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Peter C. Andersen, Brent V. Brodbeck, and Russell F. Mizell III

Diurnal variations in the chemical composition of xylem fluid have been established for many plant species exhibiting positive root pressure; similar patterns have not been well documented in transpiring plants. Diurnal changes in plant water status and xylem fluid chemistry were investigated for `Flordaking' peach [Prunus persica (L.) Batsch], `Suwannee' grape (Vitis hybrid), and `Flordahome' pear (Pyrus communis L.). Xylem tension was maximum at 1200 or 1600 hr and declined to <0.5 MPa before dawn. Xylem fluid osmolarity ranged from 10 to 27 mm and was not correlated with diurnal patterns of xylem tension. The combined concentration of amino acids and organic acids accounted for up to 70%, 45%, 55%, and 23% of total osmolarity for irrigated P. persica, nonirrigated P. persica, Vitis, and P. communis, respectively. The concentration of total organic compounds in xylem fluid was numerically greatest at 0800 or 0900 hr. For irrigated P. persica the osmolarity of xylem fluid was reduced by 45% from 0800 to 1200 hr, 1 h after irrigation, compared to only a 12% reduction from 0800 to 1200 hr for nonirrigated trees. Asparagine, aspartic acid, glutamine, and glutamic acid were mainly responsible for diurnal changes in the concentration of total amino acids and organic N for P. persica; the diurnal variation in organic N for Vitis was due to glutamine. Arginine, rather than the amides, was the primary source of organic N in xylem fluid of P. communis, and there was no consistent diurnal change in the concentration of amino acids or organic N. The predominant organic acids in all species examined were citric and malic acids. No consistent diurnal trend occurred in the concentration of organic acids or sugars in xylem fluid.

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Myong-Dong Cho, Yong-Koo Kim, and Hee-Seung Park

`Yumyeong' peach has the desirable characteristics of long shelf-life and specific non-melting nature with a long harvest period. However, some fruits harvested too late show fruit pithiness symptoms or internal breakdown. This study was conducted to analyze the differences between fruit flesh pithiness and internal breakdown symptoms and to find out the source of flesh pithiness in `Yumyeong' peach. The rate of flesh pithiness was higher in fruit harvested late in the season. Sugar and malic acid contents showed no differences between the normal and flesh pithiness fruits, but the acidity was significantly lower and was affected by low citric acid content in flesh pithiness fruit. In flesh pithiness fruits, calcium contents were low both in skin and flesh. Occurrence of flesh pithiness fruits was high in the years with low precipitation and high temperature for 2 months before harvest. In observations on morphological characteristics, the parts showing flesh pithiness consisted of smaller cells than the normal parts. Tonoplasts disintegrated and the number of dead cells was high in internal breakdown fruits, while the tonoplasts were intact, with contracted vacuoles, in flesh pithiness fruits. Tylosises were observed in vascular tissues around the flesh pithiness; therefore, it was assumed that those tylosises restrict flesh tissue development, resulting in flesh pithiness. Other varieties (`Fantasia', `Wolmi' and `Hakuto') also showed tylosis, and smaller cells were observed in the flesh tissue of these cultivars, indicating abnormal growth of the flesh part. These results suggest the possibility of the occurrence of pithiness-like symptoms in other peach varieties.

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Jonathan W. Sinclair and David H. Byrne

Carbohydrate energy source of various tissue culture media has an effect on growth and survival of the explants. Sucrose is the standard carbohydrate used in most tissue culture systems. The objective of the study was to determine the effect of five carbohydrate sources (fructose, glucose, maltose, sorbitol, and sucrose) at two levels (2% and 3%) on germination, growth, and survival of immature peach embryos (9.7 to 14.7mm) in vitro. Five cultivars were used. Overall, fructose, maltose, and sucrose each stimulated germination and growth as the primary carbohydrate energy source of peach embryo culture to the same degree; glucose and sorbitol were inferior. However, fructose was superior to sucrose in one cultivar. In general, sugar level did not affect survival, although cultivars did vary somewhat. Survival was found to be highly dependent upon embryo maturity.

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Alexandra Delgado, Angel L. Gonzalez*, and Maria Del C. Libran

Peach cultivars are being evaluated for their adaptation to the conditions of the central region of Puerto Rico. The root weevil, Diaprepes abbreviatus (L.), a common insect pest in the area, is being previously reported on peach trees. With the purpose of evaluating the potential feeding damage that this insect might represent for future peach production, choice and no-choice tests were made with leaf discs (feeding) and leaf strips (oviposition) to determine adult feeding and oviposition behavior in comparison with `Navel' orange. Larval feeding behavior on the roots was studied on a peach rootstock and `Cleopatra' mandarin planted in 18.9-L containers. In the no-choice test, adults fed significantly more on `Navel' orange foliage than on peach foliage. In the choice test, adults preferred to feed on `Navel' orange leaf discs. Oviposition occurred on both peach cultivars tested, but more egg masses were laid on Navel orange leaf strips in the no-choice test. However, given the choice, adults preferred to oviposit on peach leaf strips while fed on `Navel' orange leaf strips. In some replications this behavior was reversed. At 90 days after infestation, larval feeding damage on the roots was severe on `Cleopatra' mandarin where most of the cortex tissue on the primary root was removed and growth of roots and foliage was reduced. Larvae bore also on peach trees, but there was no sign of growth reduction on foliage or the roots compared to the control. These preliminary results indicate that D. abbreviatus will not be a primary pest on peach.

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Christina Wells and Desmond Layne

We are using a minirhizotron camera system to observe fine root dynamics beneath irrigated and nonirrigated peach trees. Our long term goals are: 1) to relate the timing of fine root production to tree phenology, soil water content, and soil temperature; and 2) to determine how fine root architecture and demography differ between trees with and without supplemental irrigation. In early 2002, minirhizotrons were constructed and installed beneath each of 72 open-center, 4-year-old `Redglobe' peach trees at the Musser Fruit Research Farm near Clemson University. Beginning in May 2002, videotaped images from each minirhizotron were collected at 2-week intervals; notes on tree phenology were also recorded biweekly. Videotapes were digitized in the lab, and information on root length, diameter, appearance and longevity was extracted from the images. Soil temperature and volumetric water content were measured in the orchard throughout the growing season. In the 2 years following minirhizotron installation, irrigated trees allocated a significantly greater percentage of their fine root length to the upper soil layers and exhibited less root branching than nonirrigated trees. Fine roots produced by irrigated trees lived significantly longer: irrigated trees had a median root life span of 165 days, while nonirrigated trees had a median root life span of only 115 days (P< 0.001; proportional hazards regression). Fine roots from irrigated trees remained in the physiologically active “white” state for an average of 10 days longer than roots from nonirrigated trees (P< 0.001). Data from 2002–03 indicate that the trees produce new root flushes at least three times during the year, with a significant flush occurring immediately after harvest.

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Yong Xu, Fan Wu, and Rong-Cai Ma

MADS box genes regulate most of the development processes in plants. Studying peach MADS box genes will provide insights into its flower and fruit development. Five MADS box cDNAs with complete coding regions were cloned in this study. PpMADS2 cDNA is 1116-bp long. RT-PCR analysis indicated that PpMADS2 is expressed in leaf, flower, fruit, and nutlet. PpMAD4 cDNA is 824-bp long, which is the homologue of Agamous. RT-PCR analysis indicated that PpMADS4 is expressed in the two inner parts of flower, fruit, and nutlet; and was absent in leaf and the two outer parts of the flower. This expression pattern is similar to that of Agamous gene in A. thaliana. PpMADS4 could promote the flowering process in A. thaliana tested by genetic transformation. PpMADS5 cDNA is 873-bp long, which is the homologue of SEP3. RT-PCR analysis indicated that PpMADS5 is expressed in the three inner parts of flower, fruit, and nutlet; and was absent in leaf and sepal, similar to the expression pattern of SEP3 gene in Arabidopsis. PpMAD6 cDNA is 1037-bp long, which is the homologue of FUL. RT-PCR analysis indicated that PpMADS6 is expressed in leaf, sepal, petal, carpel, and fruit; and was absent in stamen and nutlet. PpMAD7 cDNA is 1147-bp long, which is the homologue of SEP1. RT-PCR analysis indicated that PpMADS7 is expressed in the four parts of flower and fruit, and was absent in leaf, stamen, and nutlet. Furthermore, two SSRs were identified in the 5' UTR in the two MADS box genes, PpMADS2 and PpMADS7, respectively. The SSR in PpMAD2 was more polymorphic than that in PpMADS7 in the 39 Prunus accessions collected.

