SOME PHYSIOLOGICAL RESPONSES OF THEOBROMA CACAO VAR. CATONGO SEEDLINGS TO AIR HUMIDITY

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1 New Phytol. (\987) 107, 59\~ SOME PHYSIOLOGICAL RESPONSES OF THEOBROMA CACAO VAR. CATONGO SEEDLINGS TO AIR HUMIDITY BY A. R. SENA GOMES/ T. T. KOZLOWSKP AND P. B. REICH^ ^Centro de Pesquisas do Cacau, CEPLAC/CEPEC, Caixa Postal 7, Itabuna, Ba, Brazil and ^ Department of Forestry, University of Wiscoin, Madison, WI 53706, USA {Accepted 6 July 1987) SUMMARY The stomata of three-month-old seedlings of Theobroma cacao L. var. Catongo were more open in high relative humidity (RH) (76 to 89 %) than in low relative humidity (39 to 62 %). In both regimes, stomata closed gradually during the day, with the rate of closure accelerating in the late afternoon. Trapiration rate (TR) was correspondingly high early in the day and low late in the day. Average leaf diffusive resistance (r,) was 26% lower at the high RH. Nonetheless, TR was generally higher for plants in the low RH, because of the much greater vapour pressure gradient between the leaf and air. Abruptly lowering the RH at noon rapidly increased r,, and increasing RH decreased r^. In another experiment conducted in cotant high or low RH regimes, r, was lower, the rate of net photosynthesis (P^) was higher, leaf water potential (^) was lower (more negative), and TR was lower in the high RH regime. Water-use efficiency (WUE) was higher in high than in low RH. The relatiohips between P^ and r^ were identical at high and low RH. Thus, differences in TR and WUE at high vs low RH were a direct result of variatio in vapour pressure deficit (VPD) between the two humidity regimes. Stomatal opening and closing reflected direct effects of humidity on guard cells rather than respoes to changes in bulk leaf x/r. In addition, root to leaf hydraulic conductivity apparently was greater at low than at high RH. Key words: Cacao (Theobroma cacao), humidity, photosynthesis, water relatio. INTRODUCTION Stomatal aperture influences the rate of trapiration and the uptake of COg by plants. In turn, internal water balance and many environmental factors influence stomatal aperture (Meidner & Mafleld, 1968; Raschke, 1975; Kramer & Kozlowski, 1979; Sheriff, 1979; Kramer, 1983). For example, photon flux deity (Kanemasu & Tanner, 1969; Davies & Kozlowski, 1974), COg concentration (Fischer, 1968; Raschke, 1979), vapour pressure deflcit (VPD) (Lange et al., 1971; Pallardy & Kozlowski, 1979) and temperature (Meidner & Mafield, 1968; Raschke, 1975) independently or interactively affect stomatal opening and closing. Although leaf hydration influences stomatal aperture (Kramer & Kozlowski, 1979; Kramer, 1983), the stomata may also open or close independently of changes in the bulk leaf water potential (ijr), and stomata may function as humidity seors (Sheriff, 1977). Direct stomatal respoes to humidity have been reported for Prunus armeniaca (Schulze et al., 1975), Picea sitcheis (Grace, Malcolm & Bradbury, 1975), Pseudotsuga menziesii (Meinzer, 1982), Acer saccharum (Davies & Kozlowski, 1974), Populus spp. (Pallardy & Kozlowski, 1981) and Picea engelmannii (Johon & Ferrell, 1983), among other species (Kozlowski & Pallardy, 1984). By comparison, stomata of some species of woody plants did not X/87/ $03.00/ The New Phytologist

