WATER RELATIONS OF WHEAT ALTERNATED BETWEEN TWO ROOT TEMPERATURES

Nezo Phytol. (1979) 82, 89-96 89 WATER RELATIONS OF WHEAT ALTERNATED BETWEEN TWO ROOT TEMPERATURES BY M. B. KIRKHAM Department of Agrofiomy, Oklahoma State University^ StiHwater, Oklahoma 74074, U,S.A. (Accepted 3 May 1978) SUMMARY Water potentials, osmotic potentials and stomatal resistances were measured daily in a growth room during a 16-day period, in growing leaves of a drought-sensitive and a drought-resistant winter wheat (Tntictim aestivum L. em. Thell.), which had the roots maintained under one of three root-temperature treatments: (1) constant temperature of 24 C (about the optimum); (2) constant temperature of 34 C (10'^ above the optimum and near the high temperature limit for growth); and (3) alternating temperature of 24 h at 24 C followed by 24 h at 34 ^C. Air temperature was constant at 24 C. Water and osmotic potentials, stomatal resistances, and height of plants maintained at the 34 C root temperature were intermediate between those grown at 24 C (highest potentials, lowest resistances, tallest) and those alternated daily between the two temperatures (lowest potentials, highest resistances, shortest). Previous work showed that potentials, stomatal resistances, and growth of wheats subjected to constant root temperatures of 25, 29 and 33 C were linearly related to root temperature with lowest potentials, highest resistances, and least growth at the high temperature. Even though alternated plants in this experiment were at an average root temperature of 29 C, they grew the poorest. The wheat plants, therefore, did not integrate a root temperature stress in the same manner over 'time* as they did over 'space*. The results showed that the stomatal resistance, water and osmotic potentials, and height of wheat seedlings, maintained at a constant root temperature 10 C higher than the optimum, were about halfway between those of plants grown at the optimum and those alternated daily between the optimum and the optimum-plus-10 '^C root temperature. INTRODUCTION The root system of a plant is subject to a variety of temperatures, some of which are constantly changing. The surface centimeter of soil can vary daily by more than 20 C (Nielsen, 1974). Many studies show effects of different, but constant, root temperatures on physiological processes such as photosynthesis, respiration, water and nutrient uptake, and hormonal production (Cooper, 1973; Nielsen, 1974; Torrey, 1976). Few experiments report effects of alternating root temperatures (Walker, 1970; Cooper, 1973). Most of these experiments involve plants maintained at one temperature for several days and then changed to another temperature for several days, rather than plants shifted between temperatures on a daily basis. The studies of alternating root temperatures report growth, yield, stomatal aperture, transpiration, exudation, and nitrogen, phosphorus, and potassium uptake rates of plants, but, apparently, neither osmotic potentials nor stomatal resistances. Consequently, this study was done to determine the effect of a changing root-temperature environment on the plant water potential, osmotic potential, turgor potential and stomatal resistance of two cultivars of winter wheat, one considered drought sensitive and one considered drought 0028-646X/79/0101-0089 $02.00/0 1979 The New Phytologist

90 M. B. KiRKHAM resistant. It is important to know plant potentials and stomatal resistance because growth only occurs when potentials are high and resistances are low (Gardner, 1973). The plants were subjected to constant temperatures of 24 or 34 "C, or to a daily alternating temperature of 24 h at 24 ^C followed by 24 h at 34 ^C. These temperatures were chosen because previous work (Kirkham and Ahring, 1978) showed that 24 "^C is the optimum temperature for maximum growth of these cultivars. The high temperature was chosen because it is 10 C higher than the optimum temperature, and, when studying biological reactions, one usually determines the ratio of the rate of activity at a given temperature to the rate at a temperature 10 ""C different from the given rate (Giese, 1962, p. 199). MATERIALS AND METHODS Two cultivars of winter wheat (Triticum aestivum L. em. Thell.) seeds, one considered drought-sensitive (cv. Ponca) and one considered drought-resistant (cv. KanKing) (Todd and Webster, 1965), were germinated on wet filter paper in Petri dishes on 14 October 1977 (day 1). On 17 October one seedling was placed in each of eighteen 140 ml aluminium-foil-covered plastic jars containing a modified, half-strength Hoagland's nutrient solution (Kirkham, Gardner and Gerloff, 1969). The solution was aerated with an air pump (Hush III Aquarium Air Pump, Metaframe Corp., Elmwood Park, New Jersey, U.S.A.). The experiment was carried out in a growth room where the ffux density of incident light was 380//E m~^ sec~^ 24 h per day. There were three treatments replicated three times for each cultivar. One-third of the jars were placed in a water bath (30x33 cm internal dimensions, National Appliance Co., Portland, Oregan, U.S.A.) set at 34 C and one-third were placed at a temperature set at 24 C. Actual temperatures during the study were 33-7 + 0-4 and 24-4 ± 0-4 C. One-third of the jars were placed for 1 day (24 h) at 34 C and then placed for 1 day at 24 C. The treatments lasted for 16 days (27 Oct through 12 Nov). Leaf temperature of the lower surface was measured daily at 17.00 h, starting on day 15 (28 Oct.), with a hand-held, fine-wire thermocouple unit (chromel-constantan, 0-0762 mm diameter; Omega Engineering, Inc., Stamford, Connecticut, U.S.A.) (Perrier, 1971) attached to a Keithley Model 150B Microvolt Ammeter (Keithley Instruments, Cleveland, Ohio, U.S.A.). Resistance of the stomata on the lower leaf surface was measured, immediately after leaf temperature was measured, with a calibrated stomatal diffusion porometer (Model LI-60 and Sensor LI-15S, Lambda Instrument Corp., Lincoln, Nebraska, U.S.A.) (Kanemasu, Thurtell and Tanner, 1969). The stomatal resistance of the lower leaf surface was measured because Rawson, Gifford and Bremner (1976) reported that it is a better indicator of cultivar difference in wheat than stomatal resistance of the upper surface. After the stomatal resistances were measured, a part of a leaf was sampled for potential measurements. Water and osmotic potentials were determined with thermocouple psychrometers (Ehlig, 1961). Turgor potential was calculated as the difference between osmotic and water potential. Leaf temperatures and stomatal resistances were determined on leaf 2 between days 15 and 21 and on leaf 3 between days 22 and 30. Potentials were determined on leaf 1 between days 15 and 21, leaf 2 between days 21 and 26, and leaf 3 between days 26 and 30. Jars were w^eighed daily to determine water loss due to transpiration. Length of roots and shoots was recorded on days 5, 6, 7, 9, 14, 17, 21 and 28. On days 9, 14 and 18, plants were in the 2-, 3- and 4-leaf stage, respectively.

Water relations of wheat At harvest, dry weights of roots and shoots were determined by drying to constant weight at 70 C. Results obtained were averages of the three replications except for measurements of potentials, which were the average of leaves from two replications, randomly sampled. Standard deviations of the means, or standard errors, were calculated (Steel and Torrie, 1960) and are presented. RESULTS AND DISCUSSION Stomatal resistance and transpiration Figure 1 shows the stomatal resistance of Ponca the drought-sensitive cultivar, and KanKing, the drought-resistant cultivar, grown under the three ambient roottemperature regimes. The stomatal resistance of plants grown for 24 h at 34 ""C and then 24 h at 24 C is represented as the average of 34 and 24 (29 "C). The stomatal resistance values obtained throughout the experiment have been averaged and averages are presented in Figure 1. 