Mlchio KANECHI, Naotsugu UCHIDA, Takeshl YASUDA and Tadashi YAMAGUCHI Graduate School of Science and Technology, Kobe University, Rokko, Kobe 657

Transcription

1 Japan. J. Trop. Agr. 32 (1) : 16-21, 1988 Relationships between Leaf Water Potential and Photosynthesis of Coffea arabica L. Grown under Various Environmental Conditions as Affected by Withholding Irrigation and Re-irrigation Mlchio KANECHI, Naotsugu UCHIDA, Takeshl YASUDA and Tadashi YAMAGUCHI Graduate School of Science and Technology, Kobe University, Rokko, Kobe 657 Abstract The effects of a progressive decrease and restoration in leaf water potential (ƒõleaf) following interruption of water supply and re-watering on net photosynthesis (Pn) and transpiration (Tr) of mature coffee leaves were examined under different environmental conditions (25 Ž day/20 Ž night temperature-unshaded, 25/20 Ž-shaded and 33/28 Ž-unshaded). After withholding water supply, ƒõleaf, Pn and Tr decreased most rapidly under a combination of high-light and high-temperature conditions, however, ƒõleaf at the wilting point was the lowest in the shaded condition (about -4.5MPa). Pn decreased linearly in 33/28 Žunshaded, and curvilinearly in 25/20 Ž-shaded conditions with decreasing ƒõleaf. The pattern of Pn in 25/ 20 Ž-unshaded leaves was intermediate between the other two. After re-watering,ƒõleaf recovered completely on the next day and the restoration of Pn required five days in all the experimental plots. Stomatal conductance tended to decrease earlier than mesophyll conductance, suggesting that the initial decrease in Pn under progressive water stress was triggered by stomatal closure. Key words Coffee, Water stress, Photosynthesis, CO2 diffusive conductance, Leaf water potential Introduction Water deficiency or water stress refers to the situations in which plant water potential is reduced enough to interfere with normal functioning of plants. It is obvious that water deficiencies cause a decline or cessation of photosynthesis when leaf desiccation is severe4,9). The effect of leaf desiccation on photosynthesis may vary with plant species, developmental stage and environment etc. Thus, research dealing with photosynthesiswater stress relationships should be comprehensive, taking into account the relevant soil and atmospheric factors such as light and temperature. Little information concerning the physiology of the photosynthesis-water stress relationships in coffee plants is available2, 10, 11) In a previous paper7), a pronounced decline in the rate of light saturated photosynthesis of coffee leaves grown under water stress could be Received 3 April, 1987 mainly attributed to an increase in the stomatal resistance for unshaded leaves and in the mesophyll resistance for shaded leaves. These were obtained from the developing leaves which were imposed a water stress with constant soil water potential of about -1.6 MPa over a relatively long term. It was assumed that the effects of water stress on leaves vary significantly by the degree of maturity of leaves and the severity and the duration of water stress. This study was aimed at examining the effects of short-term water stress on the photosynthesis of mature leaves. It was designed to clarify the differences in water stress response among coffee leaves grown under different light and temperature conditions, and to analyze the factors which cause the reduction in photosynthesis on the basis of leaf diffusive conductances. 1 Plant materials Materials and Methods Coffee seeds (Coffea arabica L. var. Typica) were sown in nursery beds containing loam on 18 November After the emergence of

