1991), it seems unavoidable that many more enzymes must exhibit a circadian component to their regulation than the few so far identified. Circadian fluctuations in free cytosolic calcium (Johnson et al, 1995) may also play an important role in mediating the rhythms in these complex plant processes.…”

Circadian Stomatal Rhythms in Epidermal Peels from Vicia faba

“…Rhythms in photosynthesis and in stomatal opening generally (but not always [10]) occur in synchrony, and are likely to be strongly interrelated. Stomatal opening is regulated by internal CO2 concentration, but it can also limit photosynthesis by limiting CO2 supply.…”