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Influence of flood-preconditioning and drought on leaf gas exchange and plant water relations in seedlings of pecan
Environmentaland ExperimentalBotany,Vol. 30, No. 4, pp. 48~495, 1990
0098 8472]90 $3.00 + 0.00 ~ 1990 Pergamon Press pie
Printed in Great Britain.
I N F L U E N C E OF F L O O D - P R E C O N D I T I O N I N G A N D D R O U G H T ON LEAF GAS E X C H A N G E A N D P L A N T W A T E R R E L A T I O N S IN SEEDLINGS OF PECAN M I C H A E L W. S M I T H a n d S U S A N M . H U S L I G
Department of Horticulture and Landscape Architecture, Oklahoma State University, Stillwater, OK 74078, U.S.A.
(Received 16 October 1989; acceptedin revisedform 20 March 1990) SMITH M. W. and HUSLIGS. M. In[tuenceofflood-preconditioning and drought on leaf gas exchange and plant water relations in seedlingsof pecan. ENVIRONMENTAL ANn EXPERIMENTAL BOTANY30, 489495, 1990. Four-month old seedlings of pecan [Carya illinoensis (Wangenh) C. Koch] were either not
flooded or flooded for 14 days, then transferred to well-drained conditions for 23 days. Water was withheld from one-half of the trees for 6 days, then trees were rewatered, and water was withheld from all trees. Leaf expansion, leaf dry weight, and the number of new leaves that developed were reduced by flooding, but not trunk or root dry weights. Evapotranspiration rates of flooded trees after transfer to well-drained conditions were initially higher than those of unflooded trees, but decreased to rates of control trees after 12 days. Flooding had no effect on photosynthesis after trees had been transferred to well-drained conditions for 23 days. Droughtstressed trees with leaf water potentials as low as - 1.93 MPa had lower leaf photosynthetic CO2 assimilation rate (A), transpiration (E), and leaf conductance to CO2 (gL) as compared to wellwatered trees. Leaf internal CO2 concentration (c~) was reduced only by the most severe waterstress treatment. Water use rates and relative water content were lower at the permanent wilting point during a second drought stress when trees had been exposed previously to drought stress.
INTRODUCTION
SOIL flooding c o m m o n l y occurs in native pecan orchards a n d results in m a n y deleterious effects. These include reduced leaf expansion a n d form a t i o n of new leaves, decreased absorption a n d translocation of some elements, smaller root mass and lower A. (7'9'1°) Flooding usually occurs in the spring in O k l a h o m a a n d m a y be followed by extended periods of little or no rainfall. Flooding may predispose trees to d r o u g h t stress since root mass is reduced./9'1°) T h e objectives of this study were to determine (1) if soil flooding affected the tree's response to
subsequent d r o u g h t stress, a n d (2) the effects of d r o u g h t stress on leaf gas exchange (A, E, gL, ci), a n d water relations (RWC, ~kL, ~0~, ~p) of pecan. T h e flooding d u r a t i o n chosen was 14 days which we considered a moderate duration. Trees were then transferrred to well-drained conditions with n o r m a l watering for 23 days before d r o u g h t stress was imposed by withholding water.
