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

Abstract

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...

Study of Glucose Starvation in Excised Maize Root Tips

“…1 and 2B); it may be calculated that the times for 50% respiration recovery after glucose replenishment were 30 min, 2 h, and 10 h after 3, 10, and 72 h starvation, respectively. Such observations have already been reported for maize roots (18) and sycamore cells (12), and may be due to the synthesis (Fig. 2A); at the end of the second phase, it was close to the control rate (Fig.…”

“…1), as already reported by Saglio and Pradet ( 18). It then further decreased slowly over 200 h until it was no longer detectable.…”

“…18 10 ,L/mL) and 0.1 M Mes at pH 6.0. A gas mixture containing 50% 02 and 50% N2 was continuously bubbled through the incubation medium to maintain a partial oxygen pressure above 35 kPa, which is the critical oxygen pressure for maize roots in aqueous solution (19 The effects of 0.2 M glucose (control and replenishment experiments) and uncouplers (10 gM DNP' and 0.5-5 AM FCCP) were studied by adding them to the incubation medium during the measurement of 02 consumption.…”

“…Since the end of the seventies, carbohydrate starvation has been studied in a number of plant species: wheat (28,29), maize (14,18), barley (8), pearl millet (1), pea (20, 26,27), soybean (13, 25), sycamore (7,10,12,17), etc. These studies have shown that in most cases, sugar starvation triggers the following sequence in plant cells: (a) the depletion of intracellular carbohydrate content and the subsequent decrease of respiration (1,12,18,20,25); (b) the breakdown of lipids and proteins (1,7,12,28) and a decline in the respiratory quotient from 1 to 0.75 (18); (c) an increase in inorganic phosphate ( 12,17), phosphorylcholine (7,17), and free amino acids (10), and a concomitant decline in nucleotides (17,18) and glycolytic enzymatic activities (12); and (d) the more or less marked disappearance of some cell ultrastructures (1,29).…”

Modifications of Etioplasts in Cotyledons during Prolonged Dark Growth of Sugar Beet Seedlings (Identification of Etiolation-Related Plastidial Aminopeptidase Activities)

“…Cotyledons were excised under a dim green light at different stages of etiolation, which was usually carried out for 4,6,8,10, and 12 d, and immediately used for the isolation of plastids or exposed to light. In this latter case, 30 pairs of excised cotyledons were placed in flasks containing 20 mL of the mineral nutrient solution described by Saglio and Pradet (1980) complemented with 50 m~ Mes, pH 6.2. Exposure to light (400 pmol photon m-'s-l) was carried out at 2OOC.…”