Carbon Dioxide Exchange and Acidity Levels in Detached Pineapple, Ananas comosus (L.), Merr., Leaves during the Day at Various Temperatures, Oxygen and Carbon Dioxide Concentrations

Integrated Metabolome and Transcriptome Analysis Provide Insights into the Effects of Grafting on Fruit Flavor of Cucumber with Different Rootstocks

Rootstocks frequently exert detrimental effects on the fruit quality of grafted cucumber (Cucumis sativus L.) plants. To understand and ultimately correct this deficiency, a transcriptomic and metabolomic comparative analysis was performed among cucumber fruits from non-grafted plants (NG), and fruits from plants grafted onto different rootstocks of No.96 and No.45 (Cucurbita moschata. Duch), known to confer a different aroma and taste. We found remarkable changes in the primary metabolites of sugars, organic acids, amino acids, and alcohols in the fruit of the grafted cucumber plants with different rootstocks, compared to the non-grafted ones, especially No.45. We identified 140, 131, and 244 differentially expressed genes (DEGs) in the comparisons of GNo.96 vs. NG, GNo.45 vs. NG, and GNo.45 vs. GNo.96. The identified DEGs have functions involved in many metabolic processes, such as starch and sucrose metabolism; the biosynthesis of diterpenoid, carotenoid, and zeatin compounds; and plant hormone signal transduction. Members of the HSF, AP2/ERF-ERF, HB-HD-ZIP, and MYB transcription factor families were triggered in the grafted cucumbers, especially in the cucumber grafted on No.96. Based on a correlation analysis of the relationships between the metabolites and genes, we screened 10 candidate genes likely to be involved in sugar metabolism (Fructose-6-phosphate and trehalose), linoleic acid, and amino-acid (isoleucine, proline, and valine) biosynthesis in grafted cucumbers, and then confirmed the gene expression patterns of these genes by qRT-PCR. The levels of TPS15 (Csa3G040850) were remarkably increased in cucumber fruit with No.96 rootstock compared with No.45, suggesting changes in the volatile chemical production. Together, the results of this study improve our understanding of flavor changes in grafted cucumbers, and identify the candidate genes involved in this process.

Stomatal Biology of CAM Plants

Recent advances in the stomatal biology of CAM plants are reviewed, and key opportunities for future progress are identified.

Integrated operation of the photorespiratory cycle and cytosolic metabolism in the modulation of primary nitrogen assimilation and export of organic N-transport compounds from leaves: a hypothesis

Photorespiration is generally considered to be an essentially dissipative process, although it performs some protective and essential functions. A theoretical appraisal indicates that the loss of freshly assimilated CO2 due to photorespiration in well-watered plants may not be as high as generally believed. Even under moderately adverse conditions, these losses may not exceed 10%. The photorespiratory metabolism of the source leaves of well-watered and well-nourished crop plants ought to be different from that of other leaves because the fluxes of the export of both carbohydrates and organic N-transport compounds in source leaves is quite high. With a heuristic approach that involved the dovetailing of certain metabolic steps with the photorespiratory cycle (PR-cycle), a novel network is proposed to operate in the source-leaves of well-watered and well-nourished plants. This network allows for the diversion of metabolites from their cyclic-routes in sizeable quantities. With the removal of considerable quantities of glycine and serine from the cyclic route, the number of RuBP oxygenation events would be several times those of the formation of hydroxypyruvate. Thus, to an extreme extent, photorespiratory metabolism would become open-ended and involve much less futile recycling of glycine and serine. Conversion of glyoxylate to glycine has been proposed to be a crucial step in the determination of the relative rates of the futile (cyclic) and anabolic (open-ended) routes. Thus, in the source leaves of well-watered and well-nourished plants, the importance of the cyclic route is limited to the salvaging of photorespiratory intermediates for the regeneration of RuBP. The proposed network is resilient enough to coordinate the rates of the assimilation of carbon and nitrogen in accordance with the moisture and N-fertility statuses of the soil.