20 25 30 Ambient temperoture CO Fig. 1. Stomatal resistance of two cultivars of winter wheat grown for 16 days under three ambient root temperatures: (1) constant temperature of 24''C; (2) constant temperatuie of 34 C; (3) alternating temperature of 24 h at 24 C followed hy 24 h at 34 C. The alternating temperature regime has been represented as the average of 24 and 34 C (29 C). Ponca (O) is considered drought-sensitive and KanKing, (D) drought-resistant. Vertical lines indicate standard error. (Only half the standard error line has been drawn to avoid cluttering the figure.) Plants grown in a constantly changing root-temperature environment had a stomatal resistance that was higher than that of plants grown with a constant root temperature. The high root temperature (34 C) was near the temperature limit for growth of these two wheat cultivars because previous work (Kirkham and Ahring, 1978) showed that Ponca and KanKing do not germinate at 37 ^C, The previous study also showed that the two cultivars grew best, and had the lowest stomatal resistance, at a root temperature of 25 '^C. In the earlier work, stomatal resistance was found to be linearly related to root temperatures of 25, 29 and 32 "C. Yet, in this study, the plants maintained

M. B, KIRKHAM constantly at the high root temperature had a lower stomatal resistance than those which grew half the time at the optimum temperature and half the time at the high temperature. Plants alternated between root temperatures of 24 and 34 ^C had a stomatal resistance that was about 1-5 times higher than plants grown continuously at 24 C. The stomatal resistance of the drought-resistant plants, cv. KanKing, at the constant low or high temperature (24 or 34 ^C) were similar (9-9 vs 9 3 sec cm"^). The drought sensitive plants, cv. Ponca, had a higher stomatal resistance at the high temperature than at the low temperature (7-5 vs 9-9 sec cm~^). The amount of water lost from the containers, due to transpiration, is shown in Figure 2. Transpiration was related to the stomatal resistance (compare Figs 1 and 2), which showed that stomata were controlling the rate at which water moved out of the plants. 25 30 AmbienI temperature CO Fig. 2. Water lost from jars containing two cultivars of winter wheat grown for 16 days under three ambient root temperatures. For details, see legend of Figure 1. Water, osmotic and turgor potentials Figure 3 shows the water, osmotic and turgor potentials of the drought-sensitive and drought-resistant wheat cultivars. The figure shows the average values obtained throughout the experiment. The water potential of the drought-sensitive cultivar grown at a constant root temperature of 24 C was higher than that of plants grown at a constant root temperature of 34 C ( 8-6 vs 9*7 bar. Fig. 3). Ponca plants alternated between 24 and 34 C had a lower potential ( 13-7 bar) than those grown at a constant root temperature. The drought-resistant cultivar also had a lower potential when grown with alternating temperature ( 11-2 bar) than when grown at a constant root temperature of 24 ""C (-8-6 bar) or 34 C (-6-2 bar). The osmotic potential of Ponca grown at a constant temperature of 24 or 34 C (- 14-2 or - 15-9 bars. Fig. 3) was higher than that of Ponca grown at the alternating temperature (-197 bar). The osmotic potential of KanKing at the constant root 35

Water relations of zvheat 93 temperature of 24 C or at the alternating root temperature were similar (- 15-2 bar). KanKing at the high root temperature (34 C) had the highest (least-negative) osmotic potential ( 11-5 bar). Turgor potential of Ponca grown under the three root-temperature regimes was similar (5-5, 6-0, 5-7 bars. Fig. 3). KanKing had a lower turgor potential under the alternating temperature regimes (3-9 bar) than under either constant-temperature condition (7-2 or 5-8 bar. Fig. 3). In the previous study, when plants were grown under constant root temperatures of 25, 29 or 33 C, the water, osmotic, and turgor potentials were, in general, linearly related to root temperature. In this study the alternating root temperature between optimum and high resulted in lower potentials than a constant, high temperature. o PO - 25 30 Ambient tempefoture 35 25 30 Ambient temperature C) 35 Fig. 3 Fig.4 Fig. 3. Water (b), osmotic (c) and turgor potentials (a) of two cultivars of winter wheat grown for 16 days under three ambient root temperatures. For details see legend of Figure 1. Fig. 4. Difference between air and leaf temperature of two cultivars of winter wheat grown for 16 days under three ambient root temperatures. For details, see legend of Figure 1. Leaf temperature Figure 4 shows the difference between leaf and air temperature of the two cultivars of winter wheat grown under the three root temperatures. The figure shows the average values obtained throughout the experiment. The leaf temperature of a plant alternated between 24 and 34 "^C was similar to leaf temperatures of plants grown constantly at 34 C when the plant was at 34 C. When the plant was at 24 "^C, its leaf temperature was similar to that of plants grown constantly at 24 C. Therefore, when values were averaged, the leaf temperature of plants with alternating root temperatures was halfway between leaf temperatures of plants grown at 24 and those at

