Tillandsia recurvata (L.) L.

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Betancur 2001b: 118* Betancur 2007: 75:37 Bromtravels (Web) Cabrera 1968: 453 Castellanos 1945: 115 ,116 ,129b: 115 ,116 ,129b Die Bromelie: 1992:17 FCBS (Web) Gilmartin 1972: 264 Gomez et al. 2018: 52 Gross 1992: 73 Holst 2020: 166, 170 Howard 1979: 418 Ibisch & Vasquez 2000 Iheringia: 51(1):31 J. Bromeliad Soc.: 16:33b 21:75b 52:185: 60(2):72,75,76 Klein & Klein 2013c: 2013(3): 144, 150 Kockel 1995: 42 Leme & Marigo 1993: 163 Machado & Neto 2010: 117 Mak 1996: 25 Mendoza 2005: 102 Morren Icons: kew 19-101*: kew 12-340*: kew 12-345*: kew 12-357*: kew 12-359*: kew 12-347* NY Bot. Garden Padilla 1973, 1986: 95 Proctor 1984: 211 Rauh 1981, 1990: Abb. l , Fig. 45 Reitz 1983: 74 Roguenant 2001: 608 Rohweder 1956: 14 Shimizu & Takizawa 1998: 25

Shimizu 1992: 2 Siqueira & Leme 2006: 354 Smith & Downs 1977: 888,890: 888*: 888*: 888*: 888*: 890*: 888*: 888*: 888*: 888*: 888*: 890*: 888*: 888*: 888*: 890*: 888*: 888*: 890*: 888*: 888*: 890,997*: 1051* Steyermark & Huber 1978: 69 Tropicos U.S. Nat. Herb. Veliz 2010: 75* Wanderly & Martins 2007: 125 Wendt 2010: 44 Wien Herbarium, Inst Wilson 1977: 61

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• Stomata of the CAM Plant Tillandsia recurvata Respond Directly to Humidity by
  O.L. Lange and E. Medina in Oecologia (Berl.) 40, 357-363 (1979)
  
  Summary. Under controlled conditions, CO 2 exchange of Tillandsia recurvata showed all characteristics of CAM. During the phase of nocturnal CO 2 fixation stomata of the plant responded sensitively to changes in ambient air humidity. Dry air resulted in an increase, moist air in a decrease of diffusion resistance. The evaporative demand of the air affected the level of stomatal resistance during the entire night period. Due to stomatal closure, the total nocturnal water loss of T. recurrata was less at low than at high humidity. It is concluded that stomata respond directly to humidity and not via bulk tissue water conditions of the leaves. Such control of transpiration may optimize water use efficiency for this almost rootless, extreme epiphyte.
  
  Introduction
  It is well known that stomata of many plants respond directly to changes in ambient air humidity (Lange et al., 1971; Hall et al., 1976). Cowan and Farquhar (1977) explain this control of transpiration in terms of optimization of water use efficiency of plants under arid conditions. Most species known so far with humidity-sensitive stomata, are C 3 plants (Sheriff, 1977; Losch, 1979). It was only shown recently that the stomatal resistance of two representatives of the genus Opuntia changes with cladode-air vapor pressure difference (Conde and Kramer, 1975; Osmond et al., 1979). However, this feature may be more widespread among plants with acid metabolism since experiments under controlled conditions show that stomata of the epiphytic CAM plant Tillandsia recurvata also respond to humidity.
  