2 KANECHI et al.: Relationships between Leaf Water Potential and Photosynthesis 17 primary leaves, seedlings were transplanted to polyvinyl pots (12 cm diam. ~ 10 cm height) filled with loam, with a single plant per pot. After reaching the fifth or sixth-leaf pair stage, they were replanted to 1/5000 are Wagner pots containing soil and leaf mold (3 : 1, v/v). The plants were grown in phytotrons under three different temperature and light regimes: 1) at a day/night temperature of 25/20 Ž under natural sunlight, 2) 25/20 Ž at 20% of full sunlight, which was prepared with a black saran screen, and 3) 33/28 Ž under natural sunlight. These are referred to 25 Ž-unshaded, 25 Ž-shaded and 33 Ž-unshaded conditions, respectively. The light intensity inside the phytotrons was about 70% of full sunlight outside the phytotrons. Photoperiod was 12 to 13h and the relative humidity was maintained at 60% during the day and 80% at night. Plants were adequately watered until the onset of the water stress treatment. 2 Water stress treatment and water potential measurements of soils and leaves It has been reported that coffee leaves were completely expanded at about 30 days after the leaf emergence and that the rate of net photosynthsis reached a maximum between 60 to 90 days after the leaf emergence14). In this study, water stress treatments were imposed by withholding watering on plants with 60 day-old leaves, and restorations were made by re-watering at the wilting point. Following the last watering, water potential measurements of soils (ƒõsoil) and leaves (ƒõleaf) were made at 9:00 a.m. every two days using a thermocouple psychrometer, as previously described7). 3 Measurements of the CO2 exchange rate and estimation of leaf diffusive conductances Following the last irrigation, net photosynthesis rate (Pn) and transpiration rate (Tr) were determined every two days with an acrylic assimilation chamber (150 mm ~ 140 mm ~ 5 mm) at a constant air temperature of 25 Ž, using an infrared gas analyzer (model LIA-2, Horiba Ltd.) in an open air stream system. Pn and Tr measurements were made on the other leaf in the pair used for ƒõƒõleaf measurements under light saturated conditions above 500 ƒê mol m-2s-1ppfd. The CO2 concentrations and vapor pressure deficits of the air flow into the assimilation chamber were regulated in the same manner as the previous report7). Leaf temperature was monitored by chromel-alumel thermocouples (0.3 mm diam.) attached to the abaxial leaf surface. Leaf diffusive conductances (gs=stomatal conductance, gm=mesophyll conductance), the reciprocals of leaf diffusive resistances, were calculated according to the method of Burrows and Milthorpe5). The boundary layer resistane, determined using wet filter paper, was approximately 0.2 s cm-1 in this system. Results Fig. l-a, b and c show changes in ƒõsoil,ƒõleaf+ Pn and Tr following interruption of the water supply in mature leaves developed under 25 Ž -unshaded, 25 Ž-shaded or 33 Ž-unshaded conditions, respectively. In 25 Ž-unshaded plants (Fig. l-a),so;1 began to decrease eight days after withholding water supply but ƒõleaf remained almost unchanged until the 8 th day following the interruption. The latter decreased thereafter with accompanying declines in both Pn and Tr. Twelve days after the reduction in ƒõleaf, the plants showed severe wilting symptoms, when ƒõ deaf and ƒõsoil were approximately and -2.0 MPa, respectively. At the wilting point, Pn reduced to 3.0 mg CO2 dm-2h-1(23% of the pre-stress value) and Tr to 0.3 g H2O dm-2h-1 (30% of the pre-stress value). In 25 Ž-shaded plants (Fig. 1-b), DSO;, started decreasing two days after the withholding of water, while the obvious decline in leaf occurred six days later than that of ƒõsoil. Ten days after the reduction in ƒõleaf the plants showed wilting symptoms at a leaf of -4.5 MPa. Pn decreased rapidly under relatively mild stress conditions (ƒõleaf > -2.0 MPa) from the 4 th day to the 8 th day following the last irrigation but decreased more slowly under severe stress conditions (ƒõdeaf < -2.2 MPa). Pn at the wilting point was only 1.7 mg CO2 dm-2h-1(about 15% of the pre-stress value). Tr began to decrease immediately after the withholding of water supply and continued to decrease until the plants showed severe wilting symptoms. Tr at the wilting point was 0.1 gh2o dm-2h-1 (lower than 10% of the per-stress value).