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Michael Wisniewski, Tim Artlip, Carole Bassett, and Ann Callahan

Cold acclimation in temperate, woody plants involves distinct changes in gene activity and protein expression. We have been identifying proteins and genes that are associated with seasonal changes in cold hardiness. Seasonal changes in a 60-kDa dehydrin and its corresponding transcript have been identified, as well as seasonal changes in 16- and 19-kDa storage proteins. Further screening of a cDNA library, constructed from cold-acclimated bark tissues collected in December, identified a 700–800-bp clone that was seasonally expressed in Northern blots. The transcript began to accumulate in October, reached a peak in November–December, and then began to decline. By April, the transcript was no longer present in bark tissues. The transcript size indicates that this gene my be related to either the 16- or 19-kDa storage proteins previously identified; however, an amino acid sequence of the protein for comparison has not yet been obtained. Interestingly, the transcript is also expressed during the early stages of peach fruit development. A similar pattern between seasonal expression and fruit development has been observed for a peach dehydrin transcript. Analysis of a partial sequence of the clone has indicated a similarity to genes encoding proteinase inhibitors and thionins (a class of biocidal proteins). More definitive characterization of the gene and identification of its corresponding protein are in progress.

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Carlos H. Crisosto, Gayle M. Crisosto, Gemma Echeverria, and Jaume Puy

Cultivar segregation according to their organoleptic perception was attempted by using trained panel data evaluated by principal component analysis in four sources of 24 peach and 27 nectarine cultivars as a part of our program to develop minimum quality indexes. Source significantly affected cultivar ripe soluble solids concentration (RSSC) and ripe titratable acidity (RTA), but it did not significantly affect sensory perception of flavor, sourness and aroma by the trained panel. On two out of 51 cultivars tested, source played a role on sweetness perception. In all of these cases, when source fell out of the proposed cultivar organoleptic group it could be explained by fruit being harvested outside the commercial physiological maturity (immature or overmature). The perception of the four sensory attributes was reduced to three principal components that explain 92% for peach and 94% for nectarine of the variation in the sensory characteristics of the cultivars tested. Season did not affect significantly the classification of three cultivars that were evaluated during these two seasons. By plotting organoleptic characteristics in PC1 and PC2 (∼76%), cultivars were segregated into groups (balanced, robust, sweet, peach or nectarine aroma, and/or peach or nectarine flavor) with similar sensory attributes; nectarines were classified into five groups and peaches into four groups. Based on this information, we recommend that cultivars should be clustered in organoleptic groups and a development of a minimum quality index should be attempted within each organoleptic group rather than proposing a generic minimum quality index based on RSSC. This organoleptic cultivar classification will help to match ethnic preferences and enhance the current promotion and marketing programs.

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M.R. Pooler and R. Scorza

Shoots were regenerated from cotyledons of mature stored seed of three peach rootstock cultivars (`Flordaguard', `Nemared', and `Medaguard'). Shoot regeneration rates were highest when cotyledons were cultured for 3 weeks in darkness on Murashige and Skoog (MS) medium with 2.5% sucrose and a combination of IBA (1.25 or 2.5 μm) and TDZ (6.25 or 12.5 μm). Regeneration rates for `Flordaguard', `Nemared', and `Nemaguard' were as high as 60%, 33%, and 6%, respectively. Length of seed storage (1 to 3 years) did not affect regeneration rates. Seventy percent of regenerated shoots produced rooted plants. This regeneration method is rapid and simple, and stored seed can be used year-round. It may be a useful regeneration system for gene transfer in seed-propagated peach rootstocks. Chemical names used: 5 indole-3-butyric acid (IBA); thidiazuron (TDZ).