Search Results
The pit membrane of xylem parenchyma of peach plays an important role in deep supercooling. Enzyme hydrolysis of xylem tissue indicated that the pit membrane is rich in pectin. The objective of the present study was to determine if removal of calcium from the cell wall would effect deep supercooling by loosening the cell wall. Current year shoots of `Loring' peach were infiltrated with oxalic acid, EGTA, or sodium phosphate buffer for 24-48 hours and then prepared for either ultrastructural analysis or differential thermal analysis. The use of 5-50 mM oxalic acid resulted in a distinct reduction in the size of the low-temperature exotherm (LTE) with increasing concentration. Oxalic acid also produced a loosening and swelling of the pit membrane. The use of EGTA (100 mM) or NaP04 (150 mM) produced only a slight shift in the LTE to warmer temperatures when compared to fresh tissues. Heat treatments (30-100°C) also resulted in a gradual shift of the LTE to warmer temperatures. The data indicate that cross-linking of pectins may play a role in defining the pore structure of the pit membrane and that this area of the cell wall plays an integral role in deep supercooling of peach wood.
Abstract
The freezing behavior of peach [Prunus persica (L.) Batsch] flower buds was influenced by the temperature at which ice formation was initiated. Buds seeded with ice just below 0°C were more likely to exhibit deep supercooling, and water in the primordia would supercool to lower temperatures than in unseeded excised flower buds. This effect was not always expressed and varied with the stage of acclimation. Researchers using differential thermal analysis to estimate bud hardiness will need to evaluate this effect. Seeding specimens with ice may be warranted to obtain results comparable with field conditions.
Our previous research has indicated that the pit membrane regulates deep supercooling of xylem parenchyma in woody plants. This area of the cell wall is composed of three layers that may be rich in pectins. Since pectins may define the porosity of the cell wall they may also regulate deep supercooling. The present study examined pectin distribution in ray cells using monoclonal antibodies, that recognize un-esterified (JIM5) and methyl-esterified (JIM7) epitopes of pectin, in conjuction with immunogold electron microscopy. Antibodies were obtained courtesy of J. Paul Knox, John Innes Inst., U.K. Dormant and non-dormant tissues of Prunus persica, Cornus florida and Salix babylonica were utilized. Labelling with JIM7 revealed that methyl-esterified pectins were abundant and evenly distributed within the primary cell wall and amorphous layer. Labelling with JIM5 revealed that un-esterified pectins were located specifically within the pit membrane, in the outer region of the primary cell wall. No differences were observed between species, however, preliminary data indicated that JIM5 labelling was greater in dormant than in non-dormant tissues.
It has been previously reported that sub-lethal heat treatments of cold hardy Azalea (45C) and grape floral buds (47-50C) for 2 hours altered deep supercooling by shifting low temperature exotherms (LTE) to warmer temperatures. Heat treatments were shown to modify suberized tissues in Azalea which may function as an ice barrier required for deep supercooling. Our study examined deep supercooling of Loring peach buds subjected to heat treatments ranging from 40 - 49C. Temperatures above 43C for 2 hours resulted in visible injury to floral tissues. However, 65% of these buds continued to exhibit deep supercooling. Tissue injury was not observed at 40C, however longer exposure durations (16-24h) were required to shift LTEs to warmer temperatures. Moderately hardy buds (avg LTE -15C) were responsive to 40C treatment whereas, extremely hardy buds (avg LTE -20C) were not. Pre-disposing extremely hardy buds to deacclimating conditions by placing twigs in water in the greenhouse for 24h, elicited a response to heat treatment and promoted warm temperature shifts of the LTE. It appears that tissue-water status of floral buds may play a role in their response to sub-lethal heat treatment.
Abstract
Allium galanthum, A. drobovii, A. pskemense, A. roylei, and A. cepa type have the same chromosome number (n = 8), as A. cepa. The karyotypes of these species have been described. Each species has seven V-shaped chromosomes. These can be grouped as metacentric, median, submedian, and one subterminal. The subterminal chromosome has a satellite attached to its short arm. There are no other conspicuous morphological markers on the chromosomes, such as knobs or heterochromatic blocks. Chromomeres in the several species are of the same size and are present along the entire length of the chromosomes.
Slight differences were found in the morphology of the genomes of these species. The genome in A. roylei is the longest–63.44 µ, while that of A. cepa type is the shortest–56.18 µ. Allium cepa and A. pskemense have the same genome length, but there are slight differences in the morphology of the individual chromosomes. Allium cepa and A. cepa type have a wider and bigger satellite than that present in the other species. The differences among the members of the complement in A. cepa, A. cepa type, and A. galanthum are less pronounced than in A. pskemense, A. roylei, and A. drobovii. It seems that these species may have had a common ancestor, and that chromosomal differences have arisen due to inversions, translocations, and pairing in unequal chromosomes.
`Hull Thornless' and `Black Satin' blackberry (Rubus spp.) canes were collected from Sept. 1989 through Mar. 1990 to determine the hardiness and supercooling characteristics of buds at various stages of development. Anatomical studies were also conducted to examine the location of ice voids in buds frozen to -5 or -30C. Differentiation of the terminal flower occurred in `Black Satin' buds by 6 Nov., whereas `Hull Thornless' buds remained vegetative until early spring. As many as nine floral primordia were observed in both cultivars by 12 Mar. The hardiness of the two cultivars was similar until February. Thereafter, `Black Satin' buds were more susceptible to cold injury than those of `Hull Thornless'. Flora1 and undifferentiated buds of both cultivars exhibited one to four low temperature exotherms (LTEs) from 9 Oct. to 12 Mar. in differential thermal analysis (DTA) experiments. The stage of flora1 development did not influence the bud's capacity to supercool. The number of LTEs was not related to the stage of floral development or to the number of floral primordia. Extracellular voids resulting from ice formation in the bud axis and scales were observed in samples subjected to -5 or -30C.
A seasonal study was conducted to assess the freezing injury of `Boskoop Giant' black currant (Ribes nigrum L.) samples from Oct. 1991 through Mar. 1992. Buds were subjected to either differential thermal analysis (DTA) or one of a series of temperatures (0 to -36C). Freeze injury was then assessed either visually or with TTC. Results indicated that black currant floral buds have multiple low-temperature exotherms (LTE). Freeze injury in intact buds could not be visually quantified because of the lack of visible browning, nor assayed with TTC reduction. Excised floral primordia incubated in TTC, however, developed colored formazan following exposure to nonfreezing and sublethal freezing temperatures, but remained colorless when exposed to lethal temperatures. The percentage of floral primordia that were colored and colorless were tabulated and a modified Spearman-Karber equation was used to calculate the temperature at which 50% of floral primordia were killed (T50 The T50 temperature was correlated with the temperature at which the lowest LTE was detected (R2 = 0.62). TTC reduction assay using excised floral bud primordia was a good indicator of viability in frozen blackcurrant buds. Chemical name used: 2,3,5-triphenyltetrazolium chloride (TTC).