94 M. B. KIRKHAM 34 C. Leaf temperatures reflected the root temperature environment and heat must have been translocated quickly from the root to the leaf. The drought-resistant cultivar might have been better able to translocate heat than the drought-sensitive cultivar because at the high temperature its leaf was warmer than the leaf of the droughtsensitive cultivar. Growth Figure 5 shows the length of roots and shoots, at harvest, of the droughtsensitive and drought-resistant cultivars of winter wheat. Plants grown with the alternating root temperature had the shortest roots and shoots. Shoots of plants grown at 34 C were intermediate in height between those grown constantly at 24 C 20 25 30 Ambient temperoture CO 35 Fig. 5. Length of shoots (O, D) and roots (#, ) at harvest of two cultivars of winter wheat grown for 16 days under three ambient root temperatures. For details see legend of Figure 1. Vertical lines indicate standard deviation. or alternated between 34 and 24 ^C. Shoots of the drought-sensitive wheat were taller at the 24 C root temperature than those of the drought-resistant cultivar. But at 34 C the drought-resistant cultivar had taller shoots than did the drought-sensitive cultivar. Dry weight results (not reported) paralleled the results shown for length (Fig. 5). Number of roots per plant was less when roots were alternated between the two temperatures than when they were held at a constant temperature. The average number of main roots 1 cm below the crown were as follows at harvest for the 24 C, alternating, and 34 C root treatments respectively: cv. Ponca: 10, 7, 9; cv KanKing: 9, 7, 12. DISCUSSION The experiment showed that if one alternated wheat root temperatures between the optimum temperature (24 C) and that near the high-temperature limit (34 T), growth was inhibited more than if one exposed the roots constantly to the high root

Water relations of zvheat 95 temperature (34 C). The results were surprising because the average of 24 and 34 (29 ^C) is closer to the optimum than is 34 C. For both drought-sensitive and drought-resistant wheat cultivars during a 16-day treatment period, stomatal resistance was about 1-5 times higher, leaf water potential was about 4-6 bar more negative, and shoot growth was 6 cm less, if plants were grown under the alternating root temperature environment than if plants were grown at a constant root temperature of 34 C. The plants were not integrating the root temperature stress in the same manner over 'time' as they did over 'space' because results of another experiment (Kirkham and Ahring, 1978) showed that stomatal resistance, water potential and growth of these wheat cultivars exposed to constant root temperatures of 25, 29, or 33 C was related linearly to temperature. It is interesting to note that plants (barley) exposed to a salinity stress integrated the stress in the same way over time and space (Kirkham et al, 1969). Exposure of roots to a saline solution, followed by a non-saline solution (alternating daily), did not disrupt the osmotic-adjustment process and growth was intermediate between that of plants exposed all the time to a saline solution or those exposed all the time to a non-saline solution. Apparently, the plants exposed to a constant root temperature of 34 "^C had a chance to adjust to the high temperature and develop a new equilibrium. The production of root hormones probably affected the rate of adjustment. For example, cytokinins are produced in roots and travel to leaves where they cause stomatal opening (Clarkson, 1974; Torrey, 1976). Abscisic acid does not appear to be produced mainly in roots, but in leaves which are