3 18 Japan. J. Trop. Agr. 32 (1) 1988 Fig. 1 Changes in the soil (dashed line) and leaf (solid line) water potentials and the rates of net photosynthesis ( ) and transpiration ( œ) after withholding water supply and re-watering in different environmental conditions (a: 25/20 Ž-unshaded, b: 25/20 Ž-shaded, c: 33/28 Ž-unshaded). Data are shown as mean of five measurements. FC=field capacity, =wilting point, rewatering. In the 33 Ž-unshaded treatment (Fig. 1-c), ƒõsoil, started decreasing four days after the last irrigation, while leaf began to decrease two days earlier. Eight days after the reduction in leaf, the plants showed severe wilting symptoms at a leaf of -2.8 MPa. Plants stressed under the high temperature condition wilt faster than those under the other two conditions. Both Pn and Tr began to decrease immediately after the interruption of water supply and decreased linearly until the wilting point at ƒõsoil of -2.0 MPa. Pn and Tr at the wilting point were 2.0 mg CO2 dm-2h-1(about 13% of pre-stress value) and 0.2g H2O dm-2h-1 (about 27% of pre-stress value), respectively. On the day following restoring soil moisture to field capacity, leaf recovered rapidly in all the treatments and capacity, "leaf recovered rapidly in all the treatments and reached their original levels two days later. Although each restoration of Pn and Tr on the 2nd day after the re-watering was about 80% or about 70% of pre-stress values for both 25 Ž-unshaded and 33 Ž-unshaded, each of them was only 40% or 35% for 25 Ž-shaded. However, they recovered almost complete five days after the rewatering. Fig. 2 shows the relationship between deaf and Pn under different environmental conditions. With decreasing "ƒõleaf, Pn decreased progressively but in different patterns. Pn in both 25 Ž-unshaded and -shaded leaves decreased curvilinearly against the ƒõleaf while that in 33 Ž-unshaded leaves decreased linearly. If examined in more detail, Pn in 25 Ž-shaded leaves decreased rapidly during the initial desiccation (until leaf fell to about -2.0 MPa) and a 50% reduction in Pn occurred at approximately -2.2 MPa. On the other hand, Pn in 25 Ž-unshaded leaves decreased gradually and was reduced by 50% from the pre-stress value at -2.8 MPajeaf Thus, leaf was lower in unshaded leaves than in shaded leaves when Pn was reduced by 50%, however, Pn remained at a slightly higher level in shaded leaves under severe stress conditions (ƒõleaf< -3.5 MPa). In 33 Ž-unshaded leaves, a 50% reduction in Pn occurred at approximately -2.2 MPa and Pn was completely inhibited at about -3.0 MPa ieaf After re-irrigation, Pn recovered in the similar pattern of dehydration phase except on the next day following reirrigation in 25 Ž-unshaded and -shaded conditions. Discussion Water stress depresses the rate of photosynthesis in various plant species. However,

4 KANECHI et al.: Relationships between Leaf Water Potential and Photosynthesis 19 of the other two treatments, with consequently more rapid reduction in the leaf water content during the initial phase of developing water stress. This suggests that the decline in ƒµleaf depended largely on the differences in environmental conditions under which the water-stressed leaves were placed, and that water stress in coffee plants progressed more rapidly at higher temperatures and light intensities. On the other hand, ƒµleaf of 25 Ž-shaded leaves reached the lowest value (-5.3 MPa) among treatments and those leaves had the lowest deaf at the wilting point. This suggests that shaded leaves tend to wilt less even under conditions of severe water stress when compared with unshaded leaves. However, the mechanism of the response is unexplainable at present. The reduction in Pn after withholding water supply was further explained by considering leaf diffusive conductances, gs and gm (Fig. 3). In all the treatments, gs decreased soon after withholding water and decreased earlier than Leaf water potential (MPa) Fig. 2 Relationship between rate of net photosynthesis and the leaf water potential in different environmental conditions (a: 25/20 Ž-unshaded, b: 25/20 Ž-shaded, c: 33/28 Ž-unshaded). Symbols are dehydration phase ( ) and rehydration phase ( œ). Arrows show the values on the next day after re-watering. the sensitivity to water stress and the extent of the decrease in photosynthesis vary with plant species and environments. Since water status in leaves is represented well by ƒµleaf13), the change in ƒµleaf following interruption of water supply under three different environmental conditions were monitored in the present experiment. deaf decreased most rapidly in 33 Ž -unshaded plants, suggesting that the leaves in those plants transpired more actively following the interruption of water supply than those gm. Assuming that gs corresponds to the degree of stomatal aperture, it seems that stomatal closure occurs below approximately -2.0 MPa deaf in all the treatments (Fig. 1 and 3). It has been suggested that the main effect of water stress on plants is stomatal closure and that a reduction in Pn in water stressed plants could be mainly attributed to stomatal closure12). The severity of water stress that induces stomatal closure varies greatly among plant species, ranging from -0.7MPa ƒõleaf for tomato (Lycopersicon esculentum) to -2.5 MPa deaf for grand fir (Abies grandis)8). In coffee plants, the pre-stress ƒõleaf value is lower than those of most other plant species, which usually range from -1.5 to -1.8 MPa. In this study, stomatal closure occurred when deaf fell to about -2.0 MPa in all treatments, suggesting that stomata of coffee leaves tend to close as soon as deaf begins to decrease, and that Pn decreases due to stomatal closure during the initial desiccation. This also indicates that Pn decreased progressively with decreasing gs and gm when further reduction in deaf occurred. Pn showed only slight recovery except high temperature treatment, despite almost complete restoration of deaf 24 h after re-watering. On the 2nd day after re-irrigation,

