No abstract

The performance of Valencia sweet orange grafted onto 41 hybrid citrus rootstocks was evaluated for 11 years in rainfed cultivation under tropical savannah climate (Aw type) in Brazil, in addition to three selections of the standard drought-tolerant Rangpur lime and two selections of Sunki mandarin. Drought tolerance, assessed by visual score of leaf wilting, was directly related to the mean fruit yield. Indio and Riverside citrandarins, Tropical Sunki mandarin and the hybrid TSKC × CTSW-028 were grouped with the most productive selections of Rangpur lime, all of them inducing large tree size, intermediate fruit production efficiency, and high drought tolerance. The hybrid TSK × TR English-CO was similar except by inducing a higher mean soluble solids concentration in the orange juice. A third group of rootstocks induced high yield and drought tolerance, and a mean 30% reduction in tree size that led to high production efficiency, which comprised the hybrids HTR-053, TSKC × (LCR × TR)-017 and-059, TSKC × CTSW-041, LCR × TR-001 and San Diego citrandarin. The tree mortality on Rangpur lime selections was as least as 46%, while more than 80% of trees grafted onto the aforementioned rootstocks survived without visual symptoms of citrus sudden death disease or graft incompatibility. The selected hybrids and Tropical Sunki mandarin also induced fruit quality, mainly soluble solids, superior to the Rangpur lime and, therefore, are potential rootstocks for rainfed cultivation of Valencia sweet orange.

The Brazilian and other citrus industries will likely require, among other technologies, the use of rootstocks that are suitable for higher planting densities with high production efficiency of high quality fruits and tolerant to abiotic and biotic stresses. Considering this approach, hybrid rootstocks have been obtained by the Citrus Breeding Program of Embrapa Cassava & Fruits, in Cruz das Almas, Bahia State, Brazil. One experiment evaluating the first three commercial crops of 'Valencia' sweet orange budded on several rootstocks in Colômbia, São Paulo State, Brazil, indicated that the hybrids 'TSKC ('Sunki' mandarin) × [LCR ('Rangpur' lime) × TR (trifoliate orange)]-059', 'TSKC × CTSW ('Swingle' citrumelo)-033', 'TSKC × CTSW-041', 'LCR × TR-001', 'HTR (trifoliate hybrid)-051', 'HTR-053' and 'HTR-069' allowed planting densities higher than those attained with the use of the traditional rootstocks 'Rangpur' lime and 'Sunki' mandarin. They also induced higher production efficiency of fruits with higher or equivalent quality in comparison to fruits on 'Rangpur' lime, which is the usual rootstock in Brazil. Additionally, 'TSKC × (LCR × TR)-059' and 'LCR × TR-001' induced high tolerance to drought, with results similar to 'Rangpur' lime, and the first hybrid also induced early fruit bearing of the scion variety. In spite of not being dwarfing rootstocks, the hybrids 'TSKC × CTTR ('Troyer' citrange)-002' and 'TSKC × CTSW-028', as well as 'Rangpur' lime selection 'CNPMF-003', 'Sunki Tropical' mandarin and 'Indio' and 'San Diego' citrandarins performed well.

No abstract

Xylella fastidiosa inhabits the plant xylem, a nutrient-poor environment, so that mechanisms to sense and respond to adverse environmental conditions are extremely important for bacterial survival in the plant host. Although the complete genome sequences of different Xylella strains have been determined, little is known about stress responses and gene regulation in these organisms. In this work, a DNA microarray was constructed containing 2,600 ORFs identified in the genome sequencing project of Xylella fastidiosa 9a5c strain, and used to check global gene expression differences in the bacteria when it is infecting a symptomatic and a tolerant citrus tree. Different patterns of expression were found in each variety, suggesting that bacteria are responding differentially according to each plant xylem environment. The global gene expression profile was determined and several genes related to bacterial survival in stressed conditions were found to be differentially expressed between varieties, suggesting the involvement of different strategies for adaptation to the environment. The expression pattern of some genes related to the heat shock response, toxin and detoxification processes, adaptation to atypical conditions, repair systems as well as some regulatory genes are discussed in this paper. DNA microarray proved to be a powerful technique for global transcriptome analyses. This is one of the first studies of Xylella fastidiosa gene expression in vivo which helped to increase insight into stress responses and possible bacterial survival mechanisms in the nutrient-poor environment of xylem vessels.