Paul D. Galletta

In vitro flowering from Citrus limon lateral buds.

Summary Citrus limon (L.) Burm. f. cv. «Eureka» vegetative lateral buds from 6-year old trees, when cultured at 14 to 20 °C in vitro , flower at rates of 5 to 20 % after 16 weeks and 90 % within 32 weeks. Buds maintained at 25 °C did not flower, and attempts to chemically induce flowering at 25 °C using various growth inhibitors and promoters as well as high concentrations of sugars failed. Abscission of floral and foliar buds, as well as leaves, was common in vitro .

Production of cucumber fruits from the culture of ‘Marketmore-76’ plantlets

A procedure to produce fruits from cultured shoot tips of Cucumis sativus L. cultivar ‘Marketmore-76’ in vitro is described. Four-week-old shoot tips, derived from sterile germinated seedlings on a MS medium, were cultured in a 3.8-1 Mason jar using an automated plant culture system. Tips readily generated roots, leaves and flowers after another 4 to 8 weeks in culture. Administration of compressed air at a 300 ml/min flow rate for 30 min 10 or 15 times a day induced the development of parthenocarpic fruits from flowers. Fruits, up to 170 mm long by 35 mm diameter, were obtained within 30 to 45 d after flower opening.

In vitro culture of lemon juice vesicles

This study explores the possibility of culturing Citrus limon (L.) Burm. f. cv. ‘Eureka’ (lemon) juice vesicles as isolated intact tissues capable of retaining their unique growth structure in vitro. Isolated juice vesicles either attached to or detached from the endocarp/mesocarp (albedo) tissue of origin were grown on various nutrient media using several physical environments. Various growth responses achieved in vitro from cultured vesicles are described. Intact vesicles attached to endocarp/mesocarp tissue were found to grow up to 6 months in culture as a distinct tissue with a minimum of adverse callus proliferation. Callus formation from some cultured explants occurred on all media and physical environments tested. Callus production eventually obliterated and irreversibly altered the normal development of juice vesicles. Inherent vesicle physiology rather than the tissue culture environment was the major determining factor affecting culture growth. Reducing the concentration of carbohydrates (fructose, glucose or sucrose) added to media from 3 to 0.01% or 0.0% reduced, but still did not totally prevent, callus production. Treatment of vesicles with 1000 mg/l ascorbic acid or citric acid, or 0.1 mg/l indole-3-acetic acid or abscisic acid enhanced the occurrence of normal appearing vesicles.

Carpel Polymorphism in Citrus Fruit

Each citrus species and cultivar in the Aurantioideae exhibits a distinct range of carpel (segment) numbers within their fruits. The observed range of segments for fruit within any species or cultivar can be influenced by branch location on a tree. The carpel number for all fruits produced on a single tree may have a symmetrical, unimodal distribution, as well as a positively or negatively skewed unimodal distribution. Most citrus species produce fruits that contain eight to ten segments. However, the average segment number per fruit may be as high as 17 in some pummelos (Citrus grandis [L.] Osb.) or as low as four in some kumquats (Fortunella japonica [Thumb.] Swing.). Correspondingly, pummelos are the largest citrus fruit in terms of fresh weight and diameter, while kumquats are the smallest. Fruit geographical orientation on the tree does not affect the segment number. Abscission of fruit does not select for any particular segment number. Crossing parents with dissimilar fruit segment numbers produces progeny with a range of segment numbers.

Flower Organ Culture

A variety of factors contribute to flower induction in nature. These same factors are assumed to be responsible for in vitro flowering (I). Flower formation in tissue culture has been observed in several plant species, and arises from a variety of explant sources (2). However, the factors responsible for flowering in culture have not been extensively studied (I). Reasons for studying flower formation in tissue culture can be summarized as follows:

Adventitious juice vesicle initiation in lemon (Citrus limon L.), mandarin (Citrus reticulata Blanco), sour orange (Citrus aurantium L.), and sweet orange (Citrus sinensis (L.) Osb.) fruit explants

Adventitious juice vesicles have been obtained from lemon, mandarin and navel and sour orange juice vesicle explants cultured for prolonged periods on a nutrient medium containing 3.0% sucrose in vitro.

Fruit Organ Cultures

The culture of fruit tissues as whole organs or isolated tissue sections has been conducted with various species (1). Whole, isolated ovaries have been successfully cultured to give rise to mature fruits (e.g., strawberry). Typically, however, when an isolated portion of the fruit tissue is introduced into a sterile environment, it immediately loses structural integrity and degenerates into a rapidly dividing callus mass (2). Loss of structural integrity is correspondingly associated with an alteration of physiology that is subsequently reflected in the production of an altered metabolism. Therefore, a meaningful study of fruit development using callus derived from fruit tissues is often not possible. Recently, we studied the parameters involved in the maintenance of citrus fruit tissue integrity (2). In this paper, the culture of isolated fruit tissues, as well as half and whole fruit culture, is demonstrated using the lemon fruit (Fig. 1-3).