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J.P. Syvertsen and M.L. Smith

Four-year-old `Redblush' grapefruit (Citrus paradisi Macf.) trees on either the relatively fast-growing rootstock `Volkamer' lemon (VL) (C. volkameriana Ten. & Pasq.) or on the slower-growing rootstock sour orange (SO) (C. aurantium L.) were transplanted into 7.9-m3 drainage lysimeter tanks filled with native Candler sand, irrigated similarly, and fertilized at three N rates during 2.5 years. After 6 months, effects of N application rate and rootstock on tree growth, evapotranspiration, fruit yield, N uptake, and leaching were measured during the following 2 years. When trees were 5 years old, low, medium, and high N application rates averaged about 79,180, or 543 g N/tree per year and about 126,455, or 868 g N/tree during the following year. Recommended rates average about 558 g N/tree per year. A lysimeter tank with no tree and additional trees growing outside lysimeters received the medium N treatment. Nitrogen concentration in the drainage water increased with N rate and exceeded 10 mg·liter-1 for trees receiving the high rates and also for the no tree tank. Leachate N concentration and total N recovered was greater from trees on SO than from those on VL. Average N uptake efficiency of medium N rate trees on VL was 6870 of the applied N and 61 % for trees on SO. Nitrogen uptake efficiency decreased with increased N application rates. Trees outside lysimeters had lower leaf N and fruit yield than lysimeter trees. Overall, canopy volume and leaf N concentration increased with N rate, but there was no effect of N rate on fibrous root dry weight. Fruit yield of trees on SO was not affected by N rate but higher N resulted in greater yield for trees on VL. Rootstock had no effect on leaf N concentration, but trees on VI. developed larger canopies, had greater fibrous root dry weight, used more water, and yielded more fruit than trees on SO. Based on growth, fruit yield and N leaching losses, currently recommended N rates were appropriate for trees on the more vigorous VL rootstock but were 22% to 69 % too high for trees on SO.

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J.P. Syvertsen, M.L. Smith, J. Lloyd, and G.D. Farquhar

Five- to six-year-old `Redblush' grapefruit (Citrus paradisi Macf.) trees on `Volkamer' lemon [VL = C. volkameriana (Ten. & Pasq.)] or sour orange (SO = C. aurantium L.) rootstock, were grown individually in 7.9-m3 lysimeters for 2.5 years using low to high rates of fertilizer N. Net CO2 assimilation (ACO2) of leaves and leaf dry mass per area (DM/a) increased with leaf N concentration, whereas leaf tissue C isotope discrimination (Δ) decreased. Leaf tissue Δ was negatively related to ACO2 and DM/a. Transient effects of rootstock on leaf N were reflected by similar effects on Δ. There was no effect of leaf N on water-use efficiency (WUE) of leaves (WUEL = ACO2/transpiration); WUEL was not correlated with Δ. Although photosynthetic N use efficiency (ACO2/N) consistently decreased with increased leaf N, Δ was not consistently related to ACO2/N. Annual canopy growth, tree evapotranspiration (ET), and fruit yield increased with whole tree N uptake. Leaf tissue Δ was negatively related to all of these tree measurements at the end of the second year. By that time, whole-tree WUE (WUET, annual canopy growth per ET) also was negatively related to Δ. Larger trees on VL had higher ET than trees on SO, but there were no rootstock effects on WUET or on Δ. Leaf tissue Δ was consistently higher than Δ values of trunk and woody root tissues. Citrus leaf tissue Δ can be a useful indicator of leaf N, characteristics of leaf gas exchange, tree growth, yield, and WUET in response to N availability.

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John D. Lea-Cox and James P. Syvertsen