CO(2) and O(2) Exchanges in the CAM Plant Ananas comosus (L.) Merr: Determination of Total and Malate-Decorboxylation-Dependent CO(2)-Assimilation Rates; Study of Light O(2)-Uptake

Photosynthesis and light O(2)-uptake of the aerial portion of the CAM plant Ananas comosus (L.) merr. were studied by CO(2) and O(2) gas exchange measurements. The amount of CO(2) which was fixed during a complete day-night cycle was equal to the amount of total net O(2) evolved. This finding justifies the assumption that in each time interval of the light period, the difference between the rates of net O(2)-evolution and of net light atmospheric CO(2)-uptake give the rates of malate-decarboxylation-dependent CO(2) assimilation. Based upon this hypothesis, the following photosynthetic characteristics were observed: (a) From the onset of the light to midphase IV of CAM, the photosynthetic quotient (net O(2) evolved/net CO(2) fixed) was higher than 1. This indicates that malate-decarboxylation supplied CO(2) for the photosynthetic carbon reduction cycle during this period. (b) In phase III and early phase IV, the rate of CO(2) assimilation deduced from net O(2)-evolution was 3 times higher than the maximum rate of atmospheric CO(2)-fixation during phase IV. A conceivable explanation for this stimulation of photosynthesis is that the intracellular CO(2)-concentration was high because of malate decarboxylation. (c) During the final hours of the light period, the photosynthetic quotient decreased below 1. This may be the result of CO(2)-fixation by phosphoenolpyruvate-carboxylase activity and malate accumulation. Based upon this hypothesis, the gas exchange data indicates that at least 50% of the CO(2) fixed during the last hour of the light period was stored as malate. Light O(2)-uptake determined with (18)O(2) showed two remarkable characteristics: from the onset of the light until midphase IV the rate of O(2)-uptake increased progressively; during the following part of the light period, the rate of O(2)-uptake was 3.5 times higher than the maximum rate of CO(2)-uptake. When malate decarboxylation was reduced or suppressed after a night in a CO(2)-free atmosphere or in continuous illumination, the rate of O(2)-uptake was higher than in the control. This supports the hypothesis that the low rate of O(2)-uptake in the first part of the light period is due to the inhibition of photorespiration by increased intracellular CO(2) concentration because of malate decarboxylation. In view of the law of gas diffusion and the kinetic properties of the ribulose-1,5-bisphosphate carboxylase/oxygenase, O(2) and CO(2) gas exchange suggest that at the end of the light period the intracellular CO(2) concentration was very low. We propose that the high ratio of O(2)-uptake/CO(2)-fixation is principally caused by the stimulation of photorespiration during this period.

Minor Physiological Response to Elevated CO(2) by the CAM Plant Agave vilmoriniana

One-year-old plants of the CAM leaf succulent Agave vilmoriniana Berger were grown outdoors at Riverside, California. Potted plants were acclimated to CO(2)-enrichment (about 750 microliters per liter) by growth for 2 weeks in an open-top polyethylene chamber. Control plants were grown nearby where the ambient CO(2) concentration was about 370 microliters per liter. When the plants were well watered, CO(2)-induced differences in stomatal conductances and CO(2) assimilation rates over the entire 24-hour period were not large. There was a large nocturnal acidification in both CO(2) treatments and insignificant differences in leaf chlorophyll content. Well watered plants maintained water potentials of -0.3 to -0.4 megapascals. When other plants were allowed to dry to water potentials of -1.2 to -1.7 megapascals, stomatal conductances and CO(2) uptake rates were reduced in magnitude, with the biggest difference in Phase IV photosynthesis. The minor nocturnal response to CO(2) by this species is interpreted to indicate saturated, or nearly saturated, phosphoenolpyruvate carboxylase activity at current atmospheric CO(2) concentrations. CO(2)-enhanced diurnal activity of ribulose bisphosphate carboxylase activity remains a possibility.

Diurnal Changes in Metabolite Levels and Crassulacean Acid Metabolism in Kalanchoë daigremontiana Leaves

Diurnal changes in levels of selected metabolites associated with glycolysis, the C(3) cycle, C(4)-organic acids, and storage carbohydrates were analyzed in active Kalanchoë daigremontiana Crassulacean acid metabolism leaves. Three metabolic transition periods occurred each day. During the first two hours of light, nearly all of the metabolite pools underwent transient changes. Beginning at daylight, stomata opened transiently and closed again within 30 minutes; malate synthesis continued for about 1 hour into the light; C(3) photosynthesis began within 30 minutes; and net quantities of starch and glucan began to accumulate after 2 hours, continuing linearly throughout the rest of the day.THE SECOND TRANSITION OCCURRED IN MIDAFTERNOON: stomata reopened; malate decarboxylation nearly terminated; and the assimilation of ambient CO(2) occurred primarily via the C(3) cycle. The third transition occurred at dark: stomata transiently closed before opening again; the C(3) cycle stopped; malate synthesis started in about 1 hour; starch and glucan degradation began within 1 hour; and the bulk of carbon flow was through glycolysis leading to the synthesis and accumulation of malate throughout the night. At night, the levels of metabolites involved in acidification and glycolysis (except for phosphoenolpyruvate) generally accumulated. Phosphoenolpyruvate levels peaked near midday and were minimal at night. The ribulose 1,5-bisphosphate pool was depleted at night, while sedoheptulose 1,7-bisphosphate, fructose 1,6-bisphosphate, glucose 6-phosphate, and fructose 6-phosphate accumulated.

