The cultural practices within an orchard can materially affect root systems. In Ceylon , Gandhi cites a difference in citrus root distribution between irrigated areas and rainfall areas. However, when he grew grapefruit trees on Rough lemon with various degrees of cover crop the citrus roots would not compete with the cover crop. They either turned back into open areas or penetrated deeper. If the roots extended to the cover crop they grew under it, but not through it. Those trees without cover crop extended roots laterally 2 to 3 times the spread of the branches. Where cover crops were used the roots only extended to the periphery of the branches. A nine-year-old grapefruit tree with a cover crop of Indigofera and Ecaphylla [could not identify this plant] also had lateral roots extending a little beyond the periphery of the crown. They did not appear to appreciably mingle with roots of other trees only 4.3 M away. His observations clearly indicate citrus roots are not able to maintain the same growth rate in association with permanent cover as without it, and that for free development of their roots they require the soil beyond the drip of the tree crown to be free of competition for water by other roots. Figure 64 by Wallace and Nauriyal [figure could not be included due to copyright restrictions] shows citrus rootlet distribution as influenced by cultivation. The tilled orchard had few roots at the 0-15 cm level and most were in the 60-150 cm.
By contrast, under non-tillage there were 7 times as many roots in the 0-15 cm and the highest concentration was at the 15-30 cm zone. Deep soil was essential for making furrows, basins,how to set up a vertical farm or block-and-ridge systems for water. Such tillage severed most of the surface laterals and caused severe damage. Figure 65 shows such a root system with the laterals cut at several locations, and Figure 66 [Image could not be located] shows such a severed root which then regenerates a multitude of short stubby roots, with no single root gaining dominance. Water and nutrients are most available in the upper soil zones, which is rendered unavailable by tillage. Diseased trees have reduced root systems. Ford reports that declining grapefruit trees had 43% less roots than healthy trees. Oranges on Rough lemon at 7 years of age replanted on spreading decline soil had 18 per cent more feeder roots in the upper 25 cm of soil than good trees the same age. However, the decline trees had no roots below 75 cm. Ford says that mature citrus trees affected by the spreading decline nematodes had 40 per cent fewer feeder roots than healthy trees. Decline trees had more feeder roots in the upper 25 cm of soil but almost none below 75-105 cm. Roots examined in affected groves showed significant disintegration at the 50-64 cm level with a rapid increase in deterioration with increase in depth. Practically all roots of decline trees at 50-105 cm were dark and discolored. Trees replanted in spreading decline affected soil had no lateral roots or feeder roots below 75 cm. Montenegro found poor root systems under trees affected with exocortis or tristeza.
The sour orange, because of its tendency to develop deep tap roots, was used extensively in the West Indies as a root stock for limes. Such trees were reported by growers to withstand hurricane injury better than lime on its own roots. Fennah in a study of the root systems of budded trees at St. Lucia Island in the Caribbean found that sour orange seedlings in the nursery all had tap roots. However, out of 24 orchard trees examined at three years of age only three had tap roots and that of 110 trees at five years of age only three had tap roots, and the root system was horizontal. Fennah explained this circumstance as due mainly to damage by the citrus weevil and to injury at transplanting. Kaufmann, Boswell, and Lewis , in a spacing trial of Washington navels on Troyer citrange in California, studied root distribution at different spacings. They found that the distribution of the roots varied with the distance from the tree and the availability of soil water . The trees at a 2.7 x 4.7 M spacing at eight years of age had utilized the full rooting area and this was leading to a reduced tree performance. In Florida, Ford observed that the root system of sour orange was influenced more by freeze injury of the scion than the root system of Rough lemon, and that the root system of sour orange recovered more slowly. In California, Biely, Wallace, and Kimball report pruning citrus trees affected root distribution. In a period of five months following skeletonization there was a great reduction in feeder roots at all depths.