2 A. R. S E N A GOMES et al. respond directly to changes in air humidity (Meidner & Mafield, 1968; Osonubi & Davies, 1980). Some field observatio indicate that growth and pod yield of Theobroma cacao are respoive to seasonal fiuctuatio in soil and atmospheric moisture (McDonald, 1933). Windy conditio during the dry season markedly reduce yield of T. cacao by causing excessive water loss or by reducing leaf area (Alvim, 1977). With these coideratio in mind, experiments were conducted in controlled-environment chambers to evaluate the effects of air humidity on water and CO2 relatio of seedlings of T. cacao. M A T E R I A L S AND M E T H O D S Experiment A: effects of relative humidity on stomatal diffusive resistance and trapiration rate T. cacao var. Catongo seeds, obtained from the Centro de Pesquisas do Cacau (CEPLAC/CEPEC), Bahia, Brazil, were germinated in vermiculite and used in this and subsequent experiments. Seedlings were traplanted to 9*5 x 9-5 x 25-5 cm pots containing a 2:1:1 mix (v/v) of loam, sand and perlite, respectively, and grown in a growth chamber under the following conditio: daylength 12 h beginning at h; photosynthetic photon flux deity (PPFD) 350/zmols~^ m"^ measured at plant height; day and night temperatures 25 and 22 C, respectively, and relative humidity approximately 80%. Light was supplied by a mixture of 28 fiuorescent (Sylvania F96T12/CW/VHO, 215 W) and 12 incandescent (GE, 60 W) bulbs. The seedlings were regularly irrigated with tap water to excess and once a week with 200 ml of half-strength Hoagland's solution. Thirty-two, three-month-old seedlings were selected for uniformity of growth and development and randomly divided into four groups of eight plants each. Plants of groups 1 and 2 were placed in a high humidity walk-in growth chamber of the University of Wiscoin Biotron; those of groups 3 and 4 in a low humidity room. Plants of group 1 were maintained in the high humidity room for 5 d; those of group 3 in the low humidity room for 5 d. Plants of groups 2 and 4 were alternated from high to low relative humidity (RH) or the reverse, according to the schedule shown in Table 1. Environmental conditio in the high and low humidity rooms were cotant throughout the 8 d experimental period (Table 2). The photoperiod was 12 h beginning at h in both rooms. Light was supplied by a mixture of 18 fiuorescent (Sylvania F96T12/CW/VHO, 215 W) and three incandescent (GE, 500 W) bulbs. Air entering each room (at a rate of 56-6 m^/min) contained 10% fresh air (5*66 m^/min). Air temperature and RH were monitored continuously with thermistors and lithium chloride seors, respectively, and values were recorded at 30 min intervals. A computer-controlled feedback system maintained stable conditio in the rooms. PPFD at plant height was measured with a Lambda, Li-185 quantum meter. Plants in both rooms were automatically irrigated daily at h with deionized water (250 ml/pot). Plants were acclimated to room conditio for 3 d before the measurements began. Stomatal diffusive resistance (r^) and trapiration rate (TR) were determined with a steady state porometer (Li-Cor, Model Li 1600) for the abaxial surface of two fully expanded leaves of each of eight plants of each group (1 to 4). On the first measurement day, r^ and T R were determined six times at 2 h intervals beginning at h. On days 2 to 5, measurements were made at 08.00, and h. Movement of plants of groups 2 and 4 from one room to another required 2 to 3 min.

