Relationship between leaf and xylem water potentials in rice plants

Size: px

Start display at page:

Download "Relationship between leaf and xylem water potentials in rice plants"

Gabriel Burns
5 years ago
Views:

1 Plant & CcllPhysiol. 19(7): (1978) Short communication Relationship between leaf and xylem water potentials in rice plants Kuni Ishihara and Tadashi Hirasawa Laboratory of Crop Science, Faculty of Agriculture, Tokyo University of Agriculture and Technology, Fuchu, Tokyo 183, Japan (Received March 6, 1978) Leaf and xylem water potentials were measured in rice plants with and without transpiration using a thermocouple psychrometer and a pressure chamber. The leaf water potential practically coincided widi the xylem water potential in leaves without transpiration, while the latter was 3-5 bars lower when intense transpiration was occurring. The pressure chamber should not be used to measure leaf water potential during intense transpiration in the field. The water status in transpiring leaves is discussed. Key words: Pressure chamber Rice plant Thermocouple psychrometer Transpiration Water potential. The plant-water relationship in rice plants has aroused little interest because they are usually grown in submerged paddy fields supplied with enough water. However, we have found that the diurnal course of stomatal aperture, which sensitively responded to water status in leaf blades under enough light intensity, was affected much by the weather condition from day to day (13). On fine days, the aperture increased from early morning to reach maximum at about 8:30-9:30 a.m. then decreased very quickly to half the maximum or less in the afternoon. On cloudy days, the aperture increased slowly in the morning reaching maximum at about noon and remained widely opened for some time in the afternoon. These results clearly showed that the stomata of leaf blades in rice plants closed to a considerable extent in the afternoon on fine days due to an unbalance of water economy when accompanied by intense transpiration, even when the plants were grown in submerged paddy fields with negligible influence of water potential about the roots. Considering this stomatal behavior and the close relation between stomatal aperture and photosynthetic rate (14), the photosynthetic rate in rice plants must be much affected by closure of the stomata, reinforced by water stress in leaf blades under high light intensity. To clarify the diurnal changes of water status of leaf blades in rice plants grown under submerged conditions, the diurnal changes of leaf and xylem water potentials in rice plants (cultivar Manryo) grown in pots were measured using a thermocouple psychrometer (Wescor Inc.) and a pressure chamber (PMS Instr. Co.), respectively. The leaf water potential is determined by measuring the vapor pressure following the 1289

2 1290 K. Ishihara and T. Hirasawa attainment of equilibrium between the gas phase and small leaf disks in a sealed chamber of the psychrometer. The xylem water potential is determined by measuring the pressure around a leaf blade in the pressure chamber when xylem sap appears at the cut end of the petiole extending outside the chamber. This pressure can be considered to be the xylem water potential because the osmotic potential of the xylem sap is practically negligible. As this potential is sufficiendy close to the leaf water potential, the pressure chamber has commonly been used to estimate the leaf water potential not only in field experiments but also in physiological studies (, 8, 19). Fig. 1 shows the diurnal course of leaf water potential, xylem water potential and transpiration rate in rice plants (panicle formation stage) growing submerged in a pot covered with a styrofoam plate to prevent evaporation from the water surface as well as a rise in soil temperature. Transpiration was determined gravimetrically by weighing the loss of water at about 30-min intervals. Leaf and xylem water potentials were measured in the second expanded leaves of plants growing under the same condition as the plant whose transpiration was measured. Leaf disks and a blade were sealed, respectively, in the sample chamber of the psychrometer and the pressure chamber within 20 sec after sampling from intact leaves in the field exposed to the sun. Leaf blades with sheaths inserted between two halves of a rubber stopper and extending outside, were placed in a pressure chamber whose inside wall was "S 1.0 l 3.0 zo S- 2.0 I 1.0 = 1.0 \ LEAF WATER POTENTIAL "*~ ^ ^ d ^ XYLEM WATER POTENTIAL \ A TRANSPIRATION RATE x' X x ATMOSPHERIC VAPOR X ~x-x' x v PRESSURE DEFIC1TX- > V ^- * \ 0-1 u -2S -3 S -1 ' g -5 UJ i 10 Fig. 1. llo.s «5 - SOLAR RADIATION M HOUR OF DAY (hr) 5 S2! :c i Q_ CO t_> o sa Diurnal course of leaf water potential, xylem water potential and transpiration rate in rice plants as well as atmospheric vapor pressure deficit and solar radiation.