MATERIALS AND METHODS
Stratified " D o d d " pecan seeds were germ i n a t e d in aerated water, and planted on 19
Abbreviations: Leaf net photosynthetic rate (A), transpiration (E), leaf conductance to CO2 (gi~), leaf internal CO2 concentration (ci), relative water content (RWC), leaf water potential (0IJ), leaf osmotic potential (0s), leaf turgor potential (Op). 489
490
M.W. SMITH and S. M. HUSLIG
February 1988 in 11.3-1 containers filled with a fire-hardened calcite clay (Turface, Wyandotte, MI). The media were amended with (in g/m 3) 3530 18 N-2.6 P-10 K (Osmocote slow-release fertilizer, Sierra Chemical Co., Milpitas, CA), 4694 dolomite, 882 triple phosphate (20% P), 480 FeSO4 (25% Fe), 92 MnSO4 (27% Mn), 21 CuSO4 (25.4'~'o Cu), 3.5 NaBO2 (20.5'}(~ B), 0.5 NaMoO4 (39% Mo), and 39 ZnSO4 (36% Zn). Trees were grown in a greenhouse with temperature controls set at 21°C night and 26°C day. On 3 June 1988, trees were transferred to a growth chamber at 2°C and manually detoliated. Trees were returned to the greenhouse on 8 July 1988. (a) Flooding On 9 July 1988, one-half of the trees were flooded by submerging the pots in containers of water about 2 cm above the soil surface. Dissolved oxygen, measured with an oxygen analyzer (Cole Parmer model POM1A), decreased ti'om 8 ppm when trees were initially flooded to 5 ppm by the fourth day, then remained constant. Water temperature, measured using copper-constantan thermocouples, was near ambient temperature (27 and 21°C mean day and night temperatures, respectively). After 14 days, flooded trees were transferred to well-drained conditions with normal watering (irrigated every second or third day until water exited the container bottom). Normal watering was maintained on the unflooded, control trees. Five trees per treatment were harvested 3 and 21 days after transfer to drained conditions. Leaf number was counted and leaf area measured using a L I - C O R model 3100 area meter (Lincoln, NE). The medium was shaken from the roots, then roots were washed to remove adhering medium particles. Leaves, trunk, tap root and fine roots were dried at 80°C until constant weight, then weighed. Evapotranspiration was determined alter trees were transferred to well-drained conditions. Containers were watered, excess water drained and containers weighed, then weighed prior to the next watering. Data were analyzed using the Student's t test. (b) First drought stress After 23 days in well-drained conditions, onehalf of the flooded and control trees were exposed
to drought stress by withholding water until the permanent wilting point was reached (trees remained wilted at dawn). Each treatment combination (flooding stress by drought stress) contained five single-tree replications in a randomized complete block design. Gas exchange parameters, which included A, E, g~, and cl, were measured 2, 4 and 6 days after water was withheld, using a flow-through system/gi with 350 #l/1 CO2, 1200 #tool m z see l photosynthetic active radiation (saturating light) at the leaf surface, and 1.3 1.8 kPa vapor pressure gradient between the leaf and air. The flowthrough system used a 14 x 14.5 x 20 cm plexiglass chamber with a water-jacketed aluminum base. Chamber temperature was controlled at 26_+0.5°C by circulating water from a refrigerated water bath through the aluminum base. The chamber was equipped with a circulating fan to minimize boundary layer resistance. The air source was regulated with mass flow controllers (Brooks Instrument Division model 5850 controller and model 5876 meter, Hatfield, PA), and humidified by passing the air stream over water in a controlled temperature bath. Carbon dioxide and water vapor exchange were measured with a differential Horiba PIR 2000 infrared gas analyzer (Irvine, CA) and an EG & G model 911 dew point hygrometer (Waltham, MA), respectively. The air stream was passed through an ice bath trap to remove excess water prior to CO2 measurement. Light was supplied from a 400-W General Electric metal halide lamp. Gas exchange parameters were calculated as described by FARQUHAR,SHARKEY, and VON
CAEMMERER.(3'4) Leaf water potential (~PL), osmotic potential (0~), and turgor potential (0p) were measured using leaf cutter psychrometers (J. R. D. Merrill, Logan, UT)./9) Three 0.24-cm 2 leaf disks were cut from one leaflet near the leaf used for gas exchange measurements and sealed in three psychrometers within 3 sec. Psychrometers were placed in a 30°C water bath for 3 hr, then wetbulb depressions were read in microvohs using a Wescor HP-115 water potential data system (Logan, UT). The microvoh readings were used with calibration equations derived for each psychrometer to calculate water potential values. Osmotic potential was determined by placing the
F L O O D - P R E C O N D I T I O N I N G AND D R O U G H T ON PECAN SEEDLINGS psychrometers containing the leaf tissue in a - 4 0 ° C freezer for 24 hr, then following the procedures outlined above. T u r g o r potential was calculated as the difference in @L and Cs. Leaf relative water content ( R W C ) was determined using three 5-mm diameter leaf disks per tree] 1) Leaf disks were cut and weighed immediately. T h e y were then floated in water for 4 hr, blotted dry, and reweighed. Disks were dried at 80°C for 24 hr and weighed. Relative water content was calculated as (fresh weight - dry weight)/ (turgid weight - d r y weight) x 100. D a t a were analyzed using an analysis of variance.