3 Respoes of Theobroma cacao to humidity 593 Table 1. Schedule of exposure of four groups of seedlings to high or low relative humidity {RH) over a 5 d period Groups Day High RH High RH High RH High RH High RH High RH until h Low RH beginning at h of day 1 High RH beginning at h of day 2 High RH until h. then low RH Low RH until h, then high RH LowRH Low RH LowRH Low RH Low RH Low RH until h High RH beginning at h of day 1 Low RH beginning at h of day 2 Low RH until h, then high RH High RH until h, then low RH Table 2. Environmental conditio in the high and low relative humidity rooms {mea and absolute variatio; Expt A) {RH) High RH room Low p.h room Environmental factor Day Night Day Night RH (%) Temperature ( C) PPFD* ifimol E VPDf (kpa) 82-6 ± ±2-l ± ± ± ±ll ± ± ± ±1-5 * Photosynthetic photon flux deity. t Vapour pressure deficit. The effects of RH, time of day, day and interactio among these on rj and TR were tested by analysis of variance. The least significant difference (LSD) at 0-05 was used to test the mea (Snedecor & Cochran, 1980). Experiment B: effects of relative humidity on rates of photosynthesis, trapiration, stomatal diffusive resistance, water-use efficiency and leaf water potential Plants and pre-treatment conditio were as in Expt A. When the seedlings were 98 d old, 150 plants were randomly divided into two groups of 75 plants each. One group was assigned to the high RH room and the other to the low RH room. Environmental conditio in the high and low humidity rooms were cotant during the 12 d experimental period (Table 3). The photoperiod was 12 h beginning at h. Light was supplied by a mixture of 18 fluorescent (Sylvania F96T12/CW/VHO, 215 W) and three incandescent (GE, 500 W) bulbs. Room air temperature and RH were monitored continuously and recorded at 30 min intervals as described in Expt A. PPED at plant height was measured with a Lambda, Li-185 quantum meter. Plants in both rooms were irrigated daily at h with deionized water (250 ml/pot). Plants were acclimated to growth room conditio for 3 d before measurements were started. The rate of net photosynthesis {PJ, r^, TR and leaf water potential (^) were determined several times during days 1, 5 and 9. On all 3 days, pre-dawn f was determined at at h. On day 1, ^, P.^, TR and r^ were determined at 08.00, 11.00, and h. On days 5 and 9, in addition to pre-dawn f, all

4 594 A. R. SENA GOMES et al. Table 3. Environmental conditio in the high and low relative humidity rooms {mea and absolute variatio ; Expt B) (RH) High RH room Low RH room Environmental factor Day Night Day Night RH (%) Temperature ( C) PPFD* (jimol s-^ m-^) VPDt (kpa) 81-4 ± ± ± ± ± ±M ± ± ±2-4 * Photosynthetic photon flux f Vapour pressure deficit. deity. the above parameters were measured at 08.00, and h. At each time Pjj, rj and TR were determined for one randomly selected fully expanded leaf from each of eight plants in both high and low RH regimes. Pjj, rj and TR were measured with a portable gas exchange system (Analytical Development Corporation) using the mass balance technique (Reich, 1983). Gas exchange was calculated for the abaxial leaf surface only because T. cacao leaves lack stomata on the adaxial surface. The P^ and TR data for each leaf were used to derive water-use efficiency (WUE) values, defined as net COg uptake per unit of trapirational water loss. The i/f of excised, fully expanded leaves was determined with a pressure chamber (Scholander et al, 1965). At each measurement time, the number of leaves harvested from seedlings in each humidity regime for i/r determinatio was 9 (days 1 and 5) or 12 (day 9). To avoid possible effects of altered root shoot ratios on i/r, the plants from which two leaves were excised were removed from the experiment. All data were subjected to analysis of variance to test for significant effects of RH, time of day and interactio among these. Mea among treatments were compared using the least significant difference (LSD) at RESULTS Experiment A Stomatal aperture was correlated with the humidity of the air, the stomata being more open (lower r^) at high (group 1 plants) than low humidity (group 3 plants) [Table 4; Figs 1 (a) and 2(a)]. Average r, values were 26 % lower at high RH. During the first day, r^ of plants in both humidity regimes was lowest in early to mid-morning, increased gradually until h, and then much faster between h and h [Fig. l(a)]. On days 2 to 5, at both high and low RH the stomata were more open at h than at or h. When the plants were moved from a low to a high humidity regime the stomata opened, and they closed when there was a change from high to low relative humidity [Fig. 2(b), days 2 to 5]. A change from high to low RH at h on days 4 and 5 induced rapid stomatal closure. When the RH was abruptly increased at the end of day 1 and day 2 (allowing for overnight acclimation), the stomata closed

