Osmotic Stress in Coleoptiles and Primary Leaves of Wheat. II. Main Phosphorylated Fractions

Abstract

Seedlings of wheat (Triticum durum, cv. Balcarceno-INTA) were water-stressed in darkness with 20% polyethylene glycol (PEG) 6000 or 0-3 M mannitol added to the root medium. At different times and up to a total of 36 h of treatment the coleoptiles and primary leaves were cut and analysed for acid soluble-P, lipid-P, protein-P, alkali-stable organic-P, and nucleic acid-P. AU the phosphorylated fractions were expressed on 100 mg of total coleoptile and primary leaves dry wt. Acid soluble-P (14-6 /miol) accounted for most of the phosphorus. The lipid-P fraction remained at an unchanged level (6-2 /nnol/100 mg dry wt) in control or in mildly osmotic stressed shoots of seedlings over the period of treatment. The protein-P represented only 2-4% (0-8 //mol) of the total phosphorus found in coleoptiles and primary leaves. The alkali-stable organic-P and nucleic acid-P fractions represented 22-2% (7-2 fimai) and 11-1% (3-6 fmo\) of the total phosphorus content of shoots, respectively. All the phosphorylated fractions, excepting the lipid-P fraction, underwent a significant {P < 0-025) fall over periods of up to 36 h of wheat seedlings growth in darkness. A common characteristic found in all the phosphorylated fractions was the fact that there were not significant differences between control and 20% PEG 6000 or 0-3 M mannitol treated seedlings during 36 h of treatment and growth in darkness. However, when seedlings were pulse-labelled with 32P during imbibition, some effects of the osmotic stressants on several fractions could be seen. Specific radioactivity fell in acid soluble-P of control shoots, but increased in 20% PEG 6000 and in 0-3 M mannitol treated seedlings. Radioactive phosphorus was not found in the alkali-stable organic-P fraction. Lipid-P, nucleic acid-P, and protein-P fractions increased their specific activities during 36 h of shoot growth in control plants. Such 20% PEG 6000 and 0-3 M mannitol restricted this increase during the same period of time, the former being more active. Possible implications of phospholipid and phosphoprotein turnover in relation to water stress are discussed.