  Material and Methods
  Fresh material of Tillandsia recurvata L. was collected from its natural habitat in a humid evergreen forest at 1,400 m above sea level in the coastal range of northern Venezuela. The plants were airmailed to Wurzburg and cultivated in a greenhouse for several weeks prior to experimentation. Growth conditions were those under which this species as well as other Tillandsias have been grown successfully for years.
  Transpiration and CO 2 exchange of the plant was measured with an infrared gas analyzer and a humidity and temperature conditioned cuvette (Walz Instruments, Effeltrich), the functioning of which has been described in detail (Lange et al., 1969; Koch et al., 1971). Absolute humidity of ingoing and outgoing air as well as in the bypass of the gas exchange chamber was monitored with dew point mirrors (Walz). The air stream was humidified and subsequently passed through a Peltier cooled condensor at the inlet of the plant chamber. With this system it was possible to keep the water vapor concentration at a constant level inside the cuvette and to alter it stepwise. Illumination was provided from an air cooled xenon arc (Osram, 6,000 W). Flux density of photosynthetically active quanta at plant level was 350 µEinstein m-² s-¹ (Quantum sensor, Lambda Instruments).
  Prior to experimentation whole plants of T. recurvata (consisting of leaf rosettes and runners only) were soaked to the point of saturation in deionized water. Excess moisture was then removed by blotting. Water content of the leaves per gram dry weight was determined by removing representative leaf samples before placing the entire plants into the cuvette. After several hours during which the externally adhering water evaporated, gas exchange measurement was begun. This was continued for at least 24 h before the plants were remoistened.
  Temperature was recorded by small thermocouples inserted into leaves. Humidity conditions in the environment of the plant were defined on the basis of water vapor concentration difference (WD, in mg H 2 0 · l -¹) between leaves and air, assuming 100% relative humidity at leaf temperatures at the mesophyll cell walls where evaporation takes place. Transpiration (TR) and CO 2 exchange (NP) were expressed on a plant dry weight basis, because determination of leaf surface area of T. recurvata is hardly possible. Total diffusion resistance for water vapor (R*) was calculated from the quotient W D - TR-¹. Since TR could not be expressed on a surface area basis the dimension of R* (g - s - dm-³ · 10-¹) is unusual, and it cannot be compared with other data of diffusion resistance. However, the values of R* are a relative measure of the total diffusion resistance of the plant included. At constant wind speed within the chamber, changes of R* must reflect changes in stomatal resistance. Humidity, CO 2 exchange, temperature, and light intensity were recorded every 2 min. Five subsequent readings were averaged to produce the results shown in Figs. 1 to 3.
  Two types of experiments were conducted. With a 12:12 light/dark cycle, diurnal courses of gas exchange were investigated. Temperature and WD were held constant throughout each 12 h period at levels which were different for the night and dark period respectively. In order to study stomatal behavior, WD was changed stepwise during both periods, raising and lowering humidity at constant temperatures.
  
  Results
  In accordance with the findings of Medina et al. (1977) with the same species, T. recurvata exhibited all features of 'full-CAM' behavior (Kluge and Ting, 1978) at night temperatures of 16° C and day temperatures of 22° C (Fig. 1). After a short initial burst of CO 2 release at the beginning of the dark period, CO 2 fixation took place (phase 1; see Osmond, 1978). It reached maximum values within the first half of the night. During this phase of nocturnal dark fixation of CO 2 the stomata of T. recurvata responded sensitively to changes in ambient air humidity. In the experiment depicted (Fig. 2), WD was initially kept at low values of 4.2 and 2.0 mg H 20 ·1-¹ (corresponding to 67% and 84% relative humidity, respectively). At 23:15 h, when CO 2 fixation had normally reached a more or less stable rate, the air was dried (maximum WD7.8 mg H 20 ·1-¹; 39% rel. hum.) and subsequently humidified again. CO 2 uptake exactly reflected the changes in humidity. During the driest conditions as little as half of the initial rate of CO 2 was fixed. At low rates of water loss and with sudden alterations in humidity, accurate transpiration measurements were difficult to accomplish. Therefore, continuous data for the diffusion resistance of the plants could not be obtained during the experiments described. However, the results of representative spot measurements are indicated in Fig. 2. They clearly show that the depression of CO 2 uptake at low air humidity can be explained through stomatal action. The total diffusion resistance increased strongly with decreasing humidity and stomata opened again with increasing humidity at the end of the experiment. The same pattern of responses occurred repeatedly in many experiments with different plant samples. Increasing WD always resulted in an immediate increase of diffusion resistance and lowered rates of CO 2 uptake. Decreasing WD always had the opposite effect. However, it was not possible to establish a reproducible quantitative relationship between WD and R* as has been done with several C3 species (see Losch, 1977; Schulze et al., 1974). The range and the magnitude of response differed considerably between experiments. It depended strongly on the pretreatment of the plant material during the preceding light period and on the particular time within the dark period at which the experiments were conducted. The second effect is clearly seen in Fig. 2, where at the end of the experiment diffusion resistance was higher and CO 2 fixation was lower than initially under the same humidity conditions. Apparently the humidity-determined stomatal response is mediated by other controlling systems, most likely by the internal CO 2 concentration and/or other endogenous properties. Such factors lead to characteristic changes in the nocturnal diffusion resistance of T. recurvata even under conditions of constant humidity and temperature (see Fig. 1).
  Water vapor concentration of the air determines the level of diffusion resistance of T recurvata during the entire night period. This is shown in Fig.. 1 in which two different diurnal courses of CO2 exchange are compared. Both plant samples had similar water content at the beginning of the experiment, and the treatment differed only in the nocturnal value of WD. At high humidity, diffusion resistance increased slowly during the course of the night from about 65 to about 90 units. Dark fixation of CO2 reached the highest values around midnight. Under conditions of dry air, CO2 uptake was much lower at all times and progressively decreased after 22:00 hrs. This was apparently due to stomatal action. In both experiments, R* at the beginning of the dark period was of the same order of magnitude. However, under dry conditions diffusion resistance rose sharply and reached values more than twice as high as those reached under moist conditions. During almost the entire night, the transpiration rate was lower in dry air. Thus, the total water loss of the plant per unit dry weight during the night was greater at high air humidity than at low humidity.
  It is difficult to determine whether stomatal response to humidity in T recurvata is restricted to the dark period. During the daytime (under the described temperature and light conditions) stomata are almost fully closed for several hours (phase 3 ; Osmond, 1978), independent of air humidity. However, humidity seems to control to a certain extent the diffusion resistance during phase 4 of gas exchange, when stomata open again at the end of the day and external CO2 is assimilated. In order to obtain a sufficiently long period of stable CO2 uptake for experimentation, the light period was extended in this phase, and WD was changed (Fig. 3). In this case as well, the rate of CO2 uptake varied with humidity and suggesting stomatal reactions to be responsible.
  
