To investigate the abscisic acid (ABA) production of tomato (Mill.) plants in response to diurnal stressful temperatures, five-week old seedlings were exposed to day/night temperatures of 10/5, 15/10, 25/15, 35/25, or 45/35 C. The daylength was 16 hours with a light intensity of approximately 400 microeinsteins per meter per second. Plant tops were sampled at 12, 24, 48, and 72 hours. Free, alkaline-hydrolyzable (conjugated), and total ABA quantities were measured using standard gas chromatographic techniques. AU temperature regimes significantly increased both free and conjugated ABA levels over concentrations in control plants (25/15 C). The highest ABA levels were observed in plants exposed to the coolest temperature of 10/5 C. Since normal water potentials were obtained in plants of al treatments, the observed ABA response was not due to temperature-induced water stress. Therefore, temperature stress, like several other environmental stresses, induces the plant to produce high levels of ABA. Because of the similar involvement of ABA in temperature-induced and other environmental stresses, ABA may be a common mediator for all plant stresses.ABA is a naturally occurring compound of major importance in regulating plant growth and development. It has been implicated in a variety of physiological processes (1,11,13,15) and is found in elevated levels under several stressful conditions (3,4,7,8,16,18,23 Samples for ABA determination were harvested at 0, 12, 24, 48, and 72 h. Zero sampling time (09:00 h) was 3 h after the lights came on; therefore, all samples, except those at 12 h, were harvested 3 h after the lights came on. The 12-h samples were taken 1 h before the end of the light period (2 1:00 h). At each harvest, three single plant replicates (exclusive of the root system) were immediately frozen on dry ice and stored in a freezer at -18 C until analyzed. A representative sample (1-3 g) of each frozen and crushed plant was weighed, homogenized in ice-cold 90% methanol (10 ml/g fresh tissue), and filtered. ABA was analyzed according to the method described by Seeley and Powell (21). The alkaline-hydrolyzable ABA (conjugated ABA) was determined by adjusting the pH of the remaining aqueous phase to 11.0 with KOH, heating it at 60 C for 45 min, and re-extracting with methylene chloride. Acidic fractions were derivatized by ethereal diazomethane. ABA was quantified with a Tracor 222 Gas Chromatograph equipped with a Ni3 electron capture detector. Column packing was 3% OV-25 on Gas Chrom Q (100 to 200 mesh support). Purified N2 at a flow rate of 80 ml/min was used as the carrier gas.

Active sucrose uptake by discs of mature sugar beet (Bew vadai L cv GW-D2 and USH-20) root tssue shows a biphasc de nce on external sucrose. At concentrations up to 20 mill r sucroe, the active uptake mechanism appears to approach saturation, with an appaet K,, of 3.6 milhnmolar. At Sucrose gradients between source and sink regions can be maximized by increasing assimilate levels at the site of phloem loading in leaves (4) or by reducing the concentration of sucrose at the site of phloem unloading (13). Although the mechanism controlling phloem unloading of sucrose is not known, the concentration ofthe sucrose pool at the unloading site will presumably influence the rate of unloading. The sucrose pool into which phloem unloads may be in the cell wall free space in a number of economically important sinks, ie. soybean cotyledons (28), corn kernels (3), sugar beet taproot (27), and sugarcane (8). Therefore, the ability of economically important sinks to maintain low apoplastic sucrose concentrations either by metabolism or uptake should enhance their mobilizing ability relative to other sinks.An understanding of photosynthate movement into and within economically important sink regions of crop plants is necessary to evaluate some of the potential parameters controlling crop productivity and yield. In sugar beet, sucrose is not hydrolyzed during transport from the source leaves into the vacuoles of parenchyma cells of the root (6, 32). Wyse (32) found that sucrose uptake in sugar beet roots was linear between concentrations of I and 500 mi, occurred against a sucrose concentration gradient, and was sensitive to metabolic inhibitors. More recently, Saftner and Wyse (26) showed that sucrose is actively transported into the vacuoles of sugar beet root discs in a manner consistent with an alkali cation/sucrose co-transport mechanism. The objective of this research was to further characterize sucrose uptake into the cytoplasmic and vacuolar compartments of sugar beet taproot sink tissue.Translocation via the phloem conveys large quantities of photosynthates to specialized sink regions which compete for available assimilates. The ability of a sink region to assimilate translocated sucrose is limited by its relative ability to mobilize photosynthates from supply sources (12,28).The mobilizing ability of a sink region is a function of its relative size, potential growth rate, and capacity to take up and metabolize assimilates. This latter characteristic may influence sucrose gradients between various source/sink regions. These gradients are hypothesized to control carbon flux to sink regions (13,14). ' (25). Discs were incubated with [14Cjaorbitol for 1 h and the "C-sugar in the free space was then quantified by compartmental analysis. From the specific activity of the exteral media at the end of the incubation period and the "C in the free space, the volume of the free space was estimated. The volumes of the cytoplasm and vacuole were estimated from measurements made on electron micrographs of root parenchy...

