Regulation of Ribulose Bisphosphate Carboxylase Activity in Intact Wheat Leaves by Light, CO2, and Temperature

The activity of the enzyme ribulose bisphosphate carboxylase (RuBPCase) was estimated after rapidly extracting it from intact wheat leaves pretreated under different light and CO2 levels. No HCO3− was added to the extraction buffer since it is shown to inhibit RuBPCase. The activity increased as light intensity or CO2 concentration during pretreatment was increased. Enzyme activity increased as temperature during pretreatment was decreased. Light activation did not affect the affinity of RuBPCase for CO2. A Km of 30 μM CO2 under air level O2 was determined. CO2, light and temperature are three main limiting factors of photosynthesis. It seems that the activity of RuBPCase is regulated by these factors according to the requirements for CO2 fixatio

Influence of Position and Number of Nodal Roots on Outgrowth of Axillary Buds and Development of Branches in Trifolium repens (L.)

The implications of the presence of a root, either at the parent node or at neighbour nodes, on branch formation of Trifolium repens (white clover) was investigated. Plants were freely rooted or rooting was restricted to every sixth or every twelfth node along the parent axis. The absence of a root at the parent node had little influence on the probability of the subtending axillary bud forming a branch but, on average, delayed the outgrowth of the bud. The probability that an axillary bud, emerging from a non-rooted parent node, developed to a lateral branch (branch with elongated internodes) decreased with decreasing proximity of the parent node to a rooted node. Lateral branches emerging from non-rooted parent nodes which were two nodes distal to a rooted node had a higher rate of node appearance, a greater mean internode length and area per leaf, and were more branched than lateral branches emerging from other non-rooted parent nodes. The dry mass of each single root and of branches grown at rooted parent nodes were significantly higher in plants with restricted rooting than in freely rooted plants. Restriction in the number of rooted nodes per plant increased the number of inflorescences. It is concluded that the whole plant response to restricted root formation was continuous growth of the parent axis and compensatory growth of the branch at the rooted node. In general, growth was slow for axillary buds whose development was dependent on the basipetal movement or cross-transport within the stolons of resources exported from root

Influence of Inorganic Phosphate on Photosynthesis of Wheat Chloroplasts: II. RIBULOSE BISPHOSPHATE CARBOXYLASE ACTIVITY

Isolated wheat chloroplasts were pre-incubated in the dark in the presence of various concentrations of inorganic phosphate with or without carbon dioxide, oxaloacetate, glycerate, and 3-phosphoglycerate. The effect of subsequent illumination on photosynthetic oxygen evolution, ribulose bisphosphate carboxylase activity, ATP content, and ribulose bisphosphate content was investigated. Inorganic phosphate had little effect on ribulose bisphosphate carboxylase activity in darkness or during the initial phase of illumination, but it prevented the decline in activity that occurred during later stages of illumination, when photoreduction of CO2 was decreasing in rate. Addition of inorganic phosphate to chloroplasts illuminated without phosphate restored the ribulose bisphosphate carboxylase activity, increased the ATP, and decreased the ribulose bisphosphate in the organelles. The responses to CO2, oxaloacetate, glycerate, and 3-phosphoglycerate suggest that the decreased activity of ribulose bisphosphate carboxylase during photosynthesis results from ATP consumption. Purified ribulose bisphosphate carboxylase was activated by inorganic phosphate, but this activation did not occur in the presence of ATP. ATP inhibited ribulose bisphosphate carboxylase when it was present in combination with various photosynthetic metabolites. Inactivation of ribulose bisphosphate carboxylase in chloroplasts, illuminated in the absence of inorganic phosphate, is not due to lack of activation by inorganic phosphate or ATP. It may result from decreased stromal p

Regeneration of Ribulose 1,5-bisphosphate and Ribulose 1,5-bisphosphate carboxylase/oxygenase Activity Associated with Lack of Oxygen Inhibition of Photosynthesis at Low Temperature

The nature of the lack of oxygen inhibition of C3-photosynthesis at low temperature was investigated in white clover (Trifolium repens L.). Detached leaves were brought to steady-state photosynthesis in air (34 Pa p(CO2), 21 kPa p(O2), balance N2) at temperatures of 20°C and 8°C, respectively. Net photosynthesis, ribulose 1,5-bisphosphate (RuBP) and ATP contents, and ribulose 1,5-bisphosphate carboxylase/oxygenase (RuBPCO) activities were followed before and after changing to 2·0 kPa p(O2). At 20°C, lowering p(O2) increased net photosynthesis by 37%. This increase corresponded closely with the increase expected from the effect on the kinetic properties of RuBPCO. Conversely, at 8°C net photosynthesis rapidly decreased following a decrease in p(O2) and then increased again reaching a steady-state level which was only 7% higher than at 21 kPa p(O2). The steady-state rates of RuBP and associated ATP consumption were both estimated to have decreased. ATP and RuBP contents decreased by 18% and 33% respectively, immediately after the change in p(O2) suggesting that RuBP regeneration was reduced at low p(O2) due to reduced photophosphorylation. Subsequently, RuBP content increased again. Steady-state RuBP content at 2·0 kPa p(O2) was 24% higher than at 21 kPa p(O2). RuBPCO activity decreased by 22%, indicating control of steady-state RuBP consumption by RuBPCO activity. It is suggested that lack of oxygen inhibition of photosynthesis at low temperature is due to decreased photophosphorylation at low temperature and low p(O2). This may be due to assimilate accumulation within the chloroplasts. Decreased photophosphorylation seems to decrease RuBP synthesis and RuBPCO activity, possibly due to an acidification of the chloroplast strom

