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Plant responses to reduced air pressure: Advanced techniques and results
Adv. Spuce Res Vol. 18. No. 4/S, pp. (4/5)273-(4/5)281.
0273-1177(95)00889-6
1996 Copyright G 1995 C’XPAR Pnnted in Great Britain. All nghts reserved 027%1177/96 $9.50 + 0.00
PLANT RESPONSES TO REDUCED AIR PRESSURE: ADVANCED TECHNIQUES AND RESULTS H.-J. Daunicht and H. J. Brinkjans Technical
University Berlin, Berlin, Germany
ABSTRACT Knowledge _. _ and growth is essential for un_._ impacts.- on plant _processes _. on air pressure derstanding responses to altitude and for comprehending the way-of action of aerial gasses in general, and is of potential importance for life support systems in space. Our research on reduced air pressure was extended by help of a new set-up comprising two constantly ventilated chambers (283 L each), allowing pressure gradients of &-100 kPa. They provide favourable general growth conditions while maintaining all those factors constant or at desired levels which modify the action of air pressure, e.g. water vapour pressure deficit and air mass flow over the plants. Besides plant growth parameters, transpiration and CO2 gas exchange are determined continuously. Results are presented on young tomato plants grown hydroponically, which had been treated with various combinations of air pressure (400 - 700 - 1000 hPa), CO2 concentration and wind intensity for seven days. At the lowest pressure transpiration was enhanced considerably, and the plants became sturdier. On the other hand growth was retarded to a certain extent, attributable to secondary air pressure effects. Therefore, even greater limitations of plant productivity are expected after more extended periods of low pressure treatment. RELEVANCE AND FUNDAMENTALS OF AIR PRESSURE EFFECTS Knowledge about air pressure effects on plants is still limited, to a great extent probably due to the difficulties or costs of experimentation. At least for pressures below “normal” there are several reports, but most do not seem actually reliable due to methodological insufficiencies. On pressures above normal experimental data are almost completely missing. On the other hand such knowledge is needed to understand altitude effects and will help us to better comprehend action of gasses in general. Potential importance is also given for crop production in space life support systems, because normal atmospheric pressure ma not be optimal, lowering pressure could be an efficient mean for reducing gas losses x ough leaks, and varying the pressure could be even interesting for modulating plant processes like transpiration, e.g. for tuning transpirative cooling or matching the demand for condensed water. When thinning air in a certain space while maintaining volumetric concentrations of gasses and temperature, reduced partial gas pressures or mass concentrations are achieved, but ongoing evaporation leads to the same water vapour deficit as given before. If wind speed does not change, also air mass flow over the leaves is reduced, possibly resulting in a lower gas transfer coefficient. When changing air pressure, diffusion in the gas phase is not affected since the diffusion coefficient is inverse1 proportional to total pressure. However, pressure does affect diffusion in the liquid phase or cells and tissues for being dependent on the concentration of dissolved gas, which solution is determined by the partial pressure. Thus, the primary effects on photosynthetic or respiratory gas exchange should be limited and be governed by the extent to which * present address of ftrst author: Prof. Dr. H.-J. Daunicht, Institute of Horticultural Crop Science, Humboldt University Berlin, Koenigin-Luise-Str. 22, D-14195 Berlin (4/S )273
(4/S )274
H.-J. Daunicht and H. J. Btinkjans
