Search Results
Abstract
The capability of vesicular-arbuscular mycorrhizal (VAM) fungi, symbiotically associated with roots of citrus and most other woody perennial crops, to dramatically enhance plant growth has attracted widespread interest among horticulturists. Unfortunately, too often scientists engaging in mycorrhizal research have become overly enthusiastic with growth responses that were created under artificial experimental conditions. Mycorrhizae are the “normal” condition of roots on citrus trees and most other horticultural plants in the field (44). The nonmycorrhizal condition is “abnormal” and often the consequence of “normal” horticultural practices, such as overuse of fertilizers and pesticides, soil sterilization, and plant production in sterile soilless media. These technical advances have led to rapid production of pathogen-free plants, but VAM fungi have been sacrificed in the process. The potential impact of mycorrhizal deficiency on the “quality” of citrus and other woody crops for transplant has not been fully evaluated. With the realization that VAM fungi are essential for the growth of citrus in field soils, the question of how to reintroduce and gain maximum benefit from mycorrhizae in greenhouse production of citrus and other woody plants needs to be assessed.
Florida citrus groves of sweet orange (Citrus sinensis), tangerines (Citrus reticulata), and grapefruit (Citrus paradisi) experience an annual tree loss of 3% to 4% due to various causes of tree decline. Commonly used tree removal methods in Florida include “pushing,” which lifts most of the root system completely out of the soil, or “clipping,” which shears the tree off above the soil line leaving the tree stump and root system in place. Several operational and economic advantages and disadvantages exist for both tree removal systems. There are also potential problems with citrus resets that can occur due to foot rot (Phytophthora nicotianae) and citrus nematodes (Tylenchulus semipenetrans) that remain in the soil after tree removal. To investigate reset tree performance after “pushing” versus “clipping,” a study was conducted in three groves representative of three production regions in Florida to compare the impact of tree removal method on the pest/pathogen status and growth of resets over a period of 4 years. Based on the findings, tree removal by “pushing” or “clipping” appears to have minimal effect on subsequent pest and pathogen status and performance of citrus resets. Therefore, the method of tree removal should depend primarily on operational and economic considerations.
The effects of phosphorus (P) and of the mycorrhizal (M) fungus, Glomus intraradix, on the carbon (C) economy of sour orange (citrus aurantium L.) were determined during and following active M colonization. There were four treatments: mycorrhizal seedlings grown at standard-strength P (M1) and nonmycorrhizal (NM) plants grown at 1, 2 and 5 times standard-strength P (NM1, NM2 and NM5). Mycorrhizal colonization, tissue dry mass, P content, root length, leaf area, 14C partitioning and rate of c assimilation (A) were determined in five whole-plant harvests from 6 to 15 wks of age. In contrast to the effects of P nutrition on C economy in sour orange, M effects were generally subtle. Mycorrhizae increased the root biomass fraction, the root length/leaf area ratio, and the percent of 14C recovered from belowground components. Mycorrhizal plants had a higher percentage of belowground 14C in the respiration and soil fractions than did NM plants of equivalent P status. Mycorrhizal plants tended to have enhanced A at 8 wks but not at 7 or 12 wks. This temporarily enhanced A of M plants did not fully compensate for their greater belowground C expenditure, as suggested by apparently lower relative growth rates of M than NM plants of equivalent P status. Problems of interpreting the dynamic effects of mycorrhizae on C economy that are independent of p nutrition are discussed.
Citrus canker disease caused by the bacterial pathogen Xanthomonas axonopodis pv. citri is becoming a worldwide problem. Xa21 gene is a member of the Xa21 gene family of rice, which provides broad spectrum Xanthomonas resistance in rice. `Hamlin' sweet orange [Citrus sinensis (L.) Osbeck) is one of the leading commercial cultivars in Florida because of its high yield potential and early maturity. `Hamlin' also has a high regeneration capacity from protoplasts and is often used in transformation experiments. Since the citrus canker pathogen is in the same genus, this gene may have potential to function against canker in citrus. The wild-type Xa21 gene contains an intron, and there are some questions whether dicot plants can process genes containing monocot introns (the cDNA is intron-free). Plasmids DNA, encoding the non-destructive selectable marker EGFP (Enhanced Green Fluorescent Protein) gene and the cDNA of the Xa21 gene were transformed or co-transformed into `Hamlin' orange protoplasts using polyethylene glycol. More than 200 transgenic embryoids were recovered. More than 400 transgenic plants were developed from 75 independent transgenic events. PCR analysis revealed the presence of the cDNA of the Xa21 and the GFP genes in the transgenic plants. Some of the plants have the GFP only. Southern analysis is showing integration of the cDNA into different sites ranges from one to five sites. Western analysis is showing the expression of the cDNA of the Xa21 gene in the transgenic citrus plants. This is the first time that a gene from rice has been stably integrated and expressed in citrus plants. Canker challenge assay is in progress.