,c go0 ~ 8a0 ~_
O CONTROL ~ FLOOOEO
7OO
-J ~o ~ 50~ 400 ~ 30o ~ 200
' ~
~jo--------o ~ . o j -
RESULTS A N D DISCUSSION
(a) Flooding Flooded trees had less leaf area and leaf dry weight than unflooded trees 3 days after being returned to well-drained conditions (Table 1). Flooded trees had fewer leaves than unflooded
~
'
~ 100 ~ 0 -DRYS RFTER FLOOOING
(c) Second drought stress Trees were watered when the permanent wilting point was reached, then water was withheld from all trees. Thus trees had either not been exposed to d r o u g h t previously, or were exposed to drought prior to the second d r o u g h t stress. Evapotranspiration, OL, ~ks, ~p, and R W C were determined at d a w n 2, 4, 5, 6 and 7 days after water was withheld. Each treatment combination contained five single-tree replications in a randomized complete block design.
491
Treatment
Leaves produced (No.)
Fro. 1. Influence of flooding on post-flooding evapotranspiration rates. Treatments contain five replications. Vertical bars indicate the standard error of the mean. trees 21 days after being returned to well-drained conditions, but leaf n u m b e r was not affected after 3 days of drained conditions. Trunk, tap root, and fine root dry weights were not affected by flooding. An earlier report indicated that root dry weight was reduced by 8 3 % when trees were flooded for 31 days] l°/ Apparently 14 days of flooding was not sufficient to cause root loss or inhibit root growth when returned to drained conditions since root dry weight was not affected. Trees that had been flooded had greater evapotranspiration rates than unflooded trees 3 and 7 days after transfer to well-drained conditions (Fig. 1). Evapotranspiration rates were not affected
531 ±55I" 253±39*
6.4±0.2 6.8±0.7
Dry weights (g) Leaves
Trunk
Fine roots
Tap root
3.8±0.2 3.8-t-0.1
2.9___0.3 2.5±0.1
8.8_+ 1.1 13.1±1.1
3.7±0.2 4.1_+0.2
15.54-0.5 15.14-0.4
3 days after transfer Control Flooded Control Flooded
1404±58 1317±76
1
TERMINRTEO
Table 1. Influence of rootflooding on leaf area, leaf number, and leaf, trunk,fine root and tap root dry weights of seedlings of pecan Leaf area (cm2)
,
2.5_+0.2 1.2±0.1"
21 days after transfer 10.04-0.3 8.7±0.3 4.64-0.'1 9.2±0.4* 8.4±0.5 4.7_+0.1
* Significantly different from the control, 5 % level. I" Mean of five replications _ the standard error of the mean.
492
M . W . SMITH and S. M. HUSLIG
Table 2. Influence offlooding for 14 daysfollowed by 23 days recovery, then drought stressed 2, 4, or 6 days on photosynthesis, transpiration, leaf conductance to C02, and leaf internal C02 concentration; analysis of variance of main effects and interactions Mean squares Source
A
Flood Drought Flood x drought Error
6.52 0.16 0.05 12.57
Water withheld 2 days 0.02 127.3 0.02 0.1 0.05 99.5 1.17 2209.1
783 839 214 716
Flood Drought Flood x drought Error
8.5 170.3" 13.1 17.9
Water withheld 4 days 2.8 4101" 7.2** 10,601"* 0.2 46 0.5 491
735 3054 244 l 1998
Flood Drought Flood x drought Error
2.0 387.6*** 7.9 22.3
Water withheld 6 days 1.2 1194 20.4*** 23,336*** 0.2 628 0.9 1128
4801 23,722* 494 3786
*
E
gL
ci
** *** Significant at 5°/; (*), 1 (~,Ji~(**), or 0.1% (***)
by flooding t r e a t m e n t from 12 to 20 days after flooding. W A z m et al. ~°) reported that stomatal resistance of pecan d u r i n g flooding was greater in flooded than unflooded trees t h r o u g h 22 days of flooding, and SMITH and AeER (9/ found that flooded pecan trees h a d higher OL a n d lower gL than those of unflooded trees. I n both earlier reports, (9'~°i and the present study, leaves did not wilt while trees were flooded. This indicates that while trees were flooded, s t o m a t a were at least p a r t i a l l y closed, and leaves were turgid. However, when