Effects of season, altitude and daylength on floral initiation of two contrasting genotypes of Trifolium repens L

The effects of season and altitude on floral initiation of two Trifolium repena L. cultivars, Haifa and California Ladino, were tested in the area of Ayacucho, Peru (2730 m, 13° S), with regard to their seed production potential. Specific effects of daylength at a constant temperature (20 °C) were examined in growth chambers. In Ayacucho, the proportion of inflorescence-bearing nodes was found to be lowest between December and June (Haifa 10-20%, Ladino 0-10%) and highest in August (Haifa 30%) and September-October (Ladino 15%). Late in the cool season, floral initiation of Ladino, but not that of Haifa, increased strongly at a high altitude (3250 m). In growth chambers, Haifa initiated few inflorescences and Ladino none in a 10 h daylength. In 16 h, the floral initiation of Haifa was very limited but Ladino formed many inflorescences. Floral initiation of Haifa was most pronounced and lasted longest after a daylength shift from 10 to 13 h. It is concluded that seasonal and altitudinal variations in low temperature were the main factors influencing floral initiation in the region of Ayacucho. Haifa is considered to be an intermediate-day plant, suited for seed production in the region because of its marked and early flowering. Ladino was classified as a quantitative longday plant, unsuitable for seed production at this low latitude because of its retarded floral response to low temperatur

Influence of Temperature on the Ratio of Ribulose Bisphosphate Carboxylase to Oxygenase Activities and on the Ratio of Photosynthesis to Photorespiration of Leaves

Lehnherr, B., Mächler, F. and Nösberger, J. 1985. Influence of temperature on the ratio of ribulose bisphosphate carboxylase to oxygenase activities and on the ratio of photosynthesis to photorespiration of leaves.—J. exp. Bot. 36: 1117-1125. Rates of net and gross photosynthesis of intact white clover leaves were measured by infrared gas analysis and by short term uptake of 14CO2 respectively. Ribulose bisphosphate carboxylase oxygenase (RuBPCO) was purified from young leaves and kinetic properties investigated in combined and separate assays. The ratio of carboxylase to oxygenase activities was compared with the ratio of photosynthesis to photorespiration at various temperatures and CO2 concentrations. The ratio of photosynthesis to photorespiration at 30 Pa p(CO2) was consistent with the ratio of carboxylase activity to oxygenase activity when each was measured above 20 °C. However, the ratio of photosynthesis to photorespiration increased with decreasing temperature, whereas the ratio of carboxylase to oxygenase activity was independent of temperature. This resulted in a disagreement between the measurements on the purified enzyme and intact leaf at low temperature. No disagreement between enzyme and leaf at low temperature occurred, when the ratio of photosynthesis to photorespiration was determined at increased CO2 concentrations. The results suggest an effect of low temperature and low CO2 concentration on the ratio of photosynthesis to photorespiration independent of the enzym

Photosynthesis and Degree of Polymerization of Fructan during Reproductive Growth of Meadow Fescue at two Temperatures and two Photon Flux Densities

Accumulation of dry weight was measured in plant parts of meadow fescue (Festuca pratensis Huds.) that was grown at 16/11 °C or 26/21 °C and with 20 or 60 nE cm−2 s−1 photosynthetically active radiation. Plants reached anthesis about 3 weeks later at 16/11 °C than at 26/21 °C and had then a higher proportion of dry weight in inflorescences and less in leaf blades. Growth temperature had little effect on CO2 exchange rate (CER) but plants grown at 60 nE cm−2 s−1 had higher CER than those grown at 20 nE cm−2 s−1. The concentration of water-soluble carbohydrates (WSC) at similar growth stages was usually higher at 16/11°C than at 26/21°C. High radiation also led to higher WSC in stem and leaf tissue. Root tissue changed least and WSC did not exceed 10% of dry weight during the experiment. In all tissues, when WSC was high, the fructans were distributed into a group with a high degree of polymerization (DP) and another with a low DP. The low DP group included sucrose, reducing sugars and fructans up to about 20 units long. An apparent threshold concentration of WSC was necessary for synthesis of the high DP fructans. This concentration was near 12% for leaf tissue, about 6% for stem base tissue, and 2.5% for root tissue. The average apparent DP of the high DP fructan group was 43 to 50 for leaf tissue, 31 to 93 for stem base tissue, and 27 to 31 for roots. These characteristics appeared to be mostly tissue dependent with less effect from temperature and radiatio

Influence of Temperature and O2 Concentration on Photosynthesis and Light Activation of Ribulosebisphosphate Carboxylase Oxygenase in Intact Leaves of White Clover (Trifolium repens L.)

