This study documents changes in yield, growth, soil salinity (ECe) and leaf sodium (Na) and chlorine (Cl) concentrations in mature Valencia orange [Citrus sinensis (L.Osbeck)] trees on sweet orange (Citrus sinensis) rootstock in response to increased levels of Na and Cl in irrigation water. Four levels of salt, ranging from the river-water control (0.44 dS/m) to 2.50 dS/m, were applied over a 9-year period through an under-tree microsprinkler system to trees in the Sunraysia area of the Murray Valley in south-eastern Australia. A salt-balance model showed that evapotranspiration was reduced by salinity, whereas leaching fractions increased from an average 24% in the control to 51% in the most saline treatment. The high leaching fractions were achieved as a result of freely draining soils and good irrigation management, and allowed us to maintain low to moderate levels of soil salinity throughout the trial and minimised the effect of salt treatment on fruit yield. Soil salinity increased almost linearly in response to irrigation-water salinity during the first year, and fluctuated seasonally thereafter; however, very few readings exceeded 3 dS/m, even in the highest treatments. By contrast, leaf Na and Cl concentrations in the highest salt treatment continued to increase over the first 4 years. The relationship between yield and soil salinity was extremely weak, but yield did decrease as foliar concentrations of Na and Cl increased: in Year 9, leaf Na in the highest treatment relative to the control was associated with a predicted reduction of 17% in yield and 59% in annual trunk-diameter growth.
Citrus is regarded as a salt-sensitive crop, but its yield response to salinity is affected by variety, rootstock, duration of salt exposure, irrigation management, soil type, and climate. This study quantified the yield response of mature Valencia [Citrus sinensis (L. Osbeck)] orange trees on sweet orange (C. sinensis) rootstock to increased levels of sodium chloride in irrigation water in the Sunraysia area of the Murray Valley in south-eastern Australia. The orchard was planted on a loamy sand and trees were irrigated and fertilised with a well-managed under-tree microsprinkler system. Four levels of salt, ranging from the river-water control (0.44 dS/m) to 2.50 dS/m, were applied over a 9-year period. Overall yield effects were smaller than expected, and did not conform well to the often used bent-stick model. Relative to the control, yield was initially higher (by up to 9%) in the intermediate salt treatments, and 3% lower in the highest treatment. However, relative yields of salinised trees decreased with time, and in the final year of the experiment, yield of the highest salt treatment was 9% lower than the control. Yield increases in the intermediate treatments resulted from increases in fruit number. All 3 salt treatments decreased average fruit weight by 4% and decreased juice content but increased juice sugar and acid content. Salt treatment strongly reduced trunk growth, and the effect increased with time. Our results show that with appropriate irrigation management, soils, and rootstocks, citrus trees can maintain productivity at salinity levels of 2.0 dS/m or more, but fresh fruit profitability is likely to be lower because of a reduction in average fruit size.
The annual pattern of root growth of ‘Valencia’ orange [Citrus sinensis (L.) Osb.] trees on rough lemon (C. jambhiri Lush.) and Carrizo citrange [Poncirus trifoliata (L.) Raf. × C. sinensis] rootstocks was studied in relation to shoot growth, soil temperature, and water stress in Plexiglas-walled root observation chambers. The chambers were filled with a reconstituted profile of a fine sand soil and were installed below-ground at a field site in central Florida. The chambers periodically were raised above-ground to record root growth. Under nonlimiting soil water conditions, continuous root extension growth was evident from February to November. The overall seasonal trend in root growth was significantly correlated with soil temperature. The most intense root growth occurred when soil temperatures were above 27°C, and was limited at soil temperatures below 22°. Root growth was cyclic. During periods of shoot elongation, the number of growing roots and the rate of root elongation declined. Shoot growth was a major factor controlling the intensity of root growth when soil temperature and soil water content were nonlimiting. When the soil water content was decreased intentionally, root growth was checked at a soil matric potential of −0.05 MPa. After rewatering, there was a lag period of 2 days before root growth increased. Both number of growing roots and rate of root elongation were influenced by shoot growth, soil temperature, and soil water content; however, the growth response of the root system to these factors was mediated largely through the number of roots initiating growth. No rootstock differences were apparent in the pattern of root growth.
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