Relations between Light Level, Sucrose Concentration, and Translocation of Carbon 11 in Zea mays Leaves

The effects of ethylene evolved from ethephon on leaf and whole plant photosynthesis in Xanthium strumarium L. were examined. Ethylene-induced epinasty reduced light interception by the leaves of ethephon treated plants by up to 60%. Gas exchange values of individual, attached leaves under identical assay conditions were not inhibited even after 36 hours of ethylene exposure, although treated leaves required a longer induction period to achieve steady state photosynthesis. The speed of translocation of recently fixed "C-assimilate movement was not seriously impaired following ethephon treatment; however, a greater proportion of the assimilate was partitioned downward toward the roots. Within 24 hours of ethephon treatment, the whole plant net carbon exchange rate expressed on a per plant basis or a leaf area basis had dropped by 35%. The apparent inhibition of net carbon exchange rate was reversed by physically repositioning the leaves with respect to the light source. Ethylene exposure also inhibited expansion of young leaves which was partially reversed when the leaves were repositioned. The data indicated that ethylene indirectly affected net C gain and plant growth through modification of light interception and altered sink demand without directly inhibiting leaf photosynthesis.The extent to which ethylene can affect photosynthetic CO2 fixation and potentially alter biomass accumulation and yield has been examined in several papers; however, the results have been inconsistent (10,(12)(13)(14)(15)(16)22). Studies ofwhole plant or attached whole leaf gas exchange have indicated that net carbon uptake is reduced in some but not all of the 30 species and cultivars surveyed (10, 12-16, 21, 22 stimulated ethylene release (5). Steady state laminar gas exchange in Zea mays L. and Pisum sativum L. was not inhibited by continuous exposure to ethylene gas (12) and the stomatal aperture of intact leaves was not affected by exposure to 10 ,L L' ethylene (1) even after a 24 h exposure (4).We recently reported (22) that continuous exposure to ethylene derived from ethephon had no effect on steady state photosynthesis and transpiration rates of Lycopersicon esculentum Mill. leaves. The partitioning of recently assimilated '4CO2 into sugar, amino acid, organic acid, and starch pools in the leaf was not altered by ethylene exposure, but during the 24 h following ethephon application there was an alteration in the partitioning of current photosynthate between root and shoot sinks, (e.g. developing flower clusters). Ethephon also caused a rapid epinastic response extending through the petioles and the leaves (22) suggesting that whole plant CO2 exchange (i.e. measured in a whole plant cuvette or a whole leafcuvette enclosing the petiole) could be significantly altered due to the reduction in total light interception (21).We have selected Xanthium strumarium L. as the experimental material for this study in order to avoid potentially anomalous conclusions arising from studies (21) on a single ethylene sensitive species s...

Section: "Co2 Labeling and Speed Of Assimilate Movementmentioning

confidence: 99%

Whole Plant and Leaf Steady State Gas Exchange during Ethylene Exposure in Xanthium strumarium L.

Woodrow¹,

Jiao²,

Tsujita³

et al. 1989

“…The lower flux rate for sucrose derived from fructose compared to sucrose supplied directly as attributed to a limitation in the rate of sucrose synthesis. Recent studies have demonstrated the dependence of translocation rate upon sucrose concentration in leaves (24).…”

mentioning

confidence: 99%

Carbon Assimilation and Translocation in Soybean Leaves at Different Stages of Development