liquid phase diffusion is involved. Pronounced primary effects have to be expected on transpiration, because the vapour pressure within the stomates remains unchanged so that a raised diffusion coefficient leads to enhanced vapour transfer into the ambient air. - For growth as a long-term process, secondary effects could be even more important, e.g. as a consequence of enhanced water and mineral intake, changes in initiation and expansion of new leaves with concomitant changes in morphology and light interception or adaptation of the photosynthetic apparatus to reduced CO2 concentration within the chloroplast. METHODOLOGICAL PREREQUISITES It is not an easy task to realize trials giving meaningful results. Sufficiently pressure-proof chambers are costly and there should be at least two identical ones for allowing direct comparisons between two treatments, instead of only consecutive ones. Naturally, the nonspecific factors like irradiation, temperature and water 8z mineral supply have to be favourable and as identical as possible. Special attention has to be given to those parameters modifying air pressure effects: - Certain CO2 concentrations have to be maintained by automatic control, either equal volumetric ones or defined partial pressures. The same applies to oxygen if a closed system is used. - High attention has to be paid to achieving advanced high grade control of the water vapour pressure deficit, either for maintaining equal dew-point temperatures or other defined levels, e.g. for tuning transpiration rate to that occurring at normal pressure. - In closed systems countermeasures are to be taken to exclude accumulation of gas contaminants . - Finally, wind has to be managed either not to limit gas transfer into the leaf boundary layer or to get identical air mass flows over the leaves when wind speed is limiting this transfer. CO2 analysis should not be complicated or uncertain, as easily resulting from e.g. exposing the analyser to ambient pressure and feeding it with sample air of considerably lower pressure. Complementing the CO2 control device with CO2 dosimetry opens the attractive opportunity for following actual photosynthesis. Furthermore, transpiration should be measurable because of its high dependence on air pressure, either by determining the water uptake or by measuring the rate of water vapour given off by the plant. EXPERIMENTAL SET-UP We have assembled a set-up comprising two chambers, 283 L each (Fig. 1 and 2).Their basic parts are super-wide glass cylinders as used in the chemical industry, placed horizontally and having rugged end-plates made of stainless steel. The chambers themselves arj h$hly gastight. They are irradiated by external metal halide lamps giving up to 400 pM*m *s PAR at the site of the plants. A row of small fans is extending over the full length of the cylinder bottom, rotating the enclosed air like a roll. Appropriate dew-point air conditioning is achieved. For this purpose the glass cylinders are copiously overflown by water given the desired dew-point temperature and being recirculated through a precision brine cooler. Reheating of the air to a constant temperature is done by advanced p.i.d. control and low-mass electric heating coils. We are using an open system in that the chambers are continuously ventilated by a constant flow of atmospheric air (2 L*min-1 each), being freed of CO2 and cooled to the dew-point of the chamber air. Ventilation and air pressure are integrally controlled by a combination of mass flow controller (at the chamber inlet), mechanical precision vacuum controller (at the chamber outlet) and vacuum pump. This arrangement turns out quite favourable as allowing CO2 analysis at ambient pressure (sample air from the outlet of the. vacuum pump), as mnntaining 02 as constant as found in the atmosphere, and as excluding gas contammant accumulation within the chambers. In order to determine transpiration the condensate running off the cooled chamber walls is collected and recorded. There is no error by ventilation as this does not withdraw water.
Plant Responses to Reduced Air Pressure
Fig. 1: The glass chambers with the front end-plates slided aside
Fig. 2: Total view of the experimental set-up. From left to right: CO2-bomb, data logger, controls, chambers (background), CO2 analysers, sample air coolers, nutrient solution supply system (background) For CO2 control each chamber has its own IRGA equipped with set-point switches. Pure CO is injected in pulses of equal mass, achieved by constant pressure differential, temperature an?I pulse duration. These pulses are recorded electronically on an hourly basis. Thus, a kind of compensational photosynthesis measurement is given, being corrected for that amount of CO2 constantly withdrawn by ventilation. Dark respiration is determined eudiometrically.
