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Natalie L. Hubbard, D. Mason Pharr and Steven C. Huber

Muskmelon (Cucumis melo L.) fruit lack a stored starch reserve and therefore depend on translocated photoassimilate from the leaf canopy for sugar accumulation during ripening. The influence of canopy photosynthesis on sucrose' accumulation within muskmelon fruit mesocarp was examined. Canopy photosynthetic activities were estimated in a sweet and a nonsweet genotype. Photosynthetic rate of the nonsweet genotype, on a per-plant basis, was only 56% of that of the sweet genotype. The effect of limiting leaf area of the sweet genotype on carbohydrate concentrations and sucrose metabolizing enzymes within the fruit was evaluated. A 50% reduction of leaf area 8 days before initiation of fruit sucrose accumulation resulted in canopy photosynthesis similar to that of the nonsweet genotype. Reduced photosynthetic activity resulted in slightly lower soluble-carbohydrate concentration in the fruit; however, fruit sucrose concentration was three times higher than that reported previously for the nonsweet genotype. The extent to which `fruit sucrose phosphate synthase (SPS) activity increased during maturation was diminished by leaf removal. Acid invertase activity declined in all fruit in a similar manner irrespective of defoliation. A reduction of leaf area of a sweet genotype reduced sucrose accumulation within the fruit. Lower fruit sucrose concentration was associated with lower concentration of raffinose saccharides and lower SPS activity within the fruit. Additionally, insufficient assimilate supply was judged not to be the factor responsible for low sucrose accumulation in a nonsweet genotype.

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Jason J. Griffin, Thomas G. Ranney and D. Mason Pharr

Tolerance to high solar irradiation is an important aspect of stress tolerance for landscape plants, particularly for species native to understory conditions. The objective of this study was to evaluate differential tolerance to high solar irradiation and underlying photosynthetic characteristics of diverse taxa of Illicium L. grown under full sun or 50% shade. Eleven commercially available taxa of Illicium were evaluated for light tolerance by measuring light-saturated photosynthetic capacity (Amax), dark-adapted quantum efficiency of photosystem II (Fv/Fm), and relative chlorophyll content using a SPAD chlorophyll meter. Comparisons of Amax indicated that three of the 11 taxa (I. anisatum L., I. parviflorum Michx. ex Vent., and I. parviflorum `Forest Green') maintained similar rates of light-saturated carbon assimilation when grown in either shade or full sun. All other taxa experienced a significant reduction in Amax when grown in full sun. Chlorophyll fluorescence analysis demonstrated that Fv/Fm was similar between sun and shade plants for the same three taxa that were able to maintain Amax. These taxa appeared to experience less photoinhibition than the others and maintained greater maximum photochemical efficiency of absorbed light. SPAD readings were not significantly reduced in these three taxa either, whereas most other taxa experienced a significant reduction. In fact, SPAD readings were significantly higher in I. parviflorum `Forest Green' when grown under full sun, which also maintained the highest Amax of all the taxa. These results suggest that there is considerable variation in light tolerance among these taxa, with I. parviflorum `Forest Green' demonstrating superior tolerance to high light among the plants compared. A more rigorous examination of I. parviflorum `Forest Green' (high light tolerance) and I. floridanum Ellis (low-light tolerance) demonstrated that I. parviflorum `Forest Green' had a considerably higher Amax, a higher light saturation point, greater potential photosynthetic capacity, reduced susceptibility to photoinhibition as indicated by superior PSII efficiency following light exposure, greater capacity for thermal de-excitation as indicated by a higher rate of nonphotochemical quenching (NPQ) under full sun, greater apparent electron transport rate (ETR) at mid-day, and higher concentrations of the free-radical scavenger myo-inositol. All of these factors contribute potentially to a greater capacity to use light energy for carbon fixation while minimizing photodamage.

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Jason J. Griffin, Thomas G. Ranney and D. Mason Pharr

Net photosynthesis (Pn) of two ecotypes of redbud (Cercis canadensis L.) was studied following growth under high temperatures and increasing drought. Although mexican redbud [C. canadensis var. mexicana (Rose) M. Hopkins] exhibited greater Pn than eastern redbud (C. canadensis var. canadensis L.), Pn decreased at a similar rate under water deficit stress for both ecotypes. Mexican redbud also had greater instantaneous water use efficiency [net photosynthesis: transpiration (WUE)] than eastern redbud. Differences in both Pn and WUE might have been due to differences in leaf thickness. The optimum temperature for potential photosynthetic capacity (37 °C) was unaffected by irrigation or ecotype. Tissue osmotic potential at full turgor was more negative in eastern redbud, but was unaffected by drought stress in either ecotype. Soluble carbohydrate content was higher in eastern redbud, and in both ecotypes, d-pinitol was the major soluble carbohydrate and was considerably more abundant in the water-stressed plants. Total polyol content (myo-inositol + ononitol + pinitol) was also greater in the water-stressed plants. Both ecotypes were very tolerant of high temperatures and drought.