Silvius¹,

Kremer²,

Lee³

1978

Carbon assimilation, translocation, and associated biochemical characteristics of the second trifoiolate leaf (numbered acropetally) of chambergrown soybean, Glycine max (L.) Meff., plants were studied at selected stages of leaf development during the period from 10 to 25 days postemergence. Leaves of uniform age were selected on the basis of leaf plastochron index (LPI).The test leaf reached ful expansion (A..) and maximum CO2 exchange rates on a leaf area basis at 17 days postemergence (LPI 4.1). Maximum carbon exchange rates per unit dry weight of lamina were attained several days earlier and declined as specific leaf weight increased. Chlorophyl and soluble protein continued to increase beyond the attainment of A.., but were not accompanied by further increases in photosynthetic rates. All observations were made on the second trifoliolate leaf (T2) numbered acropetally at six stages of development from 11 to 25 days postemergence. In order to reduce ontogenetic variability, plants were selected for each stage on the basis of plastochron index (Pl)2 (7). The PI system was devised by Erickson and Michelini (7) to provide a precise numerical indication of the stage of development of a vegetative shoot and individual leaves thereon. Briefly, the PI of soybean was calculated as follows: the parameter chosen to indicate leaf size was the midvein length of the terminal leaflet. A reference length of 20 mm was arbitrarily selected. Then, the node number (n) having the youngest leaf with a reference length exceeding 20 mm was determined by counting acropetally from the cotyledonary node. The midvein length of this leaf (Ln), and that of the leaf at the adjacent node above (L+,I) were measured and the PI of the plant calculated from the following relationship: Much of the fixed carbon in leaves isThe leaf plastochron index (LPI) of a leaf at any given node, n, equals PI minus n.Carbon

“…The sugar supply of non-chlorophyllous tissues varies depending upon factors which affect the efficiency ofcarbon fixation by leaves and transport of the recently synthesized carbohydrates (5,7,13,21).…”

mentioning

confidence: 99%

Soluble Sugars, Respiration, and Energy Charge during Aging of Excised Maize Root Tips

Saglio¹,

Pradet²

1980

206

123

Oxygen uptake and energy charge were monitored during aging of excised maize root tips and related to the soluble sugar content and exogenous sugar supply.Oxygen uptake declined imsediately after excision to 50 to 30% of its initial value after 8 and 24 hours of aging at 25 C. There was also a sharp decline of the total sugar content (glucose, fructose, and sucrose). Starch content was very low at the time of excision and aimost neglgible 5 hours later. During the same period, the respiratory quotient declined from I to 0.75 and then remained stable.The addition of exogenous sugars induced a rapid rise of the respiratory rate which stabilized at a level correlated to the external sugar concentration. Addition of 0.2 molar glucose was necessary to restore the respiratory rate to the initial, also the maximum, level. These results indicate that metabolic activity of root tips is hily reliant on sugar import and carbohydrate reserves at the time of excision cannot compensate for the cessation of import. The control of respiration by substrate supply is in good agreement with the failure for dinitrophenol to stimulate oxygen uptake in aged sugar-depleted root tips.The energy charge remained constant at about 0.9, irrespective of the presence or absence of glucose and in spite of a large decline of respiratory activity in aged, sugar-depleted tissues.In normoxia as defined by Pradet and Bomsel (16), the respiratory rate of tissues is not limited by the 02 partial pressure.Under such conditions, most of the biological energy provided to the cells comes from sugar oxidation through respiratory pathways. The sugar supply of non-chlorophyllous tissues varies depending upon factors which affect the efficiency ofcarbon fixation by leaves and transport of the recently synthesized carbohydrates (5, 7, 13, 21).The question arises as to how metabolism adjusts to such fluctuation. Stimulation of root respiratory rate by light after darkness has been reported previously (6, 9). Hatrick and Bowling (10), using sunflower and barley, reported evidence for a complete dependence of root respiration on the rate of assimilate translocation from the shoot, suggesting that roots of young herbaceous plants have no reserves which might reduce the effect of fluctuations in the rate of translocation of photosynthetic sugars.The present study is part of an effort to understand the factors which limit and control the metabolic activity of roots. Our approach was to relate 02 uptake, taken as a measure of the metabolic activity, to soluble sugar and adenine nucleotide content of the tissues at various times after excision. The data reported demonstrate the influence of exogenous sugars on the metabolic activity of root tips. The significance ofenergy charge as parameter of normoxic cellular metabolism is discussed. MATERIALS AND METHODSPrimary root tips, 0.5 cm, were cut from maize seedlings (Zea mays L. INRA 402) germinated for 3 days at 25 C between sheets of filter paper soaked with 2 mm CaCl2.Determination of Gas Exchange. Measureme...