(4/.5)216
H.-J. Lhunicht and H. J. Brinkjam
PROTOCOL OF TRIALS So far we had been cultivating young tomato plants, a reactive subject, grown in quartz gravel with frequent surplus addition of full-strength nutrient solution. They are pre-cultivated within a greenhouse rfr artificial illumination depending on the season. When reaching 0.25.. .0.4 g dry weight 10 potted plants are transferred into each chamber (2 rows). Only highly uniform plants are selected, and an extra set is used for det ermining the initial status of the plants. The culture medium is covered by coarse quartz gravel and the pots are inserted into closed troughs so that very little evaporation occurs. A sophisticated arrangement is applied for supplying metered amounts of nutrient solution from outside against the pressure drop and to drain back the excess of solution. Regularly, the pressure treatments only endure 7 days (commencing at the beginning of the photoperiod), which appears sufficient in a first phase, because 85 to > 95 % of the final plant constituents are formed during the treatment period. This short period compensates for the lack of not having more chambers run simultaneously. - For the trials reported below, the following conditions had n ‘dentical: - irradiation 332 pM*m-Yei *s- PAR (a few cases with about 9 % less), - photoperiod 16 of z4 hours, - air temperature 25 C, - water vapour pressure deficit of the air 12.7 hPa (60 % r.h.), - mineral & water supply. The following parameters are determined regularly: - net photosynthesis, for the last day also per unit leaf area, - integral of dark respiration per night, - CO compensation point on the last day (CO supply interrupted for 2 hours), - dai l2 y integral of transpiration (amount of con?Iensate), - plant parameters before and after the treatment period per plant: leaf area, fresh dry weight (separately for lamina, stems & petioles and roots), length of main axis and number of side shoots. Partly, the plants are analysed for N/P/K and chlorophyll. Repeatedly, electron microscopic probes of the lamina fine structure are exploited. SELECTED RESULTS Only some of the findings can be reported here, but more details have been already published by BRINKJANS ill. Condensing the information, in all cases only the relative change of the average values pertaining to a certain series of trials are given. Plant growth parameters mostly do not refer to the final values, but to the increments produced during the treatment period (final minus initial values). Only results from trials with a treatment period of 7 days are included. Series #l : Identical volumetric CO2 concentration. Seven trials were conducted to compare 1000 hPa air pressure (= control) with either 700 or 400 hPa. In all cases an equal volum% ‘c_1CO2 concentration of 400 vpm and a specific air mass flow rate (“wind”) of 453 g*m- s were applied. Figure 3 shows the results on the average photosynthetic rate per unit leaf area during the last day of treatment. It reveals that there was almost no change at 400 hPa, while at 700 hPa an increase of 2 to 12 % occurred. Total dry matter production (Fig. 4) was reduced at 400 hPa by 4 to 14 %; at 700 hPa it was not clearly changed, in spite of the enhanced photosynthetic rate. Figures 5 demonstrates the effects on several plant growth parameters when reducing pressure from 1000 to 400 hPa. They all were negatively affected, especially the length of the stem, reflecting the sturdiness of the plants (Fig. 6). Finally, Fig. 7 shows the effects on the mean transpiration rate per unit leaf area during the last day of treatment. As to be expected from theory, it was heavily enhanced at 400 hPa (by about 30 to 40 %). But against theory no clear picture emerged for the treatment with 700 hPa.
Plant Responses to Reduced Air Pressure
y = 0,028611
(4/5 1277
x - 11.777778
X
1
700
b-h]
per unit leaf area Fig. 3: Series #l: Percent change of s on the last day of treatment compared to 1000 hPa (7 trials) +lO +5
y = 0,038889
x - 24.222222 X
B = 72%
(2)
X
I
1
400
i
700
\hPal
Fig. 4: Series #l : Percent change of &J&Idry matter produced during the treatment period compared to 1000 hPa (7 trials) +30 +20 +lO
0
-20 -30 -10 -40 1 ’
*
3
Fig. 5: Series #l : Percent change in m increments produced during the treatment period compared to 1000 hPa (7 trials); from left: leaf area, total d.w., d.w. main axis, length main axis
400 400
hPa ppm
Fig. 6: Series #l : Typical plants at the end of a trial, the right one showing the stunting effect of reduced air pressure +5@ , +40
y : -0,iX + 76 3
B = 47%
+30
r
-O,E!
ns
\
x
‘\..
i,
\.
.i
+20
\
+lO
-
-20
I
-30
‘\
< 0
L,
‘A
400
700
[Wal
Fig. 7: Series #I: Percent change of transnirdtion rate per unit leaf area on the last day of treatment compared to 1000 hPa (7 trials) Series #2: Effect of increasing volumetric CO2 concentration. Five trials were conducted to compare plant responses when increasing the CO2 concentration from 400 to 1000 vpm with the same air pressures and wind intensity as mentioned before. As can be taken from Fig. 8, “CO2 enrichment” remained effective on dry matter production under reduced air pressure and there even seemed to be an increase in CO2 growth promotion with falling pressure. Also the wellknown suppressive action on transpiration was observed under reduced pressure, even with an upward tendency (Fig. 9). Series #3: Effect of air pressure when maintaining identical CO2 partial pressure. Two trials were carried out to directly compare the combination 1000 hPa/400 vpm CO2 with 400 hPa/lOOO vpm C02, thus applyin~thelsame partial CO2 pressure of 0.40 hPa. Specific air mass flow rate was again 453 g*m *s . In general, the results were consistent with the aforementioned findings. Figure 10 shows the results on several growth parameters, revealing again a stunting effect of 400 hPa.