5 Respoes of Theobroma cacao to humidity 595 Table 4. Results of analysis of variance to test effects of relative humidity {RH) on stomatal diffusive resistance (r^) and rate of trapiration {TR). {Expt A) Source of variation Day 1 r, TR Days 1 through 5 TR RH Time of day Day F significant at P < 0-01;, not significant Time (h) Fig. 1. Effects of high (H) and low (L) relative humidity on stomatal diffusive resistance (a) and trapiration rate (b) of Theobroma cacao var. Catongo seedlings. Bars indicate LSD at P = 0*05. Day 1 of Expt A. on subsequent days (days 2 and 3). Hence, the stomata responded rapidly to humidity change, and at 1 or 24 h after the humidity change plant respoes were almost identical. Although the TR was somewhat higher in the high than in the low humidity regime during most of day 1 [Fig. l(b)], the differences were not statistically significant. However, on days 2 to 5, TR was generally higher at cotant low RH (group 1 plants) than at cotant high RH (group 3 plants) [Fig. 3(a)]. On the other hand, plants in variable RH treatments did not show a coistent TR respoe [Fig. 3(b)]. There were no significant interactio among the main factors for both r^ and TR.

6 596 A. R. SENA GOMES et al. o tn C Time (h) Fig. 2. Effects of cotant (a) and varying (b) relative humidity (RH) on leaf diffusive resistance (r,) of Theobroma cacao var. Catongo seedlings. High RH (H); low RH (L). Bars indicate LSD at P = 0-05 and arrows time of change of RH from high to low or the reverse. Change of RH on day 1 at h. Expt A., Group 1; D, group 3; A. group 2; A, group 4. Experiment B Plants experiencing different RH regimes had significantly different rates of P^, TR, Tj, iff and WUE. However, the effects of RH varied with parameters measured and time of day (Table 5). No significant interactio were shown between the main factors for all respoes. Leaf xjr was coistently lower (more negative) at high than at low RH during the day, except before dawn (Fig. 4). Daily trends in TR of plants in both high and low RH varied somewhat from one day to another, and TR was generally higher at a low than a high RH (Fig. 4). The effects of RH on TR were significant on days 5 and 9, but not on day 1 (Table 5). Daily changes in rj were very coistent, and r^ was significantly higher at low than at high RH on all three sampling days (Table 5; Fig. 4). The rate of CO2 uptake was coistently higher at high than at low RH on each day of measurement (Table 5; Fig. 4). The maximum rate of P^ varied with humidity regimes and days, but generally the highest values were recorded in the

7 Respoes of Theobroma cacao to humidity (a) 5.0 L 3-5 CM I 2-0 CC Tinne (h) Fig. 3. Effects of con.stant (a) and varying (b) relative humidity (RH) on rate of trapiration of Theobroma cacao var. Catongo seedlings. High RH (H); low RH (L). Bars indicate LSD at P = 0"05 and arrows time of change of RH from high to low or the reverse. Change of RH on day 1 at h. Expt A., Group 1; D, group 3; A, group 2; A, group 4. Table 5. Results of analysis of variance to test effects of relative humidity {RH) on rates of photosynthesis (Pjj) and trapiration {TR), leaf water potential j stomatal diffusive resistance {r^ and water-use efficiency {WUE). {Expt B) Day 1 Day 5 Day 9 RH Time of day RH Time of day RH Time of day TR P^ WUE *«#* * *# *# * P significant at P < 0-05; F < 0-01;, not significant.

