Metabolism As a Function of Water Potential in Air-Dry Seeds of Charlock (Sinapis arvensis L.)

Abstract

A new method is described for studying the metabolism of air-dry seeds. An initial pulse of '4CO2 was supplied to seeds maintained in air at controDled low water potentials for 6 months. Seeds were also infltrated with 2-'4C-acetate and with 14C-L-leucine at 0 C, redried rapiDly at 0 C, and maintained at controlled low water potentials for 4 to 6 weeks. The metabolism of the air-dry seeds was a function of the water content of the tissues, which was in equilibrium with the water potential at the seed surface. The fixation of 14CO2 and the utilization of 2-'4C-acetate increased exponentialy with water content. The incorporation of 14C-Lleucine into protein increased linearly with water content. Metabolism was not reduced to a low rate except in air-dry seeds at the lowest water potentials (-1716 to -762 bars) with 4 to 6% water.During the last phase of seed development and dehydration, structural changes occur in the embryo axis and storage tissues associated with loss of water and reduction in rate of metabolism. The cells and organelles show signs of shrinkage and possibly alteration of membrane structure and permeability. The rough endoplasmic reticulum is reduced to a small number of crescent layers in the region of organelles and the plasmalemma. The cytoplasm contains free ribosomes, but few or no free polysomes can be detected. The mitochondria have a simplified structure with an electronopaque matrix and few cristae (1,7,8,11,17). The respiration rate of air-dry seeds has been measured manometrically in a respirometer and is of the order of 100 ul h-' g-' dry weight (Qo2Qco.). The rate increases exponentially with water content, with critical values for acceleration above the water status of air-dry seeds (1, 2, 12, 15). Evidence for in vivo protein synthesis in air-dry seeds has been published by Chen (4, 5). Seeds of Avena fatua L. exposed to traces of 14C-ethanol vapor at 23 C for 1 week incorporated a significant amount of radioactivity into ethanol-insoluble residues, from which labeled amino acids were released by hydrolysis with 6 N HCI at 105 C for 18 hr or by incubation with pronase. Marcus (10) was unable to detect protein synthesis in dry wheat embryos after exposure for 19 min to 14C-leucine. Extracts (13,14). Protein synthesis in air-dry seeds apparently occurs using stored mRNA (9, 10) (Payne and Gordon, personal communication). The results of this communication show that the rate of mitochondrial activity and protein synthesis in air-dry seeds is a function of the water content of the tissues, which is in equilibrium with the water potential of the atmosphere at the seed surface. MATERIALS AND METHODSSupply of 14C to Seeds at Low Water Potentials. Charlock (Sinapis arvensis L.) seeds were surface sterilized in 96% ethyl alcohol for 5 min and dried for 1 hr under UV light. Samples of 50 seeds were placed in thin-walled polypropylene cups which were suspended with nichrome wire from the rubber caps of small glass vials under sterile conditions. The water potential at the seed sur...

Purine and pyrimidine metabolism during the partial drying treatment of white spruce (Picea glauca) somatic embryos

“…This decreased uptake rate, which suggests a reduced capability to metabolize purine and pyrimidine precursors, denotes a decline in the overall metabolism of partially dried embryos. A similar reduced incorporation rate was also reported in dry Sinapis arvensis seeds incubated with 14 CO 2 (Edwards 1976). It should be mentioned, however, that the water content of the seeds utilized in those experiments was lower than 20%, whereas that of partially dried embryos was close to 65% of that of mature embryos (data not shown).…”

Changs in pyrimidine nucleotide biosynthesis during germination of white spruce (picea glauca) somatic embryos

“…This reduced ability to take up pyrimidine precursors was observed throughout the partial drying treatment, and it was ascribed to the decrease of moisture content experienced by the white spruce somatic embryos (Stasolla et al, 2001). A similar low rate of uptake was also reported in dry Sinapis arvensis seeds fed with 14 CO 2 (Edwards, 1976). With the inception of germination, however, the gradual increase of uptake of uracil, uridine and orotic acid denotes a restored ability of the embryos to metabolize pyrimidine precursors.…”

Polysome formation and incorporation of new ribosomes into polysomes during germination of the embryonic axis of maize

“…Their presence at a low level cannot be excluded, however, since evidence for low-rate incorporation of amino-acids into protein of charlock air-dry seeds has been published by Edwards (1976).…”