Abstract
Fruit production of 11-year-old ‘CES #3 Redblush’ grapefruit Citrus paradisi Macf. trees on 13 root-stocks was best on ‘Swingle’ citrumelo (C.P.B. 4475) [Poncirus trifoliata (L.) Raf. × C. paradisi], ‘Morton’ and ‘Troyer’ [P. trifoliata × C. sinensis (L.) Osbeck] rootstock. The lowest yielding trees were on ‘Tachibana’ orange C. tachibana (Mak.) Tan. and ‘Succari’ sweet orange C. sinensis rootstock. There was wide variation in the percentage of large-sized fruit (96 and above) over 3 years; trees on ‘Troyer’ and ‘Swingle’ citrumelo (‘C.P.B. 4475’) produced the greatest quantities of large fruit. The fruit of ‘CES #3 Redblush’ was small on ‘Tachibana’ orange, ‘Cleopatra’ mandarin C. reticulata Blanco, and ‘Succari’ sweet orange rootstock. Total soluble solids was highest on ‘Tachibana’ orange, ‘Cleopatra’ mandarin, and ‘Texas’ sour orange C. aurantium L., lowest on ‘Rough’ lemon C. jambhiri Lush, and ‘Troyer’. Acids were high on ‘Tachibana’ orange and ‘Cleopatra’ mandarin rootstocks, low on ‘Rough’ lemon and ‘Christian’ trifoliate orange Poncirus trifoliata (L.) Raf. Trees were larger than average on ‘Rough’ lemon and sweet orange rootstock, smaller on ‘Christian’ trifoliate and ‘Tachibana’ orange. Leaf Mn levels were high on ‘Tachibana’ orange; B levels were relatively low on ‘Swingle’ citrumelo (C.P.B. 4475) and ‘Succari’ sweet orange rootstocks.
Citrus Tristeza Closterovirus (CTV) induces mild and/or severe symptoms on Citrus species. It may cause death of trees if the rootstock-scion combination is susceptible. It has been found in other plant/virus combinations that transformation with partial or complete viral genes (e.g., coat protein genes) can confer resistance to the resulting transgenic plants. We previously reported A. tumefaciens mediated transformation and production of two sour orange (C. aurantium L.) plants expressing the coat protein gene of CTV, which was the first report of production of transgenic Citrus using a viral gene. However, in order to properly evaluate resistance, it is necessary to obtain as many transgenic Citrus plants from single transformation events as possible. Therefore, we are currently transforming grapefruit (Citrus paradisi) `Marsh' and `Star Ruby' and sweet orange (C. sinensis) `Valencia' with CTV coat protein genes. These species are susceptible to CTV and more amenable to transformation than sour orange. Epicotyl segments of etiolated seedlings were inoculated with A. tumefaciens strain EHA101 harboring binary plasmid pGA482GG containing the coat protein gene of mild Florida CTV strain T30 (CP-T30) or severe Florida strain T36 (CP-T36). Putatively transformed shoots were regenerated on selection medium containing kanamycin. Regenerated shoots were evaluated with GUS assays; those shoots positively identified by GUS were then evaluated with PCR. We have currently identified 17 `Marsh' grapefruit, 20 `Star Ruby' grapefruit, and seven sweet orange putatively transformed plants.
Abstract
Nucellar red grapefruit, Citrus paradisi Macf., on ‘Swingle’ citrumelo, Poncirus trifoliata (L.) Raf. × C. paradisi, rootstock produced more fruit than trees on 20 other rootstocks over 9 years during the period when the trees were 6 to 14 years old. Fruit quality on ‘Swingle’ citrumelo was the same as on ‘Texas’ sour orange, C. aurantium L. Leaf analysis for 12 elements showed significant differences among rootstocks in the concentration of 7 elements, particularly a strong tendency of ‘Chinese box-orange’, Severinia buxifolia (Poir.) Ten., to accumulate Mn and of ‘Limon Real’, C. excelsa Wester, to accumulate Na.
Abstract
Interstocks significantly affected β-carotene and lycopene in the pulp and peel of ‘Redblush’ grapefruit (Citrus paradisi Macf.) on Cleopatra mandarin (C. reshni Hort. ex. Tan.) rootstocks. Fruit from non-interstocked trees consistently had the lowest levels of both pigments. Trees wit Nasnaran (C. amblycarpa Ochse.) interstocks had consistently high levels of both pigments, particularly late in the harvest season.