trees were transferred to well-drained conditions, s t o m a t a a p p a r e n t l y opened, but were unable to regulate w a t e r loss for a few days, since e v a p o t r a n s p i r a t i o n rates of trees that h a d been flooded were over twice as great as those of trees not flooded. Root flooding did not affect the gas exchange p a r a m e t e r s 23 days after trees h a d been transferred to d r a i n e d conditions then exposed to d r o u g h t stress, except gL was lower in trees that had been flooded after 4 days of d r o u g h t stress (Table 2). T h e r e were no interactions between flooding and d r o u g h t stress. SMITh a n d AoEI~/9/ reported that flooding of pecan trees for 15 days
reduced A by a b o u t 5 0 ° , and that A r e m a i n e d a b o u t 25~yo lower than that of unflooded trees 14 days after transfer to d r a i n e d conditions. T h e a d d i t i o n a l 9 days of well-drained conditions allowed complete recovery of A. (b) First drought stress L e a f R W C and gas exchange were not affected by 2 days of d r o u g h t (Table 3). However, 4 days of d r o u g h t reduced R W C by 4.2% (Table 3) and decreased ~L and ~Op, but Os was not affected (Table 4). Photosynthetic CO2 assimilation and E were 44 and 51 0/0 lower than that of control trees, respectively, after 4 days of drought. Stom a t a l c o n d u c t a n c e of stressed trees decreased 56 ~/o, but ci was not significantly different from control trees. W h e n water was withheld for 6 days, trees were n e a r the p e r m a n e n t wilting point. Relative w a t e r content was almost 10°/0 lower than that of control trees, and ~kL, ~ , and 0p were lower ( - 1 . 9 3 , - 2 . 0 6 and 0.13 M P a , respectively) than those of control trees ( - 1.43, - 1.73 and 0.30 M P a , respectively). Photosynthetic CO2 assimilation, E , gL, a n d cl of stressed trees were 63, 77, 76 and 4 2 % lower than those of control
F L O O D - P R E C O N D I T I O N I N G AND D R O U G H T ON PECAN SEEDLINGS
493
Table 3. Influence of&ought stress on photosynthesis transpiration, leaf conductance to CO2, and leaf internal CO2 concentration of seedlings of pecan
Treatment
A (pmol m -~ sec J)
E (mmol m 2 sec i)
gl. (mmol m -2 sec-i)
Water withheld 2 days 1114-9 1114-19
cl (pl/l)
R WC (0,~)
176+_9 163+13
92.5+l.2 93.14-1.4
Irrigated Not irrigated
16.02 4- 1.01 ~ 16.20+_1.25
2.99_+0.27 2.924-0.42
Irrigated Not irrigated
13.884- 1.32 7.824-0.97*
Water withheld 4 days 3.05 4-0.32 1064- 12 1.51 _+0.14"* 474-4**
197_ 11 1664- 12
92.5+ 1.2 88.34-0.9*
Irrigated Not irrigated
14.00+2.12 5.194-0.60"**
Water withheld 6 days 2.74+0.37 93_+13 0.64_+0.07*** 224-2***
172_+18 100_+ 16"
95.1+1.7 85.3_+1.6"
*, **, *** Significantly different from the control at 5°/i~ (*), 1 °/o/ (**), or 0.1 o~ (***). ~ Mean of 10 replications 4- the standard error of the mean. trees, respectively. These data indicate that moderate drought stress reduced A by limiting the mesophyll assimilation capacity since ci was not decreased significantly, but A decreased 6 3 0 . Further drought stress, near the permanent wilting point, decreased A, gL, and ci, indicating that both mesophyll assimilation capacity and CO2 diffusion into the leaf were limiting. MOLDAU/a/ concluded that reductions in A caused by drought stress were due to partial stomatal closure, since desiccation of bean leaves from - 0 . 5 to - 1 . 3 M P a did not affect mesophyll conductance. However, HUTMACHER and KRIEC,,(6) working with cotton, found that drought stress (up to - 3 . 1 MPa) caused greater reductions in A than gL, indicating that the mesophyll assimilation capacity was reduced. Similarly, drought-stressed Zea mays Ill' or Glycine max 15i had lower A and gL than those of control plants, but ci remained almost constant, indicating that the mesophyll assimilation capacity limited A more that leaf conductance to C O 2. (c) Second drought stress Trees that had been previously exposed to drought stress reached the permanent wilting point 2 days after those that had not been drought stressed (Table 3), although trees in both treatments used the same total a m o u n t of water during the drought stress period (1202 ml/tree vs 1227 ml/tree). Drought-stressed trees had lower evapo-