C(4) Acid Metabolism and Dark CO(2) Fixation in a Submersed Aquatic Macrophyte (Hydrilla verticillata)

The CO(2) compensation point of the submersed aquatic macrophyte Hydrilla verticillata varied from high (above 50 microliters per liter) to low (10 to 25 microliters per liter) values, depending on the growth conditions. Plants from the lake in winter or after incubation in an 11 C/9-hour photoperiod had high values, whereas summer plants or those incubated in a 27 C/14-hour photoperiod had low values. The plants with low CO(2) compensation points exhibited dark (14)CO(2) fixation rates that were up to 30% of the light fixation rates. This fixation reduced respiratory CO(2) loss, but did not result in a net uptake of CO(2) at night. The low compensation point plants also showed diurnal fluctuations in titratable acid, such as occur in Crassulacean acid metabolism plants. However, dark fixation and diurnal acid fluctuations were negligible in Hydrilla plants with high CO(2) compensation points.Exposure of the low compensation point plants to 20 micromolar (14)CO(2) resulted in 60% of the (14)C being incorporated into malate and aspartate, with only 16% in sugar phosphates. At a high CO(2) level, the C(4) acid label was decreased. A pulse-chase study indicated that the (14)C in malate, but not aspartate, decreased after a long (270-second) chase period; thus, the C(4) acid turnover was much slower than in C(4) plants.Phosphoenolpyruvate carboxylase activity was high (330 micromoles per milligram chlorophyll per hour), as compared to ribulose bisphosphate carboxylase (20 to 25), in the plants with low compensation points. These plants also had a pyruvate, Pi dikinase activity in the leaves of 41 micromoles per milligram chlorophyll per hour, which suggests they are not C(3) plants. NAD- and NADP(+)-malate dehydrogenase activities were 6136 and 24.5 micromoles per milligram chlorophyll per hour, respectively. Of the three decarboxylating enzymes assayed, the activities of NAD- and NADP(+)-malic enzyme were 104.2 and 23.7 micromoles per milligram chlorophyll per hour, while phosphoenolpyruvate carboxykinase was only 0.2.Low compensation point Hydrilla plants fix some CO(2) into C(4) acids, which can be decarboxylated for later refixation, presumably into the Calvin cycle. Refixation would be advantageous in summer lake environments where the CO(2) levels are high at night but low during the day. Hydrilla does not fit any of the present photosynthetic categories, and may have to be placed into a new group, together with other submersed aquatic macrophytes that have environmentally variable CO(2) compensation points.

Resistance Analysis of Nocturnal Carbon Dioxide Uptake by a Crassulacean Acid Metabolism Succulent, Agave deserti

Nocturnal CO(2) uptake by a Crassulacean acid metabolism succulent, Agave deserti Engelm. (Agavaceae), was measured so that the resistance properties of the mesophyll chlorenchyma cells and their CO(2) concentrations could be determined. Two equivalents of acidity were produced at night per mole of CO(2) taken up. The nocturnal CO(2) uptake became light-saturated at 3.5 mEinsteins cm(-2) of photosynthetically active radiation (400-700 nm) incident during the preceding day; at least 46 Einsteins were required per mole of CO(2) fixed. Variations in the daytime leaf temperature between 20 and 37 C had little effect on nocturnal CO(2) uptake. After the first few hours in the dark, the leaf liquid phase CO(2) resistance (r(liq) (CO(2) )) and the CO(2) concentration in the chlorenchyma cells (c(i) (CO(2) )) both increased, the latter usually reaching the ambient external CO(2) level at the end of the dark period. Increasing the leaf surface temperature above 15 C at night markedly increased the stomatal resistance, r(liq) (CO(2) ), and c(i) (CO(2) ).The minimum r(liq) (CO(2) ) at night was about 1.6 seconds cm(-1). Based on the ratio of chlorenchyma surface area to total leaf surface area of 82, this r(liq) (CO(2) ) corresponded to a minimum cellular resistance of approximately 130 seconds cm(-1), comparable to values for mesophyll cells of C(3) plants. The contribution of the carboxylation reaction and/or other biochemical steps to r(liq) (CO(2) ) may increase appreciably as the nighttime temperature shifts a few degrees from the optimum or after a few hours in the dark, both of which caused large increases in r(liq) (CO(2) ). This necessitates a large internal leaf area for CO(2) diffusion into the chlorenchyma to support moderate nocturnal CO(2) uptake rates by these succulent leaves.