Abstract
The dependence of fruit growth of grapefruit (Citrus paradisi Macf.) upon leaf area was investigated on girdled branches by manipulating leaf and fruit numbers. Leaf areas of 2.0 ± 0.5 m2 per fruit were found to be saturating with regard to fruit growth rate and size. Fruit on internal, shaded branches required larger leaf areas. Fruit on girdled branches weighed 44 to 119% more than fruit in ungirdled branches, which had leaf areas of 0.35 to 0.55 m2 per fruit. This indicates that leaf area is one of the factors limiting fruit growth. Starch accumulated in thin twigs during the fruit growth season, forming a saturation curve similar to those obtained for fruit size when plotted against leaf area per fruit. Increasing leaf area per fruit could involve a decrease in photosynthetic activity, a possibility which now is being investigated further.
Isozyme analysis was the basis for determining the frequency of occurrence and the characteristics of zygotic plants in Swingle citrumelo seedling populations from various sources of open-pollinated seeds, in a commercial nursery of Swingle citrumelo before and after roguing, and in commercial orchards and rootstock trials where this rootstock was used. Most zygotic seedlings identified by isozyme analysis could be distinguished by careful examination of morphological characteristics. Frequencies of zygotic seedlings varied among seedling populations, but were in the range (≈5% to 10%) found in previous studies. Roguing based primarily on size and growth habit of seedlings was effective in removing some, but not all, zygotic seedlings. Most of the remaining zygotic plants in the rogued population were found among the smaller seedlings. Trees budded on zygotic rootstock seedlings were found in two of the three groves studied, and in some instances an apparent incompatibility was developing in young trees.
Deformations consisting of longitudinal ridges in the rind of Citrus fruits have recently been found in Southern California Citrus groves. Here, we report the correlation between ridge formation and applications of chlorpyrifos (Lorsban, Dow Chemical Company, Midland, MI) during the feather-growth stage of bud break. All chlorpyrifos formulations resulted in significant ridging. Addition of agricultural oil and 2,4-D (2,4-dichlorophenoxyacetic acid (2,4-D) to chlorpyrifos resulted in the greatest ridging damage and widened the window of susceptibility by 2 weeks in 1988. In 1989, no significant difference was seen between treatments of chlorpyrifos, although all were significantly greater than the control. The susceptible stages of bud growth are described, as are the non-susceptible stages which precede and follow it. Floral buds in which carpels are initiating are susceptible to fruit ridging upon application with chlorpyrifos. These ridges are the result of an increase in cell size of the flavedo tissue which may be the result of a polyploid chimera.
Many Citrus species accumulate large amounts of flavonoids, specifically flavanone glycosides, that impart an intensely bitter flavor to the fruit. In grapefruit, this bitterness decreases the acceptability of fresh fruit and juice; in other species, these compounds entirely prevent fruit consumption. No physiological purpose for the accumulation of these compounds has been determined; they do not function in color production or, as far as is known, in defense responses. As has been found in other plants, the accumulation of specific flavonoids in citrus appears to be under genetic control, but no definitive genetic analyses have been done. The long-term objective of this research is to determine whether the production of bitter-tasting flavanone glycosides (neohesperidosides) in citrus can be manipulated using molecular genetic techniques. As a first step, cDNAs for chalcone synthase and chalcone isomerase, the first two biosynthetic enzymes specific to the flavonoid pathway, were isolated from a grapefruit leaf cDNA library using heterologous probes. Southern analyses showed that both genes appear to be part of multigene families, as expected. Northern analyses are underway to determine steady state mRNA levels in various grapefruit tissues, and Western blots to characterize protein expression are also being attempted.
Abstract
Stress caused by irrigation at long intervals or by irrigation with artificially salinized water had a similar effect on juice constituents of grapefruit (Citrus paradisi Macf.). Fruit physical parameters were affected only by irrigation water stress and not by salinity.
Abstract
Grapefruit (Citrus paradisi Macf.) trees were irrigated for 14 years at interval of 18 to 40 days. Water stress due to irrigation at long intervals reduced vegetative development and yield. Cumulative water consumption, yields and canopy volume were closely correlated.