Untitled - IRRI books - International Rice Research Institute

6 R.S. Russell and G.W. Cookemycorrhizas of the genus Endogone commonly infect the cortex of roots and develophyphae which can extend 2 cm into the soil. The fact that they are capable ofproviding roots with two to three times as much phosphate as could reach them bydiffusion through soil was first demonstrated less than ten years ago (Sanders andTinker, 1973). Field surveys in Nigeria have shown them to be present on roots ofmany crop plants (Sanders, 1981). Their importance, especially for cassava, hasalso been indicated in other regions, including South America (Howeler, 1981). Atpresent, adequate quantitative results from field experiments are lacking, butthere can be no doubt of the importance of further work on this subject.Potassium is intermediate between phosphate and nitrate in its movementthrough the soil. In some soils, it moves freely, but when 2:1 clay mineralsoccur, its rate of diffusion can be much reduced, leading to localized depletionclose to roots.Knowledge of the parts of the roots systems which are able to absorbnutrients is also important for considering the efficient passage of nutrients tothem. Improved research methods have lately shown that all parts of the entireroot systems of cereals, and some other annual crops, can absorb the three mainnutrients, N, P and K, at approximately the same rates if the external supply ofwater and nutrients does not vary. Calcium is, however, an exception. Its uptakeis confined to the apical parts of roots and it does not move downwards within theroots to the young growing parts. An adequate concentration of calcium in thezone where they extend is therefore essential.Physical Soil Stresses Which Restrict Root Growth and FunctionThe size and distribution of pores in the soil are its most importantstructural characteristic which affect the occurrence of physical stresses toroots. The following simplified classification of pores is used here:Transit pores of channels, exceeding cir. 50 µm in diameter. Water movesfreely through them under gravity; air enters and roots commonly grow downthem.Storage pores, cir. 5 - 50 µm in diameter. They hold water against gravitybut at a potential (free energy) sufficiently high for roots to extract it.Abundant smaller pores are also present, but do not enter into this discussion asthe water in them is at too low a potential to be readily available to plants.Both the pedological origin of the soil and its management can greatly influenceits pore characteristics.Water Stress: When there is a complete ground cover of vegetation, water can belost into the atmosphere at about three quarters of the rate from a free watersurface; solar radiation incident on leaves is the driving force. Thisconsiderable use of water is an inescapable consequence of the structure of leaveswhich provides for gas exchange between leaf cells and the atmosphere, on whichphotosynthesis depends. Stomatal closure, which is influenced by abscisic acid,one of the hormones produced in roots, can restrict water loss during the daylighthours when insolation is highest. Plant species differ much in the efficiency ofthis process and also in other characteristics which can reduce water use. Plantbreeding may improve their water economy but a considerable loss is inescapable inrapidly growing plants.Water moves along a gradient of decreasing water potential from soil throughthe plant and, if transpirition losses are not balanced by absorption, injury canbe rapid - much more so than would be often caused by a similar interruption in