experiencing stress (Torrey, 1976). Abscisic acid induces stomatal closure. Cytokinin production can be quick (for example, 6 h or less; see Torrey, 1976, p. 443). But, if the cytokinins must move from roots to leaves, it may take 2 to 3 days for leaf responses to be apparent (see Cooper, 1973, p. 49) because hormones travel at a rate of the order of 4 to 10 mm h~^ (see Torrey, 1976, p. 439). Cytokinin production in roots and transport to leaves of plants alternated daily between the 24 and 34 C root temperatures must have been constantly hindered. This may have caused, in part, the high stomatal resistance, low water potentials and poor growth. The shorter shoots and roots and fewer roots of these plants also suggested that abscisic acid levels were higher in them than in plants exposed to constant root temperatures because abscisic acid is a potent inhibitor of growth (Torrey, 1976). The drought-resistant wheat, cv. KanKing, had higher water and osmotic potentials, a lower stomatal resistance, and grew taller at the high root temperature (34 C) than did the drought-sensitive cultivar Ponca. This suggested that drought resistance may be related to high-temperature tolerance. KanKing grown under the alternating root temperature also had the highest * water use efficiency' i.e. growth of shoots divided by transpiration rate (see Figs 2 and 5): 27-7 cm 56-5 cm 0-49 g day-^ 1 g The water use efficiency of Ponca and KanKing grown under the other treatments was similar and averaged 26-8 cm/1 g day-^. The results have implications for field-grown plants. If weather (temperature) conditions constantly change, causing fluctuations in the temperature of the root zone, plant growth might be inhibited more than if temperature conditions, even if high and adverse, were constant. Mulches help reduce temperature variations (Willis, Larson and Kirkham, 1957) and therefore would be beneficial.

M. B. KIRKHAM REFERENCES CLARKSON, D. (1974). Ion Transport and Cell Structure in Plants. 350 pp. John Wiley, New York. COOPER, A. J. (1973). Root Temperature and Plant Grozvth: A Reviezv. Res. Rev. 4, Commonwealth Bur. Hort. and Plantation Crops, Commonwealth Agr. Bur., East Mailing, Maidstone, Kent. 73 pp. EHLIG, C. F. (1961). Measurement of the energy status of water in plants with a thermocouple psychrometer. Plant Physiology, 37, 288. GARDNER, W. R. (1973). Internal water status and plant response in relation to the external water regime. In: Plant Response to Climatic Factors (Ed. by R. O. Slatyer), pp. 221-225. Unesco, Paris. GIESE. A. C. (1962). Cell Physiology. 592 pp. W. B. Saunders Co., Philadelphia. KANEMASU, E. T.. THURTELL, G. W. & TANNER, C. B. (1969). Design, calibration and field use of a stomatal diffusion porometer. Plant Physiology, 44, 881. KIRKHAM, M. B. SC AHRING, R. M. (1978). Leaf temperature and internal water status of wheat grown at different root temperatures. Agronomy Journal, 70, 657. KIRKHAM, M. B., GARDNER, W. R. & GERLOFF, G. C. (1969). Leaf water potential of differentially salinized plants. Plant Physiology, 44, 1378. NIELSEN, K. F. (1974). Roots and root temperatures. In: The Plant Root and Its Environment (Ed. by E. W. Carson), pp. 293-333. University Press of Virginia. Charlottesville. PERRIER, A. (1971). Leaf temperature measurement. In: Plant Photosynthetic Production. Manual of Methods (Ed. by gestak, J. Catsky and P. G. Jarvis), pp. 632-67L Dr W. Junk N.V. Publishers. The Hague. RAWSON, H. M., GIFFORD, R. M. & BREMNER, P. M. (1976). Carbon dioxide exchange in relation to sink demand in wheat. Planta, 132, 19. STEEL, R. G. D. & TORRIE, J. H. (1960). Principles and Procedures of Statistics. 481 pp. McGraw-Hill. New York. TODD, G. W. & WEBSTER, D. L. (1965). Effects of repeated drought periods on photosynthesis and survival of cereal seedlings. Agronomy Journal, 57, 399. TORREY, J. G. (1976). Root hormones and plant growth. Annual Review of Plant Physiology, 27, 435. WALKER, J. M. (1970).Effectsof alternating versus constant soil temperatures on maize seedling growth. Soil Science Society of American Proceedings, 34, 889. WILLIS, W. O.. LARSON, W. E. & KIRKHAM, D. (1957). Corn growth as affected by soil temperature and mulch. Agronomy Journal, 49, 323.