5 20 Japan. J. Trop. Agr. 32 (1) 1988 Fig. 3 Changes in the leaf diffusive conductances ( : stomatal conductance and œ: mesophyll conductance after withholding water supply and re-watering in different environmental conditions (a: 25/ 20 Ž-unshaded, b: 25/20 Ž-shaded, c: 33/28 Ž-unshaded). Data are shown as relative values to the measurement before withholding water supply (mean of five measurements). Then 100% values of gs and gm are 0.16 and 0.13 s cm-i in 25/20 Ž-unshaded, 0.21 and 0.13 s cm-1 in 25/20 Ž-shaded and 0.29 and 0.15 s cm-1 in 33/28 Ž-unshaded. FC=field capacity, =wilting point, = re-watering: gm recovered to 100% and 75% of the pre-stress level for 25 Ž-unshaded and for 33 Ž-unshaded, respectively, but it was only 45% for 25 Žshaded. This suggests that extreme decline in ƒõl eaf under 25 Ž-shaded condition delays the restoration of gm after re-watering. Complete restoration of gm to the pre-stress level was observed within five days following reirrigation. However, gs did not recover completely. Several researchers noted that stomatal opening did not occur as rapidly as expected when leaves recovered from severe wilting1, 3, 6) Our results also suggest that the after-effects of severe water stress prevent stomata from fully opening and extend over the non-stomatal function explained by gm in the case of recovery from extremely low ƒõleaf. Literature Cited 1. ALLAWAY, W. G. and T. A. MANSFIELD 1970 Experiments and observations on the after of wilting on wilting on stomata of Rumex sanguineus. Can. J. Bot. 48: BIERHUIZEN, J. F., M. A. NUNES and C. PLOEGMAN 1969 studies on productivity of coffee. II Effect of soil moisture on photosynthesis and transpiration of Coffea arabica. Acta. Bot. Neerl. 18: BOUSSIBA, S. and A. E. RICHMOND 1976 Abscisic acid and the after-effect of stress in tobacco plants. Planta 129: BOYER, J. S Chapter 4 Water deficits and photosynthesis. In Water Deficits and Plant Growth, Vol. 4 (Ed.) Kozlowski, T. T., Academic Press, Mew York, BURROWS, F. J. and F. L. MILTHORPE 1976 Chapter 3 Stomatal conductance in the control of gas exchange. In Water Deficits and Plant Growth, Vol. 4 (Ed.) KozLowsxl, T. T., Academic Press, New York, FISCHER, R. A., T. C. HSIAO and R. M. HAGAN 1970 After-effect of water stress on stomatal opening potential. J. Exp. Bot. 21 : KANECHI, M., N. UCHIDA, T. YASUDA and T. YAMAGUCHI 1987 Effects of water stress during leaf development on photosynthesis of Coffea arabica L. Japan. J. Trop. Agr. 31: (in press). 8. KRAMER, P. J Chapter 11 Transpiration. In Water Relations of Plants (Ed.) KRAMER, P. J., Academic Press, New York, KRIEDEMANN, P. E. and W. J. S. DOWNTON 1981 Chapter 13 Photosynthesis. In The Physiology and Biochemistry of Drought Resistance in Plants (Ed.) PALEG, L. G. and D. ASPINALL, Academic Press, New York,

6 KANECHI et al.: Relationships between Leaf Water Potential and Photosynthesis KUMAR, D. and L. L. TIESZEN 1980 Photosynthesis in Coffea arabica. I. Effects of water stress. Expl. Agric.16: NIEGURO, N. E. and A. C. MAGALHAES 1983 Water stress affecting nitrate reduction and leaf diffusive resistance in Coffea arabica L. cultivars. J. Hort. Sci. 58: OSMOND, C. B., K. WINTER and S. B. POwLES 1980 Adaptive significance of carbon dioxide cycling during photosynthesis in water-stressed plants. In Adaptation of Plants to Water and High Temperature Stress (Ed.) TURNER, N. C. and P. J. Kramer, Wiley Interscience, New york, SLATYER, R. and S. A. TAYLOR 1960 Terminology in plant-soil-water relations. Nature 187: YAMAGUCHI, T. and D. J. C. FRIEND 1979 Effect of leaf age and irradiance on photosynthesis of Coffea arabica. Photosynthetica 13: