Autotetraploid tangor plant regeneration from in vitro Citrus somatic embryogenic callus treated with colchicine

In vitro chromosome doubling of embryogenic callus lines of the Citrus cultivars Umatilla and Dweet tangors (Citrus reticulata Blanco×C. sinensis [L.] Osb.), Caffin clementine (C. clementina Hort. ex Tan.) and Wheeny grapefruit (C. paradisi Macf) was carried out in the presence of either 0.05 or 0.1% colchicine, or 0.01, 0.05 or 0.1% oryzalin. Embryogenic callus development was partly suppressed in the presence of colchicine, and completely suppressed by oryzalin at all concentrations tested. No plants were regenerated from any of the oryzalin treatments. Ploidy level of plants regenerated from the colchicine treatments was determined using flow cytometry and chromosome squashes. Three desirable non-chimeric, autotetraploid plants of the mono-embryonic cultivar Umatilla were produced using 0.05% colchicine and one from 0.1% colchicine. One mixoploid Dweet plant was produced using 0.1% colchicine.     

Structural basis of the rind disorder oleocellosis in Washington navel orange (Citrus sinensis L. Osbeck)

Oleocellosis, a physiological rind disorder of citrus fruit, is an unattractive surface blemish caused by phytotoxic effects of released rind oils. The development of oleocellosis in Washington navel orange (Citrus sinensis L. Osbeck) was examined by following a time sequence of surface symptoms and microscopic rind changes. The two natural causes of oleocellosis were simulated: mechanical damage to the fruit and transfer of rind oil between fruit. Mechanical fruit injury resulted in rupture of the epidermis above oil glands. Released surface oil appeared to infiltrate the rind via the ruptured epidermis resulting in rapid degeneration of cortical, but not epidermal, cell contents. Oil application to the rind surface produced a more severe blemish than did mechanical damage. The oil appeared to diffuse through the cuticle causing degeneration of the contents of all cell layers, including the epidermis. Loss of membrane integrity was detected within 30 min, followed by cell content degeneration and cell collapse. The resulting blemish, characterized by rind collapse and darkening, developed substantially within 3 d and was attributed to the cellular damage.     

Plant regeneration of sweet orange (Citrus sinensis) from thin sections of mature stem segments

An efficient system for in vitro plant regeneration from thin transversal stem sections explants (1–2 mm) using mature tissues of sweet orange cv. Pera was developed. Explants were cultured in different media to evaluate the frequency of regeneration and size of buds. A high percentage of explants (54% with 3.1 buds/explant) producing large buds (1–4 mm) was observed when the explants were cultivated for 2 weeks on Murashige and Skoog medium and then transferred to Woody Plant medium (WPM). Both media were supplemented with 1.8 M 6-benzylaminopurine and 0.7 M gibberellic acid. Adventitious buds were regenerated into whole plants by in vitro shoot-tip grafting. Regenerated plants started to flower after 12 months in the greenhouse, confirming their mature nature.     

Effects of 15N application frequency on nitrogen uptake efficiency in citrus trees

Two irrigation systems were used to compare nitrogen uptake efficiency in citrus trees and to evaluate the NO₃⁻ runoff in «Navelina» orange trees [Citrus sinensis (L.) Osbeck] on Carrizo citrange rootstock (Citrus sinensis × Poncirus trifoliata Raf.). These were fertilized with 125 g N as labelled K¹⁵NO₃ and grown outdoors in containers filled with a sand-loamy soil. Two groups of 3 trees received this N dose either in five equally split applications by a flooding irrigation system or in 66 applications by drip. Trees were harvested at the end of the vegetative cycle (December) and the isotopic ratios of ¹⁵N/¹⁴N were measured in the soil-plant system. The N uptake efficiency of the whole tree was higher with drip irrigation (75 percnt;) than with flooding system (64 percnt;). In the 0-90 cm soil profile, the N immobilized in the organic fraction was similar for both irrigation methods (around 13 percnt;), whereas the N retained as NO₃⁻ was 1 percnt; of the N applied under drip and 10 percnt; under flooding. In the last case, most of NO₃⁻ remained under root system and it could be lost to leaching either by heavy rainfalls or excessive water applications. These results showed that a drip irrigation system was more efficient for improving water use and N uptake from fertilizer, in addition to potentially reduced leaching losses.     

Phytophagous Mite Populations on Tahiti Lime, Citrus Latifolia, Under Induced Drought Conditions

In the nort-western region of Venezuela, Phyllocoptruta oleivora, Tetranychus mexicanus and Brevipalpus phoenicis are common plant-feeding mites on leaves, fruits and branches of Tahiti lime, Citrus latifolia. The population dynamics of these herbivores are affected by many factors, such as weekly treatments with wettable sulphur, particularly during the wet season, maintenance pruning of plants, irrigation with microsprinklers, induction of water stress by withholding irrigation and biotic and abiotic environmental factors. During October 1994-January 1995, 31 trees in a commercial orchard were sampled weekly in order to describe population fluctuations of plant-feeding mites (mean number of mites per leaf or fruit), before (4 weeks) and after (4 weeks) a period of 6 weeks of drought stress (no irrigation). The population density of P. oleivora increased progressively during the last 3 weeks of the irrigation period and reached a maximum of 24 mites per fruit. In contrast, the populations of the other two species, T. mexicanus and B. phoenicis, remained at the same low density as before the withholding-irrigation period. After 6 weeks without irrigation, only T. mexicanus increased, to a high mean value of 11 mites per leaf. The withholding-irrigation practice appears to affect the population size of P. oleivora towards the end of this period and that of T. mexicanus at the beginning of the re-establishment of the water supply. The highest proportion of trees (32%) was infested by T. mexicanus after the withholding-irrigation period, when irrigation was resumed, whereas the highest levels of infestation of trees by P. oleivora and B. phoenicis were 16 and 10%, respectively, during the last week of the water-stress period. Although factors affecting the dynamics of the mites in the orchard are likely to be complex, irrigation management apparently plays an important role.     