We examined how N supply affected plant growth and N uptake, allocation and leaching losses from a fine sandy soil with four Citrus rootstock species. Seedlings of `Cleopatra' mandarin (Citrus reticulata Blanco) and `Swingle' citrumelo (C. paradisi × P. trifoliata) were grown in a glasshouse in 2.3-liter pots of Candler fine sand and fertilized weekly with a complete nutrient solution containing 200 mg N/liter (20 mg N/week). A single application of 15NH4 15NO3(17.8% atom excess 15N) was substituted for a normal weekly N application when the seedlings were 22 weeks old (day O). Six replicate plants of each species were harvested at 0.5, 1.5, 3.5, 7, 11, and 30 days after 15N application. In a second experiment, NH4 NO3 was supplied at 18,53, and 105 mg N/week to 14-week-old `Volkamer' lemon (C. volkameriana Ten. & Pasq.) and sour orange (C. aurantium L.) seedlings in a complete nutrient solution for 8 weeks. A single application of 15NH4 15NO3 (23.0% 15N) was substituted at 22 weeks (day 0), as in the first experiment, and seedlings harvested 3,7, and 31 days after 15N application. Nitrogen uptake and partitioning were similar among species within each rate, but were strongly influenced by total N supply and the N demand by new growth. There was no 15N retranslocation to new tissue at the highest (105 mg N/week) rate, but N supplies below this rate limited plant growth without short-term 15N reallocation from other tissues. Leaf N concentration increased linearly with N supply up to the highest rate, while leaf chlorophyll concentration did not increase above that at 53 mg N/week. Photosynthetic CO2 assimilation was not limited by N in this study; leaf N concentration exceeded 100 mmol·m-2 in all treatments. Thus, differences in net productivity at the higher N rates appeared to be a function of increased leaf area, but not of leaf N concentration. Hence, N use efficiency decreased significantly over the range of N supply, whether expressed either on a gas-exchange or dry weight basis. Mean plant 15N uptake efficiencies after 31 days decreased from 60% to 47% of the 15N applied at the 18,20, and 53 mg N/week rates to less than 33% at the 105 mg N/week rate. Leaching losses increased with N rate, with plant growth rates and the subsequent N requirements of these Citrus species interacting with residual soil N and potential leaching loss.

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Giuseppe Reforgiato Recupero, Giuseppe Russo, Santo Recupero, Roberto Zurru, Bruno Deidda, and Maurizio Mulas

; trifoliate orange [ Poncirus trifoliata (L.) Raf.], sour orange, and volkamer lemon ( C. volkameriana Pasq.) were used as male parents. In previous observations, C. latipes showed high vigor and tolerance to “mal secco.” Progenies were planted in the CRA

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Frederick S. Davies and Glenn R. Zalman

Several studies suggest that optimum N rate and application frequency differ among citrus rootstocks. `Rhode Red' valencia orange trees [Citrus sinensis (L.) Osb.] on three rootstocks, C. volkameriana Ten. and Pasq., `Carrizo' citrange [C. sinensis (L.) Osb. × Poncirus trifoliata (L.) Raf.], and `Swingle' citrumelo [C. paradisi Macf. × P. trifoliata (L.) Raf.], were used to determine if N rate and application frequency should be adjusted, based on rootstock, during the first 3 years in the field. Treatments were arranged in a 3×3×3 (rootstock, N rates, N application frequency, respectively) factorial experiment. Annual N application rates ranged from 68 to 272 g/tree depending on tree age, and N was applied biweekly, weekly or monthly. Application frequency had no effect on trunk diameter or leaf N concentration in any year. Rootstock had a significant effect on growth in all 3 years, with trees on C. volkameriana being largest and having the greatest yields, followed by those on `Carrizo' and `Swingle', respectively. Trees on C. volkameriana were larger than those on the other rootstocks because they were larger at planting, grew over a longer period during the year, and often grew at a faster rate. Nitrogen rate had no effect on growth during the first 2 years in the field, but the highest N rate increased yields in year 3 for trees on C. volkameriana and `Swingle' rootstocks. Interaction between rootstock and N rate was nonsignificant for trunk diameter, but it was significant for yield, suggesting that trees on C. volkameriana responded more to increased N than did those on the other rootstocks.

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Eybar Rojias

Flowering of `Tahiti' lime/C. volkameriana Pasq. was studied in response to several levels of hydrogen cyanamide sprayed on isolated terminal twigs of 3- year-old plants growing under field conditions. The study was performed in the central region of Venezuela at 180-m altitude (lat. 9°43'N). Hydrogen cyanamide had significant effects on floral and total activity, and on generative, mixed, floral, and total shoot flux density, as well as on defoliation of sprayed twigs. Conversely, it did not show any significant effect on vegetative development, either as activity or shoot flux density.

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Eliezer S. Louzada, Jude W. Grosseti, Frederick G. Gmitter Jr., Beatriz Nielsen, J.L. Chandler, Xiu Xin Deng, and Nicasio Tusa

Protoplast culture following polyethylene glycol-induced fusion resulted in the regeneration of vigorous tetraploid somatic hybrid plants from eight complementary parental rootstock combinations: Citrus reticulata Blanco (Cleopatra mandarin) + C. aurantium L. (sour orange), C. reticulata (Cleopatra mandarin) + C. jambhiri Lush (rough lemon), C. reticulata (Cleopatra mandarin) + C. volkameriana Ten. & Pasq. (Volkamer lemon), C. reticulata (Cleopatra mandarin) + C. limonia Osb. (Rang-pur), C. sinensis (L.) Osb. (Hamlin sweet orange) + C. limonia (Rangpur), C. aurantium (sour orange) + C. volkameriana (Volkamer lemon) zygotic seedling, C. auruntium hybrid (Smooth Flat Seville) + C. jambhiri (rough lemon), and C. sinensis (Valencia sweet orange) + Carrizo citrange [C. paradisi Macf. × Poncirus trifoliata (L.) Raf.]. Diploid plants were regenerated from nonfused callus-derived protoplasts of Valencia sweet orange and Smooth Flat Seville and from nonfused leaf protoplasts of sour orange, Rangpur, rough lemon, and Volkamer lemon. Regenerated plants were classified according to leaf morphology, chromosome number, and leaf isozyme profiles. All somatic hybrid plants were tetraploid (2n = 4× = 36). One autotetraploid plant of the Volkamer lemon zygotic was recovered, apparently resulting from a homokaryotic fusion. These eight new citrus somatic hybrids have been propagated and entered into field trials.

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Marìa Andrade-Rodrìguez, Angel Villegas-Monter, and M. Alejandra Gutièrrez-Espinosa

Polyembryony is an important characteristic for citrus that allows them to be propagated clonally through seed. Even when it is genetically controlled by a quantitative trait, the environment in which the seed is developed can affect it. The aims of this investigation were to evaluate polyembriony in two citric rootstocks in two harvest cycles and embryo germination of polyembrionic seeds. Embryos of 300 seeds of Citrus volkameriana and C. amblycarpa were counted and measured in Summer-Fall and Winter 1998 and 1999, respectively; embryo of 50 seeds of both rootstocks were germinated in vitro. The number of embryos per seed was 1.9 and 1.6 in C. volkameriana and 4.7 and 5.7 in C. amblycarpa. In C. volkameriana, we observed 42% of monoembryonic seeds during summer-fall and 67% in winter, whereas in C. amblycarpa 5.0 and 4.1% were detected, respectively. Only embryos that were larger than 1 mm long germinated. Even when germination takes similar time (5 to 6 days), further growth is faster in larger embryos (5 to 10 mm) than smaller ones. Therefore, size of embryos would need to be considered for propagation purposes.

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M.M. Khattab, A.A. Elezaby, S. ElOraby, and A.M. Hassan

This investigation was carried out on 13-year-old Valencia orange trees [Citrus sinensis (L.) Osbeck] budded on five different rootstocks. Heat unit accumulation (temperature above 12.5 °C) for fruits worked on the various rootstocks were calculated from full bloom to maturity stage. Valencia fruits on Troyer citrange (C. sinensis × Poncirus trifoliata) and Carrizo citrange (C. sinensis × Poncirus trifoliata) rootstocks matured earlier when compared to those growing on Cleopatra mandarin (C. reticulata Blanco), Volkamer lemon (C. volkameriana Ten. and Pasq.), and sour orange (C. aurantium) rootstocks. The results showed that the Valencia fruits, regardless of rootstock, could be stored for different periods under different conditions. However, in order to avoid degradation in fruit quality, storing periods should not exceed 21, 60, and 120 days under room conditions (25 °C and RH 25% to 35%), 4 °C (RH 80% to 85%), and 8 °C (RH 80% to 85%); respectively.

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Glenn C. Wright

Five rootstocks, `Carrizo' citrange, Citrus macrophylla, Rough lemon, `Swingle' citrumelo, and Citrus volkameriana, were selected for evaluation using `Limoneira 8A Lisbon' as the scion. Four years of yield and fruit packout data indicate that trees on C. volkameriana and C. macrophylla are superior to those on other rootstocks in growth and yield. `Swingle' and `Carrizo' are performing poorly, and Rough lemon is intermediate. In a similar trial, four `Lisbon' lemon selections, `Frost Nucellar', `Corona Foothills', `Limoneira 8A', and `Prior' selections of Lisbon lemon were selected for evaluation on Citrus volkameriana rootstock. Four years of yield and packout data indicate that the `Limoneira 8A Lisbon' selection has generally outperformed the other selections in both growth and yield, although `Corona Foothills' has been superior in the 1998-99 harvest season.