8 598 A. R. SENA GOMES et al o QL CJ 'e o o> CC u UJ O (M X CJ> O o E B I Day I Day 5 I I I _L I I I Time ( h ) Day 9 I I I Fig. 4. Effects of high ( ) and low (D) relative humidity on leaf water potential (r{r), rate of trapiration (TR), stomatal diffusive resistance (rj), water-use efficiency (WUE) and rate of photosynthesis (P^) of Theobroma cacao var. Catongo seedlings. Bars indicate LSD at P = Expt B. morning or near midday when the stomata were most open in both humidity regimes. Plants used water more efficiently when the RH was high than when it was low, reflecting both lower TR and higher P^ at high RH regimes (Table 5; Fig. 4). WUE (mg CO2 absorbed/g lost) of plants was approximately 60 % greater 2 in high than in low RH. The relatiohips between P^ and r^ were very similar at high and low humidity regimes (Fig. 5), indicating that greater WUE at high RH was a direct result of differences in vapour pressure deficit (VPD) and not due to any shift in the photosynthesis-leaf diffusive relatiohips. By comparison, the relatiohips between i/r and TR indicate that root-to-leaf hydraulic conductivity was greater at low RH than at high RH, with differences becoming greater over time (Fig. 6).

9 Respoes of Theobroma cacao to humidity 599 _ 6 - x: CM T3 15 Fig. 5. EflFects of humidity regimes on the relatiohips between net photosynthesis (P^) and stomatal diffusive resistance (r,) of Theobroma cacao var. Catongo seedlings. Data are observatio for day 1. Expt B. 0-2 o I ^ ^ ' ^ h\ VV Day 1 \ A "" "~-Day 9 Day 5 V, Day 9 Day Day TR ~^ s ' Fig. 6. Relation on days 1, 5 and 9 between leaf water potential (^) and trapiration rate (TR) for plants exposed to low or high relative humidity (RH). Linear regression relatiohips between i/f and TR were significant for all days and treatments, with r^ ranging from 051 to Expt B., High RH;, low RH.

10 6oo A. R. SENA G O M E S et al. DISCUSSION This study emphasized two major points: (1) stomata of T. cacao var. Catongo seedlings responded directly to air humidity by greater opening (lower r^) at high RH than at low RH, and (2) the greater stomatal conductance in high RH (low VPD) was associated with a high rate of photosynthesis, lower (more negative) leaf i/f, high WUE and lower TR. The greater stomatal opening at high than at low RH shown here contrasted with data of Wilson (1948) and Meidner & Mafield (1968), who reported that stomatal aperture of Ligustrum japonicum and Camellia japonica was relatively unaffected by changes in ambient RH. However, several other investigators showed that stomata opened as RH increased and closed as it decreased (Drake, Raschke & Salisbury, 1970; Lange et al, 1971; Schulze et al., 1972, 1974; Davies & Kozlowski, 1974; Sheriff, 1977). In the present study, the stomata were coistently more closed at low than at high RH in both experiments. In Expt A, changes in RH resulted in very coistent adjustments in conductance, regardless of whether RH was altered at the end of the preceding day or near the middle of the same day. In the present study, the importance of stomatal aperture in regulating gas exchange in two humidity regimes was shown by the strong relatiohip between photosynthesis and r^. Lower P^ at low RH was a direct result of partial stomatal closure. Moreover, WUE was lower at low than at high RH, because of a much greater evaporative demand. The guard cells of T. cacao responded to air humidity without appreciable change in the bulk leaf i/f, as shown for other species by Davies & Kozlowski (1974), Schulze et al. (1975), Grace et al. (1975), Sheriff (1977), Losch & Tenhunen (1981), Pallardy & Kozlowski (1981), Meinzer (1982), Johon & Ferrell (1983) and Appleby & Davies (1983a, b). Exposing isolated epidermal strips of leaves of Polypodium vulgare to dry air was followed by rapid stomatal closure, whereas exposure to moist air induced stomatal opening (Lange et al., 1971). The mechanism of such direct stomatal respoes to humidity is not clear, but some studies show that cuticle-free areas on the inner walls of guard cells are evaporation sites through which water may be lost rapidly enough to create localized water deficits (Mafield & Davies, 1985). Normally, plants with high T R have low ^ because the absorption of water by roots is not fast enough to keep up with trapirational losses (Kramer & Kozlowski, 1979; Kramer, 1983). However, in the present study plants in a low humidity regime with high TR rates had slightly smaller water deficits during the light period (less negative ijr). It is possible that shoot-mediated adjustments in root hydraulic conductivity (Parso & Kramer, 1974) occurred during the 2 weeks of RH treatment, which may have minimized water deficits in plants in the low RH regime. The relatiohip between i/r and T R suggests that this was true (Fig. 6). Nevertheless, maximum plant water deficits (midday i/r) in both RH treatments were always very small. The extreme seitivity of T. cacao seedlings to low RH may be a limiting factor for growth in areas where air humidity is low. In such areas, growth would be adversely affected as a result of stomatal closure and lowered P^. The inefficient use of water (low WUE) at low RH would probably lead to greater shoot water deficits under conditio of limited soil water supply. The higher photosynthetic rates and greater WUE of T. cacao in a high humidity regime are coistent with the range of the species in the wet tropics (Alvim, 1977; Wood, 1985).