Estimating the cost of flowering in a grapefruit tree

The objective of the present study is to evaluate a Citrus tree's investment in the flowering process in relation to its photoassimilate resources, as a part of its annual reproductive effort. The overall requirement for carbohydrate of a single flower of grapefruit ( Citrus paradisi Macf. cv. 'Marsh seedless') is evaluated as 8·33 × 10^–3 mol C over 3 weeks. The direct cost of production of a single flower is estimated to be 5·75 × 10^–3 mol C, most of which is allocated to the petals, anthers and style — organs designated to abscise. About 2·58 × 10^–3 mol C is consumed by respiration not associated with growth processes. Growth respiration ( R _g) occurs mostly during early stages of flower growth and development. However, the total respiration rate increases sharply during anthesis, when growth processes have almost ceased. Ethylene evolution also reaches remarkably high rates during anthesis. High temperatures increase the rate of flower respiration ( Q ₁₀ = 2·12) but shorten the duration of flowering. A grapefruit tree may bear each year 20 000–50 000 flowers, only 0·5–2·5% of which develop into mature fruit. The amount of carbohydrate invested each year in bloom at the whole-tree level is 166–400 mol C per tree (depending on the number of flowers), amounting to 10–20% of the carbohydrate consumed for fruit growth. The overall daily demand for carbohydrate by the flowers of a grapefruit tree during anthesis may exceed the daily carbohydrate production by the leaves. High temperatures lead to a further increase in the daily demand for carbohydrate. In such cases, the management of flowering must rely on carbohydrate reserves recruited from other tree organs. The ecophysiological and evolutionary aspects of Citrus flowering are discussed.     

柑桔原生质体辐射诱变筛选抗寒再生植株

用１１６１Ｃ／ｋｇ软Ｘ射线辐射“Ｐａｇｅ”桔柚（ＣｉｔｒｕｓｒｅｔｉｃｕｌａｔａＢｌａｎｃｏ×Ｃ．ｇｒａｎｄｉｓＯｓｂ．ｃｖ．Ｐａｇｅ）胚性愈伤组织分离的原生质体,经－１１℃低温选择后,将存活的原生质体培养再生植株。研究表明,“Ｐａｇｅ”桔柚胚性愈伤组织原生质体在４１２８Ｃ／ｋｇ辐射剂量处理后几乎不能恢复分裂;２０６４Ｃ／ｋｇ辐射时分裂频率低,即使分裂也难以发育为胚状体;而１１６１Ｃ／ｋｇ辐射后,经－１１℃低温处理仍有１０２％～１６９％的分裂频率,且部分细胞团能发育为胚状体。辐射的原生质体培养成胚状体后,经低温筛选,在改良的生芽培养基和生根培养基中萌发成为植株。再生植株中检测出２株的叶片原生质体的低温半致死温度（ＬＴ５０）分别比对照低１．９６℃和１６８℃。     

伏令夏橙与宁波金柑属间体细胞杂种变异研究

伏令夏橙（Ｃｉｔｒｕｓｓｉｎｅｎｓｉｓ（Ｌ．）Ｏｓｂｅｃｋ）＋宁波金柑（ＦｏｒｔｕｎｅｌａｃｒａｓｉｆｏｌｉａＳｗｉｎｇｌｅ）属间体细胞杂种在田间生长６年,树势较弱,新梢经常枯死,树冠参差不齐。染色体检查发现,除了四倍体之外,还存在其它倍性的细胞,呈嵌合体状态。酯酶同工酶图谱上,多数单株出现一条双亲没有的新带。ＲＡＰＤ分析结果表明,部分植株丢失了亲本的标记带型,表现出遗传上的不稳定性     

Somatic hybrid plantlets regeneration between Citrus and its wild relative, Murraya paniculata via protoplast electrofusion

Protoplasts isolated from `Page' tangelo (Minneola tangelo × clementine) cell suspension cultures were electrically fused with mesophyll protoplasts of orange jessamine [Murraya paniculata (L.) Jack]. Shoots were regenerated after 6 – 10 months of culture, but they were extremely recalcitrant to producing roots in root-induction medium. Complete plantlets were formed via micrografting. Chromosome counting of shoot tips revealed they were tetraploids (2n = 4x = 36). Glutamateoxaloacetate transaminase isozyme and randomly amplified polymorphic DNA analysis confirmed their hybridity. Orange jessamine is immune to citrus huanglongbin, a severe disease of citrus, but sexual incompatibility and limited graft compatibility exist between Citrus and orange jessamine. The cell fusion technique may make it possible to transfer the huanglongbin resistance trait from orange jessamine to Citrus. Received: 17 January 1998 / Revision received: 12 June 1998 / Accepted: 14 July 1998