3 Leaf and xylem water potential in rice plants 1291 covered with wet infiltration paper to prevent water loss from the leaf blades during measurement. To attain equilibrium, 3 hr in the psychrometer was necessary. Both leaf and xylem water potentials decreased with an increase of the transpiration rate due to an increase of solar radiation and atmospheric vapor pressure deficit, and increased with a decrease in the transpiration rate toward evening. A large difference was found between leaf and xylem water potentials in the daytime accompanied by intense transpiration, while both potentials were nearly the same in the early morning and evening. The maximum difference between the two potentials was about 4.5 bars at 14:30 when the transpiration rate showed the maximum value of 3.8 g H2Odm 2 hr. This disagrees with many previous reports (2-5, 9-11, 15-18, 21, 24) which state that the values of the xylem water potential obtained using a pressure chamber are close to those of the leaf water potential measured with a thermocouple psychrometer and assert the usefulness of the pressure chamber as the means of conveniently estimating leaf water potential. The relation between leaf and xylem water potentials also was examined in the flag leaf blade of rice plants growing under submerged conditions or various degrees of soil water content in pots in the sunshine or a dark room (Fig. 2). The measurement using the pressure chamber was tested on the blade after removing sample disks for psychrometer measurement from the same leaf. Leaf water potentials from the psychrometer measurements were practically the same as the xylem water po LEAF WATER POTENTIAL (bar) Fig LEAF WATER POTENTIAL v V XYLEM HATER POTENTIAL TIME (hr) Fig Fig. 2. Relationship between leaf and xylem water potentials in rice plants. Closed circles represent water potentials in the dark room plants grown under various degrees of soil water content; open ones, water potentials in the sun measured in plants grown under submerged condition; crosses, water potentials in die sun measured in plants grown under various degrees of soil water content. The line represents equipotential values. Fig. 3. Comparison between changes in leaf and xylem water potentials in rice plants exposed alternatively to light (open circles) and dark (closed ones). The light-exposure conditions were 40 klux light intensity, 12.4±0.5 mmhg atmospheric vapor pressure deficit and practical zero wind speed, and the dark conditions were 22.4±2.2 mmhg and about 3 msec, except those of the first measurement which were the same as die light-exposure ones.

4 1292 K. Ishihara and T. Hirasawa tentials for the dark room samples in which water loss did not occur, for leaves with various degrees of water deficit in their blades depending on the soil water content. On the contrary, leaf and xylem water potentials in plants supplied with enough water were very different from each other when measurements were made with leaf blades exposed to the sun on a fine day accompanied by intense transpiration, i.e., the pressure chamber values were about 3-5 bars lower than the psychrometer ones. Interestingly, when the soil water content decreased to the degree in which the leaves wilted due to intense transpiration, the pressure chamber determinations approached those of the psychrometer even under sunshine and when the leaf blades wilted severely into needles due to soil water deficit, both potentials were nearly the same value of 19 bars even when measured under sun exposure. Leaf and xylem water potentials were measured in flag leaf blades of rice plants growing submerged in pots exposed alternatively for 3-hr intervals to light and dark in a controlled-growth chamber (Fig. 3). The two potentials were much different in plants which had been exposed to light for 3 hr but the difference between them was about 2 bars or less in the dark. Leaf water potential in the dark may not have been equal to xylem water potential in the later three measurements because transpiration probably had occurred to some extent in the dark under stronger wind and higher atmospheric vapor pressure deficit conditions compared with those of the first measurement. From these results, we conclude that psychrometer determinations practically coincided with those using the pressure chamber in leaves with no transpiration, but the former were higher when transpiration was occurring intensively. We had obtained the same results with corn leaves {12). Therefore, we surveyed literature (2-5, 9-11, 15-18, 21, 24) on the experimental methods used in reports which stated that pressure chamber determinations were sufficiently close to those using the psychrometer and thus the pressure chamber could be used to measure leaf water potential. We found that these results had mostly been obtained by measuring leaf and xylem water potentials in leaf blades without transpiration; some reports did not specify the experimental conditions (3,9,10, 21,24). For example Boyer (4,5) measured both water potentials with excised shoots whose water loss was zero and the potentials in the xylem and leaf were at equilibrium, like other researchers (2, 11, 15-18). Some workers (2, 4, 5, 10, 18) measured leaf water potential with a psychrometer in leaf disks sampled from the same leaf after pressure chamber measurements. In these cases, the leaf and xylem water potentials probably reached equilibrium during the pressure chamber measurement. Our results agreed approximately with those of investigators when we measured the leaf and xylem water potentials of leaf blades of rice plants without transpiration due to dark or severe water deficit of leaves exposed to the sun. Recently, Camacho-B et al. (7) reported that pressure chamber values obtained with pear responded more to changes in the transpiration flux than leaf water potential values of the same plants. Thus, using the pressure chamber to measure leaf water potential in the field under conditions of intense transpiration may be a serious mistake, although it may be used to make relative measurements of leaf water potential in plants which do not transpire apparently. Furthermore, it probably showed the absence of an equilibrium between water in xylem and leafy cells that the leaf water potential was not equal to the xylem water