The uptake of different sugars was studied in segments of isolated phloem from petioles of celery (Apium graveolens L.) in order to determine the kinetics and specificity of phloem loading in this highly uniform conductive tissue. The uptake kinetics of sucrose and the sugar alcohol, mannitol, which are both phloem-translocated, indicated presence of a single saturable system, while uptake of non-phloem sugars (glucose and 3-O-methylglucose) exhibited biphasic kinetics with lower uptake rates than those for sucrose and mannitol. The presence of unlabeled mannitol, 3-O-methylglucose and maltose in the incubation solution did not cause inhibition of labeled-sucrose uptake, indicating high carrier specificity and lack of sucrose hydrolysis in vivo. The pH optimum for sucrose uptake was 5-6. Furthermore, a rapid and transient alkalinization of the external media by sucrose indicated a sugar/H(+)-cotransport mechanism. Dual-labeling experiments showed that sucrose influx continued at a constant rate (V max=15 μmol·h(-1)·(g FW)(-1)), whereas sucrose efflux was low and insensitive to external concentration. Therefore, the saturable uptake kinetics for sucrose did not appear to be the result of an equilibrium between rates of sucrose influx and efflux.

Fructose-1,6 bisphosphatase (FBPase) is a ubiquitous enzyme controlling a key reaction. In non-photosynthetic tissues, it regulates the rate of gluconeogenesis. In photosynthetic tissues, two FBPase isozymes (chloroplastic and cytosolic) play key roles in carbon assimilation and metabolism. The cytosolic FBPase is one of the regulatory enzymes in the sucrose biosynthetic pathway - its activity is regulated by both fine and coarse control mechanisms. Kinetic and allosteric properties of the plant cytosolic FBPase are remarkably similar to the mammalian and yeast FBPase, but differ greatly from those of the chloroplastic FBPase. Cytosolic FBPase is relatively conserved among various organisms both at amino acid and nucleotide sequence levels. There is slightly higher similarity between mammalian FBPase and plant cytosolic FBPase than there is between the two plant FBPases. Expression of plant cytosolic FBPase gene is developmentally regulated and appears to be coordinated with the expression of Rubisco and other carbon metabolism enzymes. Similar to the gluconeogenic FBPase, relatively rapid end product repression of FBPase gene occurs in plant. However, unlike the gluconeogenic FBPase, a concurrent decline in plant FBPase activity does not occur in response to increased end product levels. The physiological significance of FBPase gene repression, therefore, remains unclear in plants. Both expression and activity of the cytosolic FBPase are regulated by environmental factors such as light and drought conditions. Light-dependent modulation of FBPase activity in plants appears to involve some type of posttranslational modification. In addition to elucidating the exact nature of the presumed posttranslational modification, cloning of genomic and upstream sequences is needed before we fully understand the molecular regulation of the cytosolic FBPase in plants. Use of transgenic plants with altered rates of FBPase activity offers potential for enhanced crop productivity.

No abstract