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D. Bradley Rowe, Stuart L. Warren, Frank A. Blazich and D. Mason Pharr

Catawba rhododendron (Rhododendron catawbiense Michx.) seedlings of two provenances, Johnston County, N.C. (35°45′N, 78°12′W, elevation = 67 m), and Yancey County, N.C. (35°45′N, 82°16′W, elevation = 1954 m), were grown in controlled-environment chambers for 18 weeks with days at 18, 22, 26, or 30C in factorial combination with nights at 14, 18, 22, or 26C. Seedlings of the higher-elevation provenance generally exhibited higher net leaf photosynthetic rates (PN)s than those from the lower elevation at all temperature combinations. Thus, it appears seedlings of the high-elevation provenance possess greater relative thermotolerance, expressed as net photosynthesis, than the low-elevation provenance. Eighty-seven days after initiation (DAI) of the experiment, PN showed a quadratic response to increasing day temperature, with the maximum occurring at 22C, whereas PN decreased linearly with increasing night temperature. At 122 DAI, PN increased linearly with increasing day temperature with nights at 22 and 26C. Highest PNs were at 30/22C and 26/22C. Carbohydrate export increased with increasing day temperature, whereas the response to night temperature was minimal. High levels of nonstructural carbohydrates occurred at thermoperiods (22/22C and 26/22C) that optimize seedling growth. However, definitive trends relating seedling growth to PNs, leaf carbohydrate levels, or to the amount of carbohydrate exported from the leaves were difficult to generalize due to numerous day × night interactions.

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W. Roland Leatherwood, D. Mason Pharr, Lisa O. Dean and John D. Williamson

Sugars and sugar alcohols have well-documented roles in salt tolerance of whole plants and maturing seeds. Less is known, however, about possible effects of these compounds during germination. Seeds from mannitol-accumulating salt-tolerant celery [Apium graveolens L. var. dulce (P. Mill.) DC], non-mannitol-accumulating salt-tolerant cabbage (Brassica oleracea L. var. capitata L. ‘Golden Acre’), and salt-sensitive non-mannitol-accumulating tobacco (Nicotiana tabacum L.) and arabidopsis [Arabidopsis thaliana (L.) Heynh.] were placed on vertical Phytagel plates containing 0 to 300 mm NaCl. Germination percentage, root elongation, and carbohydrate content of seeds and seedlings were assessed. With the exception of cabbage, there was no positive relationship between ability to germinate in NaCl and the reported species salt tolerance of the mature plant. For instance, while cabbage seeds germinated in 300 mm NaCl, germination of two celery cultivars was inhibited completely by 150 mm NaCl. In contrast, seeds from salt-sensitive tobacco and arabidopsis germinated in 200 mm NaCl. There was also no obvious relationship between the observed salt tolerance and total soluble carbohydrates in either non-imbibed seeds or in seedlings germinated in salt. For example, the most-salt tolerant species in these studies, cabbage, had the third highest seed and seedling carbohydrate concentration, while the next most tolerant, arabidopsis, had the lowest. However, both species contained significant amounts of the osmoprotective oligosaccharides raffinose or stachyose. In addition, although celery seedling mannitol concentration initially increased at low NaCl concentrations (50 mm), germination and mannitol concentration decreased at higher NaCl concentrations (100 mm). Finally, the broadest response observed was a large increase in seedling sucrose at the lowest salt concentration that significantly inhibited germination. Although most seeds, with the notable exception of cabbage, did not germinate at 150 mm NaCl, they were still metabolically active because the sucrose content was two to eight times higher than in non-imbibed seeds, suggesting a possible role for sucrose in salt-stressed germinating seeds. These results not only suggest that mechanisms providing salt tolerance in mature plants are different from those in germinating seeds, but also that, even when the same mechanisms are employed, they may be less effective in seeds.

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John D. Williamson, Dianne B. Jennings, Wei-Wen Guo, D. Mason Pharr and Marilyn Ehrenshaft

The traditional use of polyols as osmotica in plant culture media is based on the assumption that polyols are not taken up or metabolized by cells. In reality, polyols are significant photosynthetic products and efficiently utilized metabolites in a large number of plants. In addition to these metabolic roles, initial interest in polyols focused primarily on their function as osmoprotectants. This was hypothesized to be due to their ability to act as compatible solutes. More recent research, however, indicates much broader roles for polyols in stress responses based on their significant antioxidant capacity. These include protection against salt and photooxidative stress as well as a potential role in plant pathogen interactions.

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Thomas G. Ranney, James D. Burton, James F. Walgenbach, D. Mason Pharr, Cindy A. Patton and Eleanor P. Maness

No-choice feeding trials were conducted with adult Japanese beetles on leaves from 14 taxa of rosaceous trees. Feeding intensity (leaf area consumption) ranged from 0.08 to 6.1 cm2/day for Prunus virginiana and P. sargentii, respectively. Analysis of endogenous chemical constituents suggested that the mechanisms of resistance varied for the different plant genera. Among the Prunus taxa there was a significant negative correlation between cyanide potential and feeding intensity (r = –0.56). Tissue toughness (resistance to tearing) was also negatively correlated with feeding intensity (r = –0.39) for all taxa. Soluble sugars (glucose, fructose, sucrose, and sorbitol) had no significant phagostimulatory effect, separately or in combination.