J. Nösberger

Storage and Remobilization of Carbohydrates in Meadow Fescue (Festuca pratensis Huds.)

Summary Vegetative meadow fescue plants ( Festuca pratensis Huds.) were steady state labelled with 14 CO 2 for one light period under standard conditions, as well as at decreased CO 2 partial pressure or at increased temperature. Storage and remobilization of labelled and unlabelled fructan and starch in blades, stubbles, and roots was studied at the end of the light period and at the end of the subsequent dark period, respectively. The content of polysaccharides in the leaf blades was high under standard conditions. Polysaccharides were associated with high sucrose contents and showed low turnover activity. Diurnal fluctuation of carbohydrate content was due more to sucrose than to polysaccharides. However, polysaccharides were remobilized readily both at decreased CO 2 partial pressure and at increased temperature. Remobilization of starch was inhibited at low temperature. Monosaccharides decreased during the light period and increased during the dark period, due probably to increased remobilization of polysaccharides in the dark. The content of polysaccharides in the stubbles was lower than in the blades and associated with a low sucrose content. Polysaccharides showed higher turnover activity than in the blades. Diurnal fluctuation of carbohydrate content was due more to polysaccharides than to sucrose; however, remobilization of polysaccharides at decreased CO 2 partial pressure or increased temperature occurred less readily than in the blades. Remobilization of starch was inhibited at low temperature. The monosaccharide content was not affected by the diurnal cycle, by decreased CO 2 partial pressure or by increased temperature. The content of polysaccharides and sucrose in the roots was low. Monosaccharides were the predominant carbohydrates. The monosaccharide content was not affected by the diurnal cycle or by decreased CO 2 partial pressure. However, some decrease in monosaccharides occurred at increased temperature.

Sink control of assimilate partitioning in meadow fescue festuca pratensis huds

Summary Vegetative tillers of meadow fescue ( Festuca pratensis Huds.) were excised from plants. One set to tillers was stripped to leave only the youngest fully developed leaf. Another set of tillers was stripped to leave the youngest fully developed leaf and the part of the growing leaf which had not yet emerged. The leaf blades of these isolated tillers were illuminated during one light period (14 h) in the presence of 14 CO 2 of constant specific activity. Temperature of the leaf blade on the one hand and of the sheath and the growing leaf base on the other hand was varied independently. 14 C in monosaccharides, sucrose, fructan, starch, and structural polysaccharides in the blade, the sheath, and the base of the growing leaf was analyzed at the end of the light period. Sink activities of the leaf sheath and of the growing leaf base were compared and temperature effects were studied. The synthesis of starch, fructan, and structural polysaccharides was related to the sucrose contents. Export of photoassimilates from the leaf blade was strongly affected by the temperature of the sink tissues (sheath and growing base) but only little by the temperature of the photosyn-thesizing blade. The increase in sink activity with temperature was associated with increased sucrose contents in the sink tissues suggesting that sucrose unloading from the phloem was increased. Synthesis of fructan, starch, and structural polysaccharides in the sheath increased with sucrose content. Sucrose saturated polysaccharide synthesis increased with temperature. Synthesis of starch increased more with temperature than synthesis of fructan. 14 C partitioning to the sheath and 14 C sucrose content in the sheath increased as the supply from the blade was increased. 14 C partitioning to the growing base and 14 C sucrose in the growing base were independent of assimilate supply. The growing base was a much stronger sink than the sheath. Sucrose unloading in the sheath appeared to be inhibited if the growing base competed for assimilates. The ratios of 14 C fructose to 14 C glucose were distinct for blades, sheaths and growing leaf bases and appeared to be related to the different metabolic activities in source, storing sink, and growing sink tissues.

Light Response of Photosynthesis and of RuBP Carboxylase Oxygenase Activity in Young, Mature, and Senescing Leaves of Red Clover (Trifolium pratense L.)

Summary The light response of the CO 2 exchange rate (CER) and of the activity of ribulose bis-phosphate carboxylase/oxygenase (RuBPCO) after rapid extraction (activity in vivo ) and after activation with CO 2 and Mg ++ (activity in vitro ) was determined in leaves of red clover ( Trifolium pratense L.) 1, 2 and 4 weeks after the petioles were first visible. They were referred to as young, mature, and senescent leaves, respectively. The age of the leaf affected CER at high but not at low photosynthetic photon flux density (PPFD) and thus influenced the shape of the light response curve. At high PPFD, CER was highest in mature leaves and lowest in those which were senescing. Responses to PPFD and to the age of the leaf were similar for CER and for in vivo activity of RuBPCO suggesting that, even at low PPFD, CER was controlled by RuBPCO activity. At PPFD higher than 400 °mol quanta m −2 s -1 , RuBPCO seemed to be fully activated in vivo . At high PPFD, differences in CER and in activity of RuBPCO in vivo between leaves of different ages were related to varying RuBPCO contents as estimated from the activity of RuBPCO after activation with CO 2 and Mg ++ in vitro . At lower PPFD, CER and in vivo activity of RuBPCO were independent of the age of the leaf despite differences in RuBPCO content. This was due to an effect of age on the degree of activation of RuBPCO in vivo below 300 °mol quanta m -2 s -1 .