Genetic and environmental variation in carbon isotope discrimination (A), photosynthetic gas exchange, growth and activities of phosphoenolpyruvate carboxylase (PEPC) and ribulose-1,5-bisphosphate carboxylase (Rubisco) were studied in four sugarcane clones grown under three different irrigation regimes for 65 d in a greenhouse. A of the uppermost fully expanded leaf increased with decreasing irrigation frequency and exhibited genetic variation at all irrigation frequencies. Concurrent measurements of leaf gas exchange characteristics suggested that variation in \ was attributable to variation in bundle sheath leakiness to CO 2 (

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Sugarcane cell suspensions took up sugar from the medium at rates comparable to or greater than sugarcane tissue slices or plants in the field. This system offers an opportunity for the study of kinetic and energetic mechanisms of sugar transport in storage parenchyma-like cells in the absence of heterogeneity introduced by tissues. The following results were obtained: (a) The sugar uptake system was specific for hexoses; as previously proposed, sucrose was hydrolyzed by an extracellular invertase before the sugar moieties were taken up; no evidence for multiple sugar uptake systems was obtained. - (b) Uptake of the glucose-analog 3-O-methylglucose (3-OMG) reached a plateau value with an intracellular concentration higher than in the medium (approximately 15-fold). - (c) There was a balance of influx and efflux during steady state; the rate of exchange influx was lower than the rate of net influx; the Km value was higher (70 μM) than for net influx (24 μM); the exchange efflux is proposed to be mediated by the same transport system with a Km value of approximately 2.6 mM for internal 3-OMG; the rate of net efflux of hexoses was less than a third of the rate of exchange efflux. - (d) The uptake of hexoses proceeded as proton-symport with a stoichiometry of 0.87 H(+) per sugar; during the onset of hexose transport there was a K(+) exit of 0.94 K(+) per sugar for charge compensation. (It was assumed that the "real" stoichiometries are 1 H(+) and 1 K(+) per sugar.) The Km values for sugar transport and sugar-induced proton uptake were identical. Sucrose induced proton uptake only in the presence of cell wall invertase. - (e) There was no net proton uptake with 3-OMG by cells which were preloaded with glucose though there was significant sugar uptake. It is assumed, therefore, that the exit of hexose occurs together with protons. - (f) The protonmotive potential of sugarcane cells corresponded to about 120 mV: pH-gradient 1.1 units, membrane potential of-60 mV (these values increased if vacuolar pH and membrane potential were also considered). It was abolished by uncouplers, and the magnitude of the components depended on the external pH value. We present evidence for the operation of a proton-coupled sugar transport system in cell suspensions that were derived from, and have characteristics of, storage parenchyma. The quantitative rates of sugar transport suggest that the role of this transport system is not limiting for sugar storage.

Heat stable (STa) enterotoxin from E. coli reduced fluid absorption in vivo in the perfused jejunum of the anaesthetized rat in Krebs-phosphate buffer containing lactate and glucose (nutrient buffer), in glucose saline and in glucose free saline. Bicarbonate ion enhanced fluid absorption of 98 +/- 7 (6) microl/cm/h was very significantly (P< 0.0001) reduced by STa to 19 +/- 4 (6) microl/cm/h, but net secretion was not found. When impermeant MES substituted for bicarbonate ion, net fluid absorption of 29 +/- 3 (6) microl/cm/h was less (P < 0.01) than the values for phosphate buffer and bicarbonate buffer. With STa in MES buffer, fluid absorption of 3 +/- 2 (6) microl/cm/h was less than (P < 0.001) that in the absence of STa and not significantly different from zero net fluid absorption. E. coli STa did not cause net fluid secretion in vivo under any of the above circumstances. Neither bumetanide nor NPPB when co-perfused with STa restored the rate of fluid absorption. In experiments with zero sodium ion-containing perfusates, STa further reduced fluid absorption modestly by 20 microl/cm/h. Perfusion of ethyl-isopropyl-amiloride (EIPA) with STa in zero sodium ion buffers prevented the small increment in fluid entry into the lumen caused by STa, indicating that the STa effect was attributable to residual sodium ion and fluid uptake that zero sodium-ion perfusates did not eradicate. These experiments, using a technique that directly measures mass transport of fluid into and out of the in vivo proximal jejunum, do not support the concept that E. coli STa acts by stimulating a secretory response.

Although D-galactose is normally toxic to sugarcane (Saccharm sp.) cells, a cell line that grows on 100 mM galactose has been propagated. Nonadapted cells in a medium containing galactose instead of sucrose accumulate UDP-galactose; these cells also have much lower UDP-galactose 4epimerase (EC plants (e.g. 4, 8, 26, 30). The exact cause of this toxicity is still in doubt. Although galactose is taken up by wheat roots (4), tomato roots (9), and sugarcane cells (16), and is respired by oat coleoptiles (26) and tomato roots (22), it or its metabolites (I 1, 30) prevent synthesis ofcell walls (26) and prevent cell expansion (14), possibly by feedback regulation (1). Galactose also promotes auxin-dependent ethylene evolution, and it has been suggested that ethylene might be the cause of its toxic symptoms (6).In cell cultures galactose appears to support limited growth in some instances, even when it is the only carbohydrate in the medium (17). However, growth rates have been measured only with Lolium (20) 2 Abbreviations: Gal-l-P: galactose 1-phosphate; UDP-Gal: UDP-galactose; G-6-P: glucose 6-phosphate; G-l-P: glucose 1-phosphate; F-6-P: This cell line has lost the ability to differentiate. The stock culture was maintained by transferring an inoculum at 14-day intervals to fresh M-3 medium (25) containing 50 mm sucrose and modified only in 2,4-dichlorophenoxyacetic acid concentration (9 gm instead of 27 ,M).Galactose adaptation of the cell line described above was accomplished over a period of approximately 5 months without mutagenic treatment. The sucrose cell line was transferred to a medium containing 100 mM galactose instead of sucrose. After 6 days the normally clear culture fluid turned opaque. Cells were kept on this medium for an additional week before they were transferred to a similar medium containing 1% agar. At time of transfer to the agar medium, respiration rates were determined (model 53 oxygen monitor, Yellow Springs Instrument Co., Yellow Springs, Ohio). An undetermined proportion of cells in the culture was viable. However, these cells remained quiescent for several weeks. Upon transfer to a similar agar medium with fresh galactose, a few small colonies gradually formed among the cell clumps. These colonies were removed and repeatedly subcultured on fresh galactose medium containing agar. When sufficient callus mass developed, pieces of calli were transferred back to a galactose liquid medium to establish the galactose-adapted suspension culture.Following several transfers to fresh liquid medium, the galactose-adapted cells subsequently were subcultured at regular 14-day intervals. Although cells continued to grow well on 100 mm galactose, 50 mm galactose was substituted for the higher concentration used during the adaptation period. Except as otherwise noted, experiments reported here were carried out in the presence of either 25 mM sucrose or 50 mm galactose.We will refer to cell types throughout the text as follows: stock sucrose cultures (S); stock galactose-propagated cultures...