5 Leaf and xylem water potential in rice plants 1293 potential when transpiration was occurring intensively. That is, negative pressure in the vessels probably was not transmitted to living mesophyll cells due to resistance to water flow between these tissues in plants when water was transported quickly in the vessles. The water path in the leaf blades from vessels in the leafy vein to transpiring sites remains unknown. The classical view is that water is supplied to the vein and via the ramifications finds its way to the different tissues (20). A recent proposal which contradicts this classical view states that water moves rapidly from the small veins along the bundle sheath parenchyma to the bundle sheath extension parenchyma and then laterally through the epidermis to the substomatal cavity from which it is transpired through the stomatal pore (6", 22, 25, 26). The sites of evaporation close to the stomatal cavity are inner epidermal walls, especially subsidiary and guard cell walls, because they are closest to air spaces with highest water vapor deficits, and less water than is traditionally supposed evaporates from the mesophyll cell walls (20, 23). From our results and this new point of view for the water path in leaf blades as well as water evaporation sites in leafy tissues, water in the mesophyll cells may be isolated from the transpiring stream from small veins through the bundle sheath extension parenchyma and epidermis to the stomatal cavity. It might be favorable for plants that the water potential of mesophyll cells, which are the physiologically active sites of leaf tissue, may be kept higher than that of water-transporting tissue during intense transpiration under strong sunshine. We wish to express our thanks to Prof. T. Ogura for his valuable suggestions throughout this study. Thanks are also due to Prof. T. Tazaki for reading the manuscript. This investigation was partially supported by a grant from the Ministry of Education, Japan. References ( 1 ) Ackerson, R. C. and D. R. Krieg: Stomatal and nonstomatal regulation of water use in cotton, corn and sorghum. Plant Physiol. 60: (1977). ( 2) Barrs, H.D., B. Freeman, J. Blackwell and R.D. Ceccato: Comparisons of leaf water potential and xylem water potential in tomato plants. Aust. J. Biol. Sci. 23: (1970). ( 3) Blum, A., C. Y. Sullivan andj. D. Eastin: On the pressure chamber technique for estimating leaf water potential in sorghum. Agron. J. 65: (1973). ( 4 ) Boyer, J. S.: Leaf water potentials measured with a pressure chamber. Plant Physiol. 42: (1967). ( 5) Boyer, J. S. and S. R. Ghorashy: Rapid field measurement of leaf water potential in soybean. Agron. 7.63: (1971). (6") Burbano, J. L., T. D. Pizzolato, P. R. Morey and J. D. Berlin: An application of the prussian blue technique to a light microscope study of water movement in transpiring leaves of cotton (Gossypium hirsutum L.). J. Exp. Bot. 27: (1976). (7) Camacho-B, S. E., A. E. Hall and M. R. Kaufmann: Efficiency and regulation of water transport in some woody and herbaceous species. Plant Physiol. 54: (1974). (8) Chu, A. C. P. and H. G. McPherson: Sensitivity to desiccation of leaf extension in prairie grass. Aust. J. Plant Physiol. 4: (1977). ( 9 ) De Roo, H. C.: Leaf water potentials of sorghum and corn estimated with pressure chamber. Agron. J. 61: (1969). ' (10) Duniway, J. M.: Comparison of pressure chamber and thermocouple psychrometer determinations of leaf water status in tomato. Plant Physiol. 48: (1971). () Frank, A. B. and D. G. Harris: Measurement of leaf water potential in wheat with a pressure chamber. Agron. J. 65: (1973).

1294 K. Ishihara and T. Hirasawa (12) Hirasawa, T., K. Ishihara and T. Ogura: Relationship between environmental factors and water potential in leaf blades of corns. Jap. J. Crop Sci.

6 1294 K. Ishihara and T. Hirasawa (12) Hirasawa, T., K. Ishihara and T. Ogura: Relationship between environmental factors and water potential in leaf blades of corns. Jap. J. Crop Sci. 46, Extra issue 1: (1977). (13) Ishihara, K., Y. Ishida and T. Ogura: The relationship between environmental factors and behavior of stomata in the rice plant II. On the diurnal movement of the stomata. Proc. Crop Sci. Soc. Japan 40: (1971). (14) Ishihara, K., R. Sago, T. Ogura, T. Ushijima and T. Tazaki: The relationship between environmental factors and behavior of stomata in the rice plant IV. The relation between stomatal aperture and photosynthetic rate. ibid. 41: (1972). (75) Kaufmann, M. R.: Evaluation of the pressure chamber method for measurement of water stress in Citrus. Amer. Soc. Hort. Sci. 93: (1968). (16) Kaufmann, M. R.: Evaluation of the pressure chamber technique for estimating plant water potential of forest tree species. Forest Sci. 14: (1968). (17) Klepper, B. and H. D. Barrs: Effects of salt secretion on psychrometric determinations of water potential of cotton leaves. Plant Physiol. 43: (1968). (18) Klepper, B. and R. D. Ceccato: Determinations of leaf and fruit water potential with a pressure chamber. Hort. Res. 9: 1-7 (1969). (79) Ludlow, M. M. and T. T. Ng: Effect of water deficit on carbon dioxide exchange and leaf elongation rate of Panicum maximum var. trichoglumc. Aust. J. Plant Physiol. 3: (1976). (20) Meidner, H.: Water supply, evaporation and vapour diffusion in leaves. J. Exp. Bot. 26: (1975). (21) Millar, B. D. and G. K. Hansen: Exclusion errors in pressure chamber estimates of leaf water potential. Am. Bot. 39: (1975). (22) Pizzolato, T. D., J. L. Burbano, J. D. Berlin, P. R. Morey and R. W. Pease: An electron microscope study of the path of water movement in transpiring leaves of cotton (Gossypium hirsulum L.). J. Exp. Bot. 27: (1976). (23) Sheriff, D. W. and H. Meidner: Water movement into and through Tradcscantia virginiana L. leaves, ibid. 26: (1975). (24) Spomer, L. A. and R. W. Langhans: Evaluation of pressure bomb and dye method measurements of tissue water potential in greenhouse Chrysanthemum. Hort. Sci. 7: (1972). (25) Tanton, T. W. and S. H. Crowdy: Water pathways in higher plants III. The transpiration stream within leaves. J. Exp. Bot. 23: (1972). (26) Wylie, R. B.: The role of the epidermis in foliar organization and its relations to the minor venation. Amer. J. Bot. 30: (1943).

Relationship between Leaf Water Potential and Photosynthesis in Rice Plants

Relationship between Leaf Water Potential and Photosynthesis in Rice Plants By KUNI ISHIHARA and HIDEO SAITO Faculty of Agriculture, Tokyo University of Agriculture and Technology (Saiwaicho,Fuchu, Tokyo,