(4/5x79
Plant Responses to Reduced Air Pressure y = -0,01416?
x + 31,016667
B = 82%
:<
I
400
700
[hPal
1000
Fig. 8: Series #2: Percent change of fntal dry matter produced during the treatment period by increasing volumetric CO2 concentration from 400 to 1000 vpm at different air pressures (5 trials)
-30
1 0
I 400
700
[hPal
1000
Fig. 9: Series #2: Percent change of B per unit leaf area on the last day of treatment b increasing the C concentration from 400 to 1000 vpm at did erent air pressures s( trials) +30
-30
Fig. 10: Series #3: Percent change of m increments by reducing air pressure from loo0 to 400 hPa, but maintaining the same partial CO2 pressure of 0.4 hPa (2 trials); from left: leaf area, length main axis, total f.w., total d.w.
(4/.5)280
H.-J. Dumicht and H. J. Brinkjam
#4: Responses to wind at different air pressures. Three trials served for exployg_he Impact of enhancing specific air mass flow rate from 388 to !I05 (1020 in trial 3) g*m- *s at 400 and 1000 hPa. As visualized by Fig. 11, at 400 hPa transpiration rate underwent a considerable increase (30 to 40 X) by wind enhancement, while remaining more or less unchanged at normal pressure. So, obviously the elevated drffusion coefficient at reduced air pressure leads to the need for a higher gas transfer coefficient for achieving unrestricted transpiration. Wind intensity improved dry matter production under both air pressures, while the effect on the length of the main axis was rather low (Fig. 12).
S&es
+50 +40
-
+30 +20 +lO
0 ~~
J
-10
-20 -30 -40
-
400
hPa
1000
hPc
3
Fig. 11: Series #4: Percent change of transniration rate per unit leaf area on the last day of treatment by increasing specific air mass flow at 2 air pressures (3 trials)
+40
+30
+20 +lO 0
-10
-20
-30
Fig. 12: Series #4: Percent change of prowth parameters by increasing specific air mass flow at 2 air pressures (3 trials); left: total d.w., right: length main axis
Plant Responses to Reduced Air Pressure
(4/5)281
CONCLUSIONS The experimental set-up appears adequate for studying air pressure effects (below and above normal pressure) without being masked by impacts of other factors. Additions are needed if investigations are desired with non-atmospheric 02 concentrations . The cooling system becomes insufficient when lower than normal dew-point depressions shall be used for normalizing transpiration at greatly reduced air pressures. Favourable growth conditions were provided and the experimental treatment period was sufficiently long enough for the vast mayority of the plant mass being created during that time. When leaving the volumetric CO2 & 02 concentrations unchanged, the air pressure reduction down to 400 hPa predominantly affected transpiration which is considerably increased, thus principally following theory. There were only minor, slightly negative effects on net photosynthesis, and somewhat greater ones on plant growth. Besides that, a substantial morphogenetic plant response was noted as low pressure plants formed shorter internodes and therefore became sturdier. In general words, these findings indicate that on one hand greatly lowered air pressure is restricting plant growth, and that on the other hand these restrictions are mainly brought about by secondary effects not being explainable by diffusion theory. Therefore, even more detrimental impacts have to be expected after more extended periods of treatment. - Certain hints on an air pressure optimum somewhat below normal atmospheric pressure have been found, which has to be investigated more thoroughly. The effect of elevating volumetric CO2 concentration on growth is obviously greatly independent of the air pressure prevailing, and transpiration appears to be more sensitive to wind under reduced pressure. Literatur cj&d / 1/ H. J. BFUNKJANS, 1992 : Wirkungen des Luftdruckes auf Gaswechsel und Wachstum von Tomatenpflanzen (Lycopersicon esculentum Mill.), doctoral thesis, Technical University Berlin, Institute of Crop Science
0273-1177(95)00889-6
1996 Copyright G 1995 C’XPAR Pnnted in Great Britain. All nghts reserved 027%1177/96 $9.50 + 0.00