Detached leaves of white clover (Trifolium repens L.) were kept for 1 h under various conditions of temperature, oxygen concentration and light intensity. Rates of photosynthesis were measured where appropriate and then ribulosebisphosphate carboxylase oxygenase (RuBPCO) was extracted rapidly and its initial activity measured immediately. The extracted activity increased with increased intensity of illumination of the leaves. Where leaves were pretreated at low light intensity, the lower the temperature of the leaves the higher the extracted activity of RuBPCO. At high light intensity temperature did not affect the activity of subsequently extracted RuBPCO but the light intensity which was necessary for maximum activity increased with temperature. Activity of RuBPCO from leaves pretreated in the dark was least when CO2 was low and temperature high. Leaves, pretreated at low temperatures and high light intensity in 20% O2, yielded higher activity in extracts than leaves pretreated under similar conditions but in 2% O2. A relatively weak temperature response of photosynthesis at low irradiances was associated with a decrease in extractable RuBPCO activity with increasing temperature. A strong temperature dependence of the oxygen inhibition of photosynthesis was associated with lower extractable RuBPCO activity in leaves pretreated at low oxygen concentration at low temperatures. With leaves from plants grown at low temperatures prior to treatment of leaves, oxygen inhibition of photosynthesis was less temperature dependent and activity of RuBPCO in extracts was not decreased by low O2 at low temperatures. Differences in the activation of RuBPCO appear to influence photosynthesis and account for an absence of oxygen inhibition of photosynthesis at low temperatures in plants grown in warm condition

Influence of Inorganic Phosphate on Photosynthesis of Wheat Chloroplasts: I. PHOTOSYNTHESIS AND ASSIMILATE EXPORT AT 5 °C AND 25 °C

Chloroplasts were isolated from 10 d old wheat seedlings and illuminated at 5 °C or 25 °C in various concentrations of PO43-. Photosynthetic oxygen evolution, ATP content, and export of triose phosphates and 3-phosphoglycerate were measured. Incorporation of 14C from NaH14CO3 into pentose monophosphates, fructose monophosphate, and glucose monophosphates was determined. The ATP content in illuminated chloroplasts decreased when the PO43- concentration in the medium was low. The ATP content increased when the PO43- concentration was increased. A higher PO43-. concentration in the medium was needed to increase the ATP at 5 °C than at 25 °C. This would suggest that PO43- deficiency occurs more readily at low than at high temperatures. More 14C was incorporated into photosynthetic metabolites within the chloroplasts at 5 °C than at 25 °C, indicating decreased assimilate export when the temperature was low. Dihydroxyacetone phosphate was preferentially exported when the PO43- concentration enabled a high rate of photosynthesis at 25 °C. However, under conditions of PO43- deficiency, either due to low PO43- concentration in the medium or due to low temperature, 3-phosphoglycerate was preferred for export. The results suggest that the relatively high photosynthetic rates at low temperature are due to increased concentrations of photosynthetic metabolites. The assimilate export at low temperature seems to be decreased due to decreased concentrations of dihydroxyacetone phosphate in the stroma. Preferential export of 3-phosphoglycerate at low temperature or at low PO43- concentration in the medium may be a consequence of high stromal concentrations of this metabolite. On the other hand, it could also be due to decreased stromal p

Effect of Carbohydrate Demand on the Remobilization of Starch in Stolons and Roots of White Clover (Trifolium repens L.) after Defoliation

White clover plants were grown from stolon tips in growth cabinets and then defoliated. Thereafter, changes in the contents of non-structural carbohydrates such as starch, sucrose, glucose, fructose, maltose, and pinitol in stolons and roots were monitored. Initial contents of carbohydrate reserves, photosynthetic supply of new carbohydrates and carbohydrate demand after defoliation were varied by growing the plants at various CO2 partial pressures, by varying the extent of defoliation and by removing either roots or stolon tips at the time of defoliation. Remobilization of carbohydrate reserves in stolons increased proportionally to their initial contents and was greater when plants had been severely defoliated, suggesting that carbohydrates were remobilized according to availability and demand. Starch was the predominant reserve carbohydrate. Starch degradation was associated with decreased contents of sucrose, glucose and fructose in young stolon parts and roots but not in old stolon parts suggesting that starch degradation was not strictly controlled by the contents of these sugars. A decrease in the demand for carbohydrates by removal of roots did not decrease starch degradation but increased the contents of sucrose, glucose, and fructose. Removal of stolon tips decreased starch degradation and contents of sucrose, glucose, and fructose. The results suggest that starch degradation was controlled by a factor other than sucrose, glucose, and fructose which was exported from stolon tips, e.g. gibberelli