Effects of Sodium Chloride on Water Status and Growth of Sugar Beet

Abstract

The effects of sodium chloride on the water status, growth, and physiology of sugar beet subjected to a range of soil water potentials were studied under controlled conditions. Sodium chloride increased plant dry weight and the. area, thickness, and succulence of the leaves. It increased the water capacity of the plant, mainly the shoot, but there was no evidence that it altered the relationships between leaf relative water content and the leaf water, osmotic, and turgor potentials or changed the way stomatal conductance arid photosynthesis responded to decreasing leaf water potential. The greater leaf expansion in sodium-treated plants is thought to be the consequence of adjustments made by leaf cells to accommodate changes in ions and water in a way that minimizes change in water and turgor potentials. It is also suggested that the greater water capacity of treated plants buffers them against deleterious changes in leaf relative water content and water potential under conditions of moderate stress. INTRODUCTION Growth and yield of sugar beet are increased by sodium chloride (Draycott, 1972). Sodium accumulates mainly in the leaves where it is thought to improve water balance and reduce wilting. The evidence for this from field experiments is contra dictory. Sodium fertilizer may have no visible effects on wilting or it may increase or decrease it depending on the season (Draycott and Farley, 1971; Farley and Draycott, 1974; Durrant, Draycott, and Milford, 1978). Similarly, Draycott, Durrant, and Webb (1974) found no statistical interaction between the effects of sodium fertilizer and irrigation on sugar yield and concluded that sodium was unlikely to have affected growth by changing plant water status. But subsequent reappraisal of this and other of their data has shown that sodium and soil moisture do interact to affect leaf water balance and growth (Durrant et al., 1978). Few detailed measurements have been made of the effects of sodium on plant water status. Durrant et al. (1978) showed that sodium increased the relative water content, water potential, and diffusive conductances of leaves only under conditions of moderate soil moisture deficit; it had no effects when deficits were small and decreased leaf water potentials when drought was severe. A major effect of sodium is to increase leaf area (Draycott and Farley, 1971). Lawlor and Milford (1973) found with plants in solution culture that sodium produced a small, but signi ficant, change in leaf turgor potential sufficient to have increased leaf expansion. This content downloaded from 157.55.39.35 on Mon, 29 Aug 2016 05:16:02 UTC All use subject to http://about.jstor.org/terms Milford, Cormack and Durrant—Salt and Water Growth Effects 1381 Even so, the large increases in cation and water content caused by sodium in leaves are rarely accompanied by significant changes in leaf water or osmotic potentials (Lawlor and Milford, 1973; Durrant et al., 1978) so it is not clear how turgor is increased. The work reported here investigated the influence of sodium on the water status, growth, and physiology of leaves of soil-grown sugar beet subjected to a range of soil water potentials in controlled environments. MATERIALS AND METHODS The experimental plants were a commercial monogerm cultivar (Bush Mono G) of sugar beet (Beta vulgaris L.), each grown for 15 weeks in a glasshouse in 5 kg of Ashley Variant, a gravelly sandy clay loam from Broom's Barn Experimental Station, Suffolk containing (kg1 air-dried soil) additional Ca(NC>3)2 (0-24 g), NH4H2PO4 (0-12 g), K2SO4 (0-30 g), and H3BO3 (0-4 mg). Half the plants were given NaCl (0-60 g kg-1 soil), the rest none. Three days before the start of the experiment the plants were placed in a growth room maintained at a visible irradiance of 115 W m-2, a temperature of 15 °C, and an absolute humidity of 7-7 g m-3 during a 12 h day (equivalent to a saturation deficit of 680 Pa) and at 11 °C and 8-9 g m-3 (130 Pa) absolute humidity at night. After acclimatizing the plants, the soil was brought to pot capacity (water content 24 % of soil dry weight) and measurements were made on groups of plants, half grown with sodium and half without, at intervals during the 10 d required to dry the soil to a water content of around 6 % of soil dry weight at which the plants were severely wilted. On each occasion transpiration was estimated over a period of 2 h from the change in weight of pot and plant; because the soil was covered with a layer of polythene granules it was assumed that evaporation from the soil surface was negligible. At the end of the 2 h period a series of measure ments were made on two mature, fully expanded leaves on each plant, these usually being the fifteenth and sixteenth leaves produced. Leaf thickness was measured with