PLANT RESPONSES TO REDUCED AIR PRESSURE: ADVANCED TECHNIQUES AND RESULTS H.-J. Daunicht and H. J. Brinkjans Technical
University Berlin, Berlin, Germany
ABSTRACT Knowledge _. _ and growth is essential for un_._ impacts.- on plant _processes _. on air pressure derstanding responses to altitude and for comprehending the way-of action of aerial gasses in general, and is of potential importance for life support systems in space. Our research on reduced air pressure was extended by help of a new set-up comprising two constantly ventilated chambers (283 L each), allowing pressure gradients of &-100 kPa. They provide favourable general growth conditions while maintaining all those factors constant or at desired levels which modify the action of air pressure, e.g. water vapour pressure deficit and air mass flow over the plants. Besides plant growth parameters, transpiration and CO2 gas exchange are determined continuously. Results are presented on young tomato plants grown hydroponically, which had been treated with various combinations of air pressure (400 - 700 - 1000 hPa), CO2 concentration and wind intensity for seven days. At the lowest pressure transpiration was enhanced considerably, and the plants became sturdier. On the other hand growth was retarded to a certain extent, attributable to secondary air pressure effects. Therefore, even greater limitations of plant productivity are expected after more extended periods of low pressure treatment. RELEVANCE AND FUNDAMENTALS OF AIR PRESSURE EFFECTS Knowledge about air pressure effects on plants is still limited, to a great extent probably due to the difficulties or costs of experimentation. At least for pressures below “normal” there are several reports, but most do not seem actually reliable due to methodological insufficiencies. On pressures above normal experimental data are almost completely missing. On the other hand such knowledge is needed to understand altitude effects and will help us to better comprehend action of gasses in general. Potential importance is also given for crop production in space life support systems, because normal atmospheric pressure ma not be optimal, lowering pressure could be an efficient mean for reducing gas losses x ough leaks, and varying the pressure could be even interesting for modulating plant processes like transpiration, e.g. for tuning transpirative cooling or matching the demand for condensed water. When thinning air in a certain space while maintaining volumetric concentrations of gasses and temperature, reduced partial gas pressures or mass concentrations are achieved, but ongoing evaporation leads to the same water vapour deficit as given before. If wind speed does not change, also air mass flow over the leaves is reduced, possibly resulting in a lower gas transfer coefficient. When changing air pressure, diffusion in the gas phase is not affected since the diffusion coefficient is inverse1 proportional to total pressure. However, pressure does affect diffusion in the liquid phase or cells and tissues for being dependent on the concentration of dissolved gas, which solution is determined by the partial pressure. Thus, the primary effects on photosynthetic or respiratory gas exchange should be limited and be governed by the extent to which * present address of ftrst author: Prof. Dr. H.-J. Daunicht, Institute of Horticultural Crop Science, Humboldt University Berlin, Koenigin-Luise-Str. 22, D-14195 Berlin (4/S )273
(4/S )274
H.-J. Daunicht and H. J. Btinkjans
liquid phase diffusion is involved. Pronounced primary effects have to be expected on transpiration, because the vapour pressure within the stomates remains unchanged so that a raised diffusion coefficient leads to enhanced vapour transfer into the ambient air. - For growth as a long-term process, secondary effects could be even more important, e.g. as a consequence of enhanced water and mineral intake, changes in initiation and expansion of new leaves with concomitant changes in morphology and light interception or adaptation of the photosynthetic apparatus to reduced CO2 concentration within the chloroplast. METHODOLOGICAL PREREQUISITES It is not an easy task to realize trials giving