transpiration rates than those of control trees 4 days after water was withheld. Relative water content was 85 °/o when trees that had not been exposed to drought reached the permanent wilting point, but previous exposure to drought lowered their R W C to 8 0 % at permanent wilting point. The decline in R W C of unstressed trees was linear, whereas the decline in R W C of previously stressed trees was curvilinear. Trees that had been exposed to drought stress had higher 0p and lower 0~ than unstressed trees when the second drought was initiated, but OL was not affected. Trees that had been exposed to drought maintained a positive pressure potential 2 days longer than did unstressed trees. Results of this study indicate that although flooding caused m a n y deleterious effects on pecans, trees recovered quickly if the flooding duration was not excessive. High evapotranspiration rates following flooding suggest that stomatal regulation was impaired. This could cause leaf damage by desiccation if the leaf-toair vapor pressure gradient were high, and root conductance to water were decreased] z) Pecan response to drought was similar to that of other plant species. (5'6'11/ Leaf photosynthetic C O 2 assimilation rate declined as the severity of drought stress increased. The decline in A during moderate stress was primarily due to a reduction in mesophyll assimilation capacity, then as the severity of stress increases A is limited by reduced
494
M.W.
SMITH
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FLOOD-PRECONDITIONING AND D R O U G H T ON PECAN SEEDLINGS mesophyll assimilation capacity a n d partial stomatal closure resulting in decreased CO2 diffusion into the leaf. Pecans which had been d r o u g h t stressed had lower rates of water loss t h a n those that were not exposed to d r o u g h t stress. Tissue h y d r a t i o n u p o n rewatering after d r o u g h t stress was not significantly different from that not exposed to drought.
Plant physiology 5.
6.
7. REFERENCES
1. BARR H. D. (1968) Determination of water deficits in plant tissues. Pages 235 267 in T. T. KOZLOWSKI, ed. Water deficits and plant growth. Academic Press, New York. 2. BRADFORDK.J. and Hsmo T. C. (1982) Stomatal behavior and water relations of waterlogged tomato plants. Pl. Physiol. 70, 1508 1513. 3. FARQUHARG. D. and SHARKEY T. D. (1982) Stomatal conductance and photosynthesis. A. Rev. PI. Physiol. 33, 317-345. 4. FARQUHARG. D. and VON CAEMMERERD. (1982) Modeling of photosynthetic response to environmental conditions. Pages 549-587 in O. L. FANGE, P. S. NOBEL, C. B. OSMONDand H. ZIEGLER, eds
8.
9.
10.
11.
495
New series, Vol. 12B. Springer, Berlin. HUBER S. C., ROGERS H. H. and MOWRY F. L. (1984) Effects of water stress on photosynthesis and carbon partitioning in soybean (Glycine max (L.) Merr.) plants grown in the field at different CO2 levels. Pl. Physiol. 76, 244-249. HUTMACHER R. B. and KmEo D. R. (1983) Photosynthetic rate control in cotton. P1. Physiol. 73, 658-661. LOUSTALOTA.J. (1945) Influence of soil moisture conditions on apparent photosynthesis and transpiration of pecan leaves. J. Agric. Res. 71, 519532. MOLDAU H. (1973) Effects of various water regimes on stomatal and mesophyll conductances of bean leaves. