Effects of Weak Light on the Ultrastructural Variations of Phloem Tissues in Source Leaves of Three-Year-Old Nectarine Trees(Prunus persica L.var. nectarina Ait.)

Leaves from three_year_old solar greenhouse nectarine trees ( Prunus persica L. var. nectarina Ait. “Zao Hong Yan”) were used as materials in this study. It was the first time that the ultrastructural characteristics of phloem tissues of source leaves were observed and compared in normal and weak light intensities using the transmission electron microscopy. Results showed that the average diameters of companion cells (CC) and sieve elements (SE) of all kinds of veins were bigger in normal than that in weak light intensity, indicating that light could influence the cell development and growth. Dense cytoplasm with abundant mitochondria, endoplasmic reticulums, multivesicular bodies, vesicles and plastids were observed in normal light intensity. On the contrary, CC with small vacuolar structures and few mitochondrias, endoplasmic reticulums were shown in weak light. Misalignment of grana thylakoid margins of nectarine leaves also was seen in weak light. The sieve pores of SEs were obstructed in weak light. Chloroplasts with numerous starch grains and few mitochondrias were noticed in the mesophyll cell (MES) surrounding the bundle sheath in weak light. The storage of starch grains appeared to result from an unbalance between photosynthate production and export of photosynthates. This observation provided a strong support to the point that most leaves export the most of assimilates in the light time. Plasmodesmal densities between SE/CC, CC/PP (phloem parenchyma cell), PP/PP and PP/BSC (bundle_sheath cell) decreased in weak light. Plasmodesmata were observed between CC/SE (NS) (nacreous_walled sieve element), PP/BSC in branch veins in normal light intensity, but not in weak light. Thus apoplasmic pathway may be the main mode of transport of assimilates in weak light, however symplasmic pathway may be the main mode of transport of assimilates in normal light intensity. These results demonstrated that the solar greenhouse nectarine trees could be adapted to the weak light via the ultrastructure variation of phloem tissues of the source leaves.

Effects of Short-term Freezing on Endodormancy Release in Nectarine Bud

[Objective] This study aimed to investigate the endodormancy release in nectarine bud treated by short-term freezing. [Method] Through short-term freezing at seven different temperatures for three different periods on bud, the livability, burst, ratio of free water to bound water and membrane permeability of ＇Shuguang＇ nec- tarine bud were studied. [Result] On November 10, compared with non-freezing treatment （CK）, the bud burst, ratio of free water to bound water and membrane permeability treated by freezing at -5 and -8 ℃ were almost the same as CK. But the rest freezing treatments advanced the date of endodormancy release, while the bud burst, ratio of free water to bound water and membrane permeability were high- er than CK. on November 20 and 30, the effects of the freezing treatment on en- dodormancy release were the same when the treatment on November 10, and the effect was better as the treatment was later. [Conclusion] The correlation of the rate of bud burst, ratio of free water to bound water, and membrane permeability of the different freezing treatments indicated that the change from bound water to free wa- ter and the increase of membrane permeability were probably the signal of endodor- mancy release.

Respiratory Response of Dormant Nectarine Floral Buds on Chilling Deficiency

Changes in main biochemical respiratory pathways in dormant nectarine floral buds were studied with nectarine trees （Prunus persica.var, nectariana cv. Shuguang） in order to determine the function of respiration in dormancy release. Oxygen-electrode system and respiratory inhibitors were used to measure total respiratory rates and rates of respiratory pathways. Results showed that chilling deficiency blocked the transition of respiratory mode, and made buds stay in a state of high level pentose phosphate pathway （PPP） and low level tricarboxylie acid cycle （TCA）. The decline of PPP and activation of TCA occurred synchronously with the release of dormancy. In addition, the inhibition of PPP stimulated a respiration increase related with TCA. It could be concluded that the function of PPP activation in dormancy release might be limited and PPP declination inducing TCA activation might be part of respiration mode transition mechanism during bud sprouting.

Effect of Photoperiod on Key Enzyme Activities of Respiration in Nectarine Buds During Dormancy Induction

Shuguang （Prunus persica var. nectariana cv. Shuguang） nectarine was used to study effects of photoperiod on key-enzyme activities of respiration during dormancy induction. The dormancy status was determined with sprouting ability. Spectrophotometry was used to investigate activities of phosphohexose isomerase （PGI）, malic dehydrogenase （MDH）, and glucose-6-phosphate dehydrogenase （G6PDH）. The results revealed that short day （SD） treatment promoted dormancy induction while long day （LD） treatment postponed the process. During dormancy induction, PGI activities declined, MDH activities changed little, and G6PDH activities increased both in flower buds and leaf buds. PGI activities and MDH activities in SD treatment were lower than control, and G6PDH activities were higher, which was opposite with LD treatment. The changes of respiratory key-enzyme activities were adjusted by photoperiod and correlated with the development of dormancy induction.