  Discussion
  Stomatal responses due to changes in air humidity can result from two different mechanisms. On the one hand, evaporative demand determines the actual rate of water loss of a plant and may thus alter the bulk water conditions of the leaf. Leaf water potential may then effect a hydroactive stomatal response (see Stalfelt, 1956). On the other hand, in addition to this negative feedback system, there exists a feedforward control of transpiration (as defined by Cowan, 1977) which enables stomata to respond directly to atmospheric dryness, independent of water conditions in the rest of the leaf (see also Hall et al., 1976). When analyzing actual stomatal behavior it is often difficult to distinguish which of the two mechanisms is involved. The results of the present investigations, however, allow the definite conclusion that stomata of T. recurvata respond to humidity directly. Having no water supply from roots, this plant depends exclusively on water uptake following external moistening of its leaves. During a period of gas exchange measurements, the initially saturated plants continuously lost water through transpiration, the effect of which must be a continuous drop in tissue water. It is possible that a single stomatal closure in response to a drop in ambient air humidity could result from a change in the turgor relations of the bulk leaf. However, since an improvement in the water status of the plant is not conceivable, reopening of stomata after a subsequent increase in humidity followed in turn by a second dryness-induced increase in resistance cannot be explained by feedback but only by direct control. In the same way, a direct stomatal response must be responsible for the "apparent paradox" (Raschke, 1975, p. 327) found also with T. recurvata, that water loss in humid air was higher and consequently water content lower than in dry air (see Schulze et al., 1972).
  The main ecological importance of CAM is its significance for improving water use efficiency in arid and otherwise dry environments. Fixation of CO2 with stomata open predominantly at night results in less water loss than if uptake of CO2 occurs during daylight hours with naturally higher evaporative demand. However, evaporative conditions during the night may vary to a large extent. This would be particularly important for extreme, almost rootless epiphytes such as Tillandsia species. Tillandsia recurvata is found growing on trees, cacti, and rocks in subtropical and tropical areas, under locally semi-arid conditions, from the southern parts of the United States to Argentina and Chile (Rauh, 1970). It is moistened by rain, dew, or fog, but it must also survive periods during which it receives no water. Biebl (1964) reports measurements of potential evaporation found for the microclimate of T. recurvata in the crown of a tree in Costa Rica. Conditions varied extremely from one day to another. From this one can conclude that nocturnal air humidity, temperature, and wind relations are decisive for the water economy of the plant. There were humid nights with almost vapor saturated air which certainly must allow for CO2 uptake together with very low transpiration loss. However, at higher temperatures, air often seemed to stay dry during most of the night. Under such conditions dark fixation of CO2 with low stomatal resistance would mean excessive water loss for T recurvata. The same is true for uptake of external CO2 under dry atmospheric conditions at the end of the light period. In such circumstances, it would be more advantageous for the plant to conserve tissue water, thus maintaining a favorable water potential. This water would be available for more effective CO2 fixation under more humid conditions during one of the next dark periods. The same could not be achieved by stomatal control via internal water status because this would not allow recovery without additional water supply. The only system that could conserve water and optimize water use would be one controlled by the direct response of stomata to atmospheric dryness. These laboratory experiments have proven that such a mechanism does exist in T. recurvata. Field work will have to show whether and how it works under natural conditions.