a Mercer dial micrometer, diffusive resistance with a LI-COR sensor (Model LI-153) and meter, and gross photosynthesis measured by determining the uptake of radioactive carbon dioxide (Milford and Lawlor, 1975). Three groups of five 0-75 cm2 discs were punched from each leaf; relative water contents were measured by the method of Barrs and Weatherley (1962) on one group, carbohydrates measured as described by Milford and Pearman (1975) 011 the second group, and the osmotic concentration of the sap expressed from the third group measured with a Wescor C52 sample chamber psychrometer. The water potentials of the remaining parts of the leaves and of similar adjacent undamaged leaves were then measured, some with a pressure bomb (Scholander, Hammel, Hemmingsen, and Bradstreet, 1964) and some with thermo couple psychrometers (Lawlor, 1972). Turgor potentials were estimated as the difference be tween the psychrometrically measured water and osmotic potentials. Finally, the plants were removed from the pots, their leaf area and fresh and dry weights determined and sodium (Na+) and potassium (K+) contents of their parts measured by atomic absorption and emission spectrophotometry after dry ashing and acid extraction. Duplicate soil samples were taken from the top, middle, and bottom of each pot, their moisture contents measured, and their matric potentials determined by thermocouple psychrometry and a pressure membrane apparatus : the average of these samples was taken as an estimate of the water potential of the bulk soil. The experiment also contained two further groups of untreated and sodium-treated plants for analysis of plant growth. The soil of one group was allowed to dry out as above and in the other group it was maintained close to pot capacity by weighing and watering twice daily. Dry weights and leaf areas of these plants were measured at the start and end of the treatment cycle. The whole experiment was repeated twice with similar results, the data have been combined for presentation. RESULTS Plant dry weights and leaf areas and the sodium, potassium, and water contents of plant organs at the beginning of the drying treatment are given in Table 1. The sodium chloride treatment increased Na+ concentration in all organs 10-fold and decreased the K+ concentration by a small, but not stoichiometrically equivalent, This content downloaded from 157.55.39.35 on Mon, 29 Aug 2016 05:16:02 UTC All use subject to http://about.jstor.org/terms 1382 Milford, Cormack and Durrant—Salt and Water Growth Effects Table 1. Effects of sodium chloride (a) on dry weights, leaf areas, and sodium, potassium, and water contents of sugar beet at the start of a 10 d drying treatment and (6) on groivth in watered and unwatered plants Without sodium With sodium L.S.D. chloride chloride (P = 0-05) (a) At start of drying treatment Dry weight (g) Laminae 4-7 5-9 0-31** Petioles 2-5 2-7 0-10* Root 1-4 1-7 0-14* Plant 8-7 10-2 1-56* Water content Laminae 5-99 6-96 0-43** (g g-1 dry matter) Petioles 6-93 8-44 0-66** Root 4-56 5-18 0-64 NS Sodium content Laminae 0-24 1-64 0.172*** (mmol g_1 dry Petioles 0-07 1-12 0-074*** matter) Root 0-02 0-22 0-048*** Potassium content Laminae 1-26 0-98 0-164* (mmol g_1 dry Petioles 1-20 1-06 0-232 NS matter) Root 0-54 0-58 0-144 NS Number of leaves 12-5 12-0 1-21 NS Leaf area (dm2) 9-83 11-84 1-55* Leaf thickness (mm) 0-33 0-39 0-014*** Leaf succulence 2-86 3-47 Q-24*** (gwaterdnw2) (b) At end of drying treatment Unwatered plants Plant dry weight (g) 12-9 16-9 2-13** Leaf area (dm2) 7-1 10-4 «1-56*** Watered plants Plant dry weight (g) 15-7 19-3 Leaf area (dm2) 10-5 12-4 NS, *, **, and *** indicate non-significance, and significance at the 5, 1, and 0-1% levels of probability, respectively. ° Least significant differences between means of both sodium chloride and watering treat ments. amount. It increased the dry weight of all parts of the plant, the leaf area, and the thickness and succulence of the leaves, i.e. water content per unit area (Jennings, 1976), but not leaf number. During the 10 d of the experimental drying period the soil moisture content decreased from 24 to 6 %, of soil dry weight equivalent to a change in matric potential from —20 kPa to —1-5 MPa (Fig. 1a). In this time the dry weight of sodium-treated plants increased by 90 % when continuously watered and by 65 % when unwatered, while the corresponding figures for untreated plants were 80 and 45 % respectively. Leaf areas of watered plants increased by 6 % irrespective of sodium treatment but those of unwatered plants decreased because their mature leaves died ; leaf area decreased by almost 30 % in untreated plants and by 10 % in sodium-treated plants. There was only a small change in matric potential with change in soil moisture content over the range 25 to 10 % in the sandy clay loam, i.e. from —20 kPa to Q. l 1 dry 1