Respiratory Response of Dormant Nectarine Vegetative Buds to High Temperature Stress

High temperature stress （HT） is efficient in breaking endo-dormancy of perennial trees. The effects of HT （50°C） on the respiration of dormant nectarine （Prunus persica var. nectariana cv. Shuguang） vegetative buds were evaluated in the research. We found that bud respiration was transiently inhibited by HT and the pentose phosphate pathway （PPP） and the cytochrome C pathway （CYT） were significantly affected. On the substrate level, PPP was activated in the HT-treated buds compared with the control group. However, the activation did mot occur until hours after HT treatment. The tricarboxylic acid cycle （TCA） in both the HT-treated buds and in the control group proceeded at a low level most of the time compared with total respiration. On the electron transfer level, CYT was transiently inhibited by HT but became significantly active in the later stage. CYT operation in the control group exhibited an attenuation process. The alternative pathway （ALT） fluctuated both in the HT-treated samples and in the control. The results suggest that the temporary CYT inhibition and the following PPP activation may be involved in HT-induced bud dormancy release and budburst mechanisms.

Effects of Photoperiod on Alternative Respiration Pathway in Nectarine Flower Buds During Dormancy Induction

Characteristics of dormancy induction and alternative respiration pathway （also known as cyanide-resistant respiration） of nectarine flower buds in different photoperiods were studied to determine the function of photoperiod and alternative respiration pathway in dormancy induction. Oxygen-electrode system and respiratory inhibitors were used to measure total respiratory rates and rates of alternative respiration pathway. The results showed that total respiration rate （Vt） in flower buds showed to be double hump-shaped curves. Short day raised, brought the first-hump of Vt forward and delayed the second-hump, while long day delayed the whole curve. The capacity （Vast） and activity （pValt） of SD and LD changed synchronously and both showed to be double hump-shaped curves. Short day made the first climax of Vast and pValt existed much earlier, while long day increased their rates significantly. The length of day had little effects on the later period climax. Long day also increased the contributions of altemative respiration pathway in total respiration rate （pValt/Vt）. The changes in alternative respiration pathway were correlated with the induction of dormancy and adjusted by photoperiod. Short day promoted dormancy induction of nectarine trees, while long day delayed it.

Relation Between Endodormancy Induction and Changes in Two Main Electron Transport Pathways of Nectarine Buds

Operation regulations of two main electron transport pathways in nectarine （Prunus persica var. nectariana cv. Shuguang） buds during endodormancy induction were studied to understand possible roles which two main electron transport pathways played in the buds of deciduous fruit trees during endodormancy induction. Respiratory inhibitors （KCN and SHAM） were used to investigate total respiration rate （Vt）, the development and operation of the alternative pathway and partitioning of electrons between the cytochrome and alternative pathways in nectarine buds during endodormancy induction. Results indicated that changes of Vt in flower and leaf buds showed single and double hump-shaped curves, respectively. In endodormancy induction, the capacity （Valt,） and activity （ρValt） of the alternative pathway rapidly increased, but changes of them had different patterns during the entire measuration. At the same time, changes of engagements of the alternative （ρValt/Vt） and cytochrome pathway （ρ′Vcyt/Vt） were opposite, and ρ′Vcyt/Vt was always further higher than ρValt/Vt during the entire measuration. All these results indicated that the development and operation of the alternative pathway played important roles in endodormancy induction, but the cytochrome pathway was the main pathway for mitochondrial electron transport in buds during endodormancy induction.

Dynamics of Growth of the Introduced Nectarine Cultivars

A research on the topic was conducted at the Experimental Station of Agriculture--Khan Krum during the period 2009-2011. The dynamics of growth and thickening of the cultivars Independence, Nectagrand 2, Aureliogrand, Fantasy, Kassiopeia and Caldessi 2000 grafted on seedling rootstock （Elbert） and clonal GF-677 have been studied in the second year of nursery. The aim of the study was to investigate the growth characteristics of the introduced cultivars of nectarines grafted on traditional seedling rootstock （Elbert） and GF-677. At the beginning of the vegetation period of the seed cultivar rootstock （Elbert） has strong growth and thickening of the stem. The dynamics of growth and thickening of the stem depend on the used rootstock and cultivar. The obtained planting material corresponds to the existing quality standards.

Sugar and Acid Content and Their Genotypic Correlations of Unselected P.persica x P.davidiana Progenies as Affected by Hairness and Flesh Color

Fruit sugar and acid content were determined in a group of hybrids from one family,including 22 peach and 85 nectarine genotypes, or 75 white-fleshed and 32 yellow-fleshedgenotypes. Sucrose accounted for about 65% of the total sugar, and malate for about 50%of the total acid. Mean sucrose, fructose, sorbitol, total sugar, soluble solid andquinate content were significantly higher in nectarines than in peaches, while nosignificant difference was found in mean malate, citrate and total acid content betweennectarines and peaches. Shikimate content was very low and ranged from 0.011-0.046 meq100 g-1 FW in all genotypes studied. Flesh color did not affect sugar and acid content.Glucose and fructose had a close positive correlation (r =0.63***). Malate had positivecorrelations with sucrose, sorbitol, total sugar and soluble solid content. The correlationbetween citrate and sugars were not significant. Glucose, fructose, sorbitol and malatecontent seemed to be higher in late-maturing genotypes. Sucrose, total sugar and totalacid content had a parabolic relationship with developmental period (from full bloomingdate to maturity date).