11 Respoes o/theobroma cacao to humidity 601 ACKNOWLEDGEMENTS This research was supported by the College of Agricultural and Life Sciences, University of Wiscoin, Madison, Wiscoin, USA and the Centro De Pesquisas do Cacau (CEPLAC/CEPEC), Itabuna, Bahia, Brazil. REFERENCES (1977). Cacao. In: Ecophysiology of Tropical Crops (Ed. by P. de T. Alvim & T. T. Kozlowski), pp Academic Press, New York. APPLEBY, R. F. & DAVIES, W. J. (1983a). A possible evaporation site in the guard cell wall and the influence of leaf structure on the humidity respoe by stomata of woody plants. Oecologia, 56, APPLEBY, R. F. & DAVIES, W. J. (1983b). The structure and orientation of guard cells in plants showing stomatal respoes to changing vapour pressure differences. Annals of Botany, 52, DAVIES, W. J. & KOZLOWSKI, T. T. (1974). Stomatal respoes of five woody angiosperms to light inteity and humidity. Canadian Journal of Botany, 52, DRAKE, B. G., RASCHKE, R. & SALISBURY, F. B. (1970). Temperature and trapiration resistances of Xanthium leaves as affected by air temperatures, humidity, and windspeed. Plant Physiology, 46, FISCHER, R. A. (1968). Stomatal opening in isolated epidermal strips of Vicia faba. I. Respoe to light and COg-free air. Plant Physiology, 43, GRACE, J. D., MALCOLM, C. & BRADBURY, I. K. (1975). The effect of wind and humidity on leaf diffusive resistance in Sitka spruce seedlings. Journal of Applied Ecology, 12, JOHNSON, J. D. & FERRELL, W. K. (1983). Stomatal respoe to vapour pressure deficit and the effect of plant water stress. Plant, Cell and Environment, 6, KANEMASU, E. T. & TANNER, C. B. (1969). Stomatal diffusion resistance of snap bea. II. Effects of light. Plant Physiology, 44, KOZLOWSKI, T. T. (1982). Water supply and tree growth. Part I. Water deficits. Forestry Abstracts, 43, KOZLOWSKI, T. T. & PALLARDY, S. G. (1984). Effects of flooding on water, carbohydrate, and mineral relatio. In: Flooding and Plant Growth (Ed. by T. T. Kozlowski), pp Academic Press, New York. KRAMER, P. J. (1983). Water Relatio of Plants. Academic Press, New York. KRAMER, P. J. & KOZLOWSKI, T. T. (1979). Physiology of Woody Plants. Academic Press, New York. LANGE, O. L., LOSCH, R., SCHULZE, E. - D. & KAPPEN, L. (1971). Respoes of stomata to change in humidity. Planta, 100, LOSCH, R. & TENHUNEN, J. D. (1981). Stomatal respoes to humidity - phenomenon and mechanism. In: Stomatal Physiology (Ed. by P. G. Jarvis & T. A. Mafield), pp Cambridge University Press, Cambridge. MANSFIELD, T. A. & DAVIES, W. J. (1985). Mechanisms for leaf control of gas exchange. BioScience, 3, MCDONALD, J. A. (1933). An Environmental Study of the Cacao Tree, pp Annual Report of Cacao Research, Imperial College of Tropical Agriculture, St Augustine, Trinidad. MEIDNER, H. & MANSFIELD, T. A. (1968). Physiology of Stomata. McGraw-Hill, London. MEINZER, F. C. (1982). Models of steady-state and dynamic gas exchange respoes to vapour pressure and light in Douglas-fir (Pseudotsuga menziesii) saplings. Oecologia, 55, OsoNUBi, O. & DAVIES, W. J. (1980). The influence of plant water stress on stomatal control of gas exchange at different levels of atmospheric humidity. Oecologia, 46, 1-6. PALLARDY, S. G. & KOZLOWSKI, T. T. (1979). Relatiohips of leaf diffusion resistance oi Populus clones to leaf water potential and environment. Oecologia, 40, PALLARDY, S. G. & KOZLOWSKI, T. T. (1981). Water relatio oi Populus clones. Ecology, 62, PARSONS, L. R. & KRAMER, P. J. (1974). Diurnal cycling in root resistance to water movement. Physiologia Plantarum, 30, RASCHKE, K. (1975). Stomatal action. Annual Review of Plant Physiology, 26, RASCHKE, K. (1979). Movement of stomata. In: Encyclopedia of Plant Physiology, New Series, vol. 7 (Ed. by W. Haupt & M. E. Frinleib), pp Springer-Verlag, New York. REICH, P. B. (1983). Effects of low concentration of O3 on net photosynthesis, dark respiration, and chlorophyll contents of aging hybrid poplar leaves. Plant Physiology, 73, ScHOLANDER, P. F., HAMMEL, H. T., BRADSTREET, E. D. & HEMMINGSEN, E. A. (1965). Sap pressure in vascular plants. Science, 148, SCHULZE, E. - D., LANGE, O. L., BUSCHBOM, U., KAPPEN, L. & EVENARI, M. (1972). Stomatal respoes to changes in humidity in plants growing in the desert. Planta, 108, ALVIM, P. D E T.

12 6o2 A. R. SENA GOMES et al. SCHULZE, E.-D., LANGE, O. L., EVENARI, M., KAPPEN, L. & BUSCHBOM, U. (1974). The role of air humidity and leaf temperature in controlling stomatal resistance of Prunus armeniaca L. under desert conditio. I. A simulation of the daily course of stomatal resistance. Oecologia, 17, SCHULZE, E.-D., LANGE, O. L., KAPPEN, L., EVENARI, M. & BUSCHBOM, U. (1975). The role of air humidity and leaf temperature in controlling stomatal resistance of Prunus armeniaca L. under desert conditio. II. The significance of leaf water status and internal carbon dioxide concentration. Oecologia, 18, SHERIFF, D. W. (1977). The effect of humidity on water uptake by, and viscous flow resistance of, excised leaves of a number of species: physiological and anatomical observatio. Journal of Experimental Botany, 28, SHERIFF, D. W. (1979). Stomatal aperture and the seing of the environment by guard cells. Plant, Cell and Environment, 2, SNEDECOR, G. N. & COCHRAN, W. G. (1980). Statistical Methods. Iowa State University Press, Ames, Iowa. WILSON, C. C. (1948). The effects of some environmental factors on the movements of guard cells. Plant Physiology, 23, WOOD, G. A. R. (1985). Environment. In: Cocoa (Ed. by G. A. R. Wood & R. A. Lass), pp Longman, London.

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