meaningful results. Sufficiently pressure-proof chambers are costly and there should be at least two identical ones for allowing direct comparisons between two treatments, instead of only consecutive ones. Naturally, the nonspecific factors like irradiation, temperature and water 8z mineral supply have to be favourable and as identical as possible. Special attention has to be given to those parameters modifying air pressure effects: - Certain CO2 concentrations have to be maintained by automatic control, either equal volumetric ones or defined partial pressures. The same applies to oxygen if a closed system is used. - High attention has to be paid to achieving advanced high grade control of the water vapour pressure deficit, either for maintaining equal dew-point temperatures or other defined levels, e.g. for tuning transpiration rate to that occurring at normal pressure. - In closed systems countermeasures are to be taken to exclude accumulation of gas contaminants . - Finally, wind has to be managed either not to limit gas transfer into the leaf boundary layer or to get identical air mass flows over the leaves when wind speed is limiting this transfer. CO2 analysis should not be complicated or uncertain, as easily resulting from e.g. exposing the analyser to ambient pressure and feeding it with sample air of considerably lower pressure. Complementing the CO2 control device with CO2 dosimetry opens the attractive opportunity for following actual photosynthesis. Furthermore, transpiration should be measurable because of its high dependence on air pressure, either by determining the water uptake or by measuring the rate of water vapour given off by the plant. EXPERIMENTAL SET-UP We have assembled a set-up comprising two chambers, 283 L each (Fig. 1 and 2).Their basic parts are super-wide glass cylinders as used in the chemical industry, placed horizontally and having rugged end-plates made of stainless steel. The chambers themselves arj h$hly gastight. They are irradiated by external metal halide lamps giving up to 400 pM*m *s PAR at the site of the plants. A row of small fans is extending over the full length of the cylinder bottom, rotating the enclosed air like a roll. Appropriate dew-point air conditioning is achieved. For this purpose the glass cylinders are copiously overflown by water given the desired dew-point temperature and being recirculated through a precision brine cooler. Reheating of the air to a constant temperature is done by advanced p.i.d. control and low-mass electric heating coils. We are using an open system in that the chambers are continuously ventilated by a constant flow of atmospheric air (2 L*min-1 each), being freed of CO2 and cooled to the dew-point of the chamber air. Ventilation and air pressure are integrally controlled by a combination of mass flow controller (at the chamber inlet), mechanical precision vacuum controller (at the chamber outlet) and vacuum pump. This arrangement turns out quite favourable as allowing CO2 analysis at ambient pressure (sample air from the outlet of the. vacuum pump), as mnntaining 02 as constant as found in the atmosphere, and as excluding gas contammant accumulation within the chambers. In order to determine transpiration the condensate running off the cooled chamber walls is collected and recorded. There is no error by ventilation as this does not withdraw water.
Plant Responses to Reduced Air Pressure
Fig. 1: The glass chambers with the front end-plates slided aside
Fig. 2: Total view of the experimental set-up. From left to right: CO2-bomb, data logger, controls, chambers (background), CO2 analysers, sample air coolers, nutrient solution supply system (background) For CO2 control each chamber has its own IRGA equipped with set-point switches. Pure CO is injected in pulses of equal mass, achieved by constant pressure differential, temperature an?I pulse duration. These pulses are recorded electronically on an hourly basis. Thus, a kind of compensational photosynthesis measurement is given, being corrected for that amount of CO2 constantly withdrawn by ventilation. Dark respiration is determined eudiometrically.
(4/.5)216
H.-J. Lhunicht and H. J. Brinkjam
PROTOCOL OF TRIALS So far we had been cultivating young tomato plants, a reactive subject, grown in quartz gravel with frequent surplus addition of full-strength nutrient solution. They are pre-cultivated within a greenhouse rfr artificial illumination depending on the season. When reaching 0.25.. .0.4 g dry weight 10 potted plants are transferred into each chamber (2 rows). Only highly uniform plants are selected, and an extra set is used for det ermining the initial status of the plants. The culture medium is covered by coarse quartz gravel and the pots are inserted into closed troughs so that very little evaporation occurs. A sophisticated arrangement is applied for supplying metered amounts of nutrient solution from outside against the pressure drop and to drain back the excess of solution. Regularly, the pressure treatments only endure 7 days (commencing at the beginning of the photoperiod), which appears sufficient in a first phase, because 85 to > 95 % of the final plant constituents are formed during the treatment period. This short period compensates for the lack of not having more chambers run simultaneously. - For the trials reported below, the following conditions had n ‘dentical: - irradiation 332 pM*m-Yei *s- PAR (a few cases with about 9 % less), - photoperiod 16 of z4 hours, - air temperature 25 C, - water vapour pressure deficit of the air 12.7 hPa (60 % r.h.), - mineral & water supply. The following parameters are determined regularly: - net photosynthesis, for the last day also per unit leaf area, - integral of dark respiration per night, - CO compensation point on the last day (CO supply interrupted for 2 hours), - dai l2 y integral of transpiration (amount of con?Iensate), - plant parameters before and after the treatment period per plant: leaf area, fresh dry weight (separately for lamina, stems & petioles and roots), length of main axis and number of side shoots. Partly, the plants are analysed for N/P/K and chlorophyll. Repeatedly, electron microscopic probes of the lamina fine structure are exploited. SELECTED RESULTS Only some of the findings can be reported here, but more details have been already published by BRINKJANS ill. Condensing the information, in all cases only the relative change of the average values pertaining to a certain series of trials are given. Plant growth parameters mostly do not refer to the final values, but to the increments produced during the treatment period (final minus initial values). Only results from trials with a treatment period of 7 days are included. Series #l : Identical volumetric CO2 concentration. Seven trials were conducted to compare 1000 hPa air pressure (= control) with either 700 or 400 hPa. In all cases an equal volum% ‘c_1CO2 concentration of 400 vpm and a specific air mass flow rate (“wind”) of 453 g*m- s were applied. Figure 3 shows the results on the average photosynthetic rate per unit leaf area during the last day of treatment. It reveals that there was almost no change at 400 hPa, while at 700 hPa an increase of 2 to 12 % occurred. Total dry matter production (Fig. 4) was reduced at 400 hPa by 4 to 14 %; at 700 hPa it was not clearly changed, in spite of the enhanced photosynthetic rate. Figures 5 demonstrates the effects on several plant growth parameters when reducing pressure from 1000 to 400 hPa. They all were negatively affected, especially the length of the stem, reflecting the sturdiness of the plants (Fig. 6). Finally, Fig. 7 shows the effects on the mean transpiration rate per unit leaf area during the last day of treatment. As to be expected from theory, it was heavily enhanced at 400 hPa (by about 30 to 40 %). But against theory no clear picture emerged for the treatment with 700 hPa.
Plant Responses to Reduced Air Pressure
y = 0,028611
(4/5 1277
x - 11.777778
X
1
700
b-h]
per unit leaf area Fig. 3: Series #l: Percent change of s on the last day of treatment compared to 1000 hPa (7 trials) +lO +5
y = 0,038889
x - 24.222222 X
B = 72%
(2)
X
I
1
400
i
700
\hPal
Fig. 4: Series #l : Percent change of &J&Idry matter produced during the treatment period compared to 1000 hPa (7 trials) +30 +20 +lO
0
-20 -30 -10 -40 1 ’
*
3
Fig. 5: Series #l : Percent change in m increments produced during the treatment period compared to 1000 hPa (7 trials); from left: leaf area, total d.w., d.w. main axis, length main axis
400 400
hPa ppm
Fig. 6: Series #l : Typical plants at the end of a trial, the right one showing the stunting effect of reduced air pressure +5@ , +40
y : -0,iX + 76 3
B = 47%
+30
r
-O,E!
ns
\
x
‘\..
i,
\.
.i
+20
\
+lO
-
-20
I
-30
‘\
< 0
L,
‘A
400
700
[Wal
Fig. 7: Series #I: Percent change of transnirdtion rate per unit leaf area on the last day of treatment compared to 1000 hPa (7 trials) Series #2: Effect of increasing volumetric CO2 concentration. Five trials were conducted to compare plant responses when increasing the CO2 concentration from 400 to 1000 vpm with the same air pressures and wind intensity as mentioned before. As can be taken from Fig. 8, “CO2 enrichment” remained effective on dry matter production under reduced air pressure and there even seemed to be an increase in CO2 growth promotion with falling pressure. Also the wellknown suppressive action on transpiration was observed under reduced pressure, even with an upward tendency (Fig. 9). Series #3: Effect of air pressure when maintaining identical CO2 partial pressure. Two trials were carried out to directly compare the combination 1000 hPa/400 vpm CO2 with 400 hPa/lOOO vpm C02, thus applyin~thelsame partial CO2 pressure of 0.40 hPa. Specific air mass flow rate was again 453 g*m *s . In general, the results were consistent with the aforementioned findings. Figure 10 shows the results on several growth parameters, revealing again a stunting effect of 400 hPa.
(4/5x79
Plant Responses to Reduced Air Pressure y = -0,01416?
x + 31,016667
B = 82%
:<
I
400
700
[hPal
1000
Fig. 8: Series #2: Percent change of fntal dry matter produced during the treatment period by increasing volumetric CO2 concentration from 400 to 1000 vpm at different air pressures (5 trials)
-30
1 0
I 400
700
[hPal
1000
Fig. 9: Series #2: Percent change of B per unit leaf area on the last day of treatment b increasing the C concentration from 400 to 1000 vpm at did erent air pressures s( trials) +30
-30
Fig. 10: Series #3: Percent change of m increments by reducing air pressure from loo0 to 400 hPa, but maintaining the same partial CO2 pressure of 0.4 hPa (2 trials); from left: leaf area, length main axis, total f.w., total d.w.
(4/.5)280
H.-J. Dumicht and H. J. Brinkjam
#4: Responses to wind at different air pressures. Three trials served for exployg_he Impact of enhancing specific air mass flow rate from 388 to !I05 (1020 in trial 3) g*m- *s at 400 and 1000 hPa. As visualized by Fig. 11, at 400 hPa transpiration rate underwent a considerable increase (30 to 40 X) by wind enhancement, while remaining more or less unchanged at normal pressure. So, obviously the elevated drffusion coefficient at reduced air pressure leads to the need for a higher gas transfer coefficient for achieving unrestricted transpiration. Wind intensity improved dry matter production under both air pressures, while the effect on the length of the main axis was rather low (Fig. 12).
S&es
+50 +40
-
+30 +20 +lO
0 ~~
J
-10
-20 -30 -40
-
400
hPa
1000
hPc
3
Fig. 11: Series #4: Percent change of transniration rate per unit leaf area on the last day of treatment by increasing specific air mass flow at 2 air pressures (3 trials)
+40
+30
+20 +lO 0
-10
-20
-30
Fig. 12: Series #4: Percent change of prowth parameters by increasing specific air mass flow at 2 air pressures (3 trials); left: total d.w., right: length main axis
Plant Responses to Reduced Air Pressure
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CONCLUSIONS The experimental set-up appears adequate for studying air pressure effects (below and above normal pressure) without being masked by impacts of other factors. Additions are needed if investigations are desired with non-atmospheric 02 concentrations . The cooling system becomes insufficient when lower than normal dew-point depressions shall be used for normalizing transpiration at greatly reduced air pressures. Favourable growth conditions were provided and the experimental treatment period was sufficiently long enough for the vast mayority of the plant mass being created during that time. When leaving the volumetric CO2 & 02 concentrations unchanged, the air pressure reduction down to 400 hPa predominantly affected transpiration which is considerably increased, thus principally following theory. There were only minor, slightly negative effects on net photosynthesis, and somewhat greater ones on plant growth. Besides that, a substantial morphogenetic plant response was noted as low pressure plants formed shorter internodes and therefore became sturdier. In general words, these findings indicate that on one hand greatly lowered air pressure is restricting plant growth, and that on the other hand these restrictions are mainly brought about by secondary effects not being explainable by diffusion theory. Therefore, even more detrimental impacts have to be expected after more extended periods of treatment. - Certain hints on an air pressure optimum somewhat below normal atmospheric pressure have been found, which has to be investigated more thoroughly. The effect of elevating volumetric CO2 concentration on growth is obviously greatly independent of the air pressure prevailing, and transpiration appears to be more sensitive to wind under reduced pressure. Literatur cj&d / 1/ H. J. BFUNKJANS, 1992 : Wirkungen des Luftdruckes auf Gaswechsel und Wachstum von Tomatenpflanzen (Lycopersicon esculentum Mill.), doctoral thesis, Technical University Berlin, Institute of Crop Science