Essentially all the feeder roots had disappeared. One year later the amount of rootlets under skeletonized trees was greater in number than before the trees were severely pruned. In a Florida root stock experiment, Castle and Krezdorn report that the depth of rooting was correlated with tree height; the tallest trees had the deepest root systems. Feeder root weight and tree height were not correlated. The tallest scion trees were on Rough lemon, Palestine sweet lime and Cleopatra mandarin. The shortest scion trees were on Rusk citrange and various trifoliate orange selections. Scion trees on Carrizo and Troyer citranges, sour orange and sweet orange were intermediate. The depth of rooting varied from 465 cm for Rough lemon to 377 cm on trifoliate orange and 366 cm on Rough lemon to 206 cm on Rusk citrange. Rootstocks had a pronounced effect on total feeder root weight. Deep-rooted trees on Rough lemon and sweet lime had more than 50 per cent of their feeder roots below 75 cm. Intermediate-sized trees were about the same as the tall trees above and below the 75 cm level. Exceptions were Cleopatra, Rusk citrange, and Rubidoux trifoliate which had over 60% of their roots above 75 cm. Rough lemon was the only root stock with roots below 457 cm. Reports on drought resistance are somewhat variable. Webber says sour orange is tolerant. Hume confirms this and states Rough lemon is more susceptible. Evans states Rough lemon is tolerant and grapefruit and sour orange are more susceptible. Brown indicates the Egyptian lime is tolerant. Ford reports sweet orange to require more frequent irrigations than Rough lemon. Bhattacharya and Dutta indicate the Soh-myndong to have drought resistance. Various visitors to the Citrus Research Center have indicated that the Palestine Sweet Lime and the Rangpur lime have drought resistance. All of this may vary with soil type, temperatures, etc. Hilgeman et al. compared Rough lemon, Rangpur lime, Ocklawaha sour orange, Sacaton citrumelo, Troyer citrange, Koethen sweet orange,plastic pots for planting arid Wilking mandarin as root stocks for Lisbon lemon in a sandy soil near Yuma, Arizona, and a sandy loam near Phoenix. They found that young trees on sour orange and Troyer citrange developed greater moisture stress between irrigations than trees on Rough lemon and Rangpur lime. The growth of the fruit was restricted at the differential irrigation levels. They suggested the root growth on the Rough lemon was more rapid than on sour orange stock and hence a larger soil volume was available to provide water to the trees on Rough lemon than those on sour orange. Gardner and Horanic in Florida’s humid climate compared the transpiration of young trees on four root stocks by weighing container-grown trees at frequent intervals. The rate of transpiration of Hamlin orange tops was lowest on Rough lemon, highest on sweet orange and intermediate on Cleopatra mandarin and sour orange, although the actual amount of water transpired was essentially the same for all four root stocks. The transpiration rates were inversely proportional to the leaf areas of the scion variety tops developed by these root stocks, indicating the tops carried the greatest leaf area on Rough lemon and had the least resistance to transpiration. The effect of the root stocks on transpiration rates apparently resulted from differences in leaf resistance. Horanic and Gardner , following a prolonged dry period in Florida during September 1958, found a marked difference in wilting of Parson Brown and Valencia oranges in a root stock experiment on a Lakeland fine sand.
Marked differences between root stocks were evident. Sour orange, Rough lemon, and Cleopatra mandarin were outstanding in their survival resistance to drought as compared to trees on grapefruit, sweet orange, and Rusk citrange. Unbudded Parson Brown seedlings in the planting also showed severe wilting. They felt the differences in apparent drought resistance were attributable to the root stock effect rather than to a greater or less soil moisture depletion by trees of different size. Ongun and Wallace , using the water weight loss in plants grown in containers in a greenhouse, studied the effect of both root stocks and root temperature on the transpiration rate of Washington Navel orange scions grafted on rooted cuttings of a number of root stock varieties. They reported that of the root stocks used, the transpiration rate was highest on Rough lemon and lowest on sour orange and that while root temperatures influenced transpiration, not all the root stocks were affected to the same extent. It descending order the combinations were rated best on Rough lemon, then Poncirus trifoliata, Troyer citrange, grapefruit, sweet orange, Cleopatra mandarin and sour orange. Mendel in Israel agrees that trees on Rough lemon transpire at a faster rate and, although a large tree, shows considerable drought resistance. He remarks of the remarkable drought resistance of trees on Rough lemon in spite of the high transpiration rate. Trees on sour orange had a strong tendency toward lower transpiration losses than trees budded on sweet lime. O’Byrne lists Rough lemon as most drought resistant followed in descending order by sweet orange, sour orange and grapefruit. Crocker, Bell, and Bartholic using a modified Scholander pressure bomb found it was sensitive enough to detect significant differences in relative leaf water stress on Orlando tangelo trees on several root stocks.In descending order the trees ranked Rough lemon, Palestine sweet lime, sour orange, Carrizo citrange, and trifoliate orange. Perhaps the largest contingent of trees observed for drought resistance was made in Brazil by Moreira et al. on nucellar Bara and Valencia Late orange, Dancy tangerine and Eureka lemon budded on 77 tristeza-tolerant root stocks which were grown under nonirrigated conditions in Brazil. They categorized the drought resistance into three groups: low, fair and good. If one ignores the early reports from Florida on the drought resistance of sour orange which seemed to be a special circumstance, it would appear that those root stocks which have the greatest amounts of fibrous roots appear to be the most drought tolerant irrespective of the tap root system, and that conversely those root stocks lacking quantities of fibrous roots regardless of the depth of penetration are more susceptible to drought.Undoubtedly, the good drought resistance of Rough lemon root stock in Florida is the deep root penetration into the warm sandy subsoil of the “ridge” area. These soils are very deep and [text incomplete] drained in the poorly drained soils in eastern Florida the drought resistance of Rough lemon is not so good. In California during the summer months, there are periods of high temperatures and low humidity accompanied by hot, dry, desiccating winds. Under these extreme conditions leaves of trees may suffer severely from mesophyll collapse. After one such occasion in Coachella Valley, several of the author’s root stock plantings were rated in 1960 for severity of mesophyll collapse . Leaf damage was very severe on Brazilian sour orange, Bessie sweet orange, Batangas mandarin, Cleopatra mandarin, Suen Kat mandarin, #653 tangor, Carrizo citrange, Savage citrange, Troyer citrange, Uvalde citrange, Calamondin, and Shekwasha. It was much less on stocks of #343 grapefruit, Ponkan mandarin, Sunshine tangelo, Citrus macrophylla, C. pennivesiculata, Rangpur lime and Rough lemon. This correlates very well with observations on the extent of fibrous roots for the various stocks. One of the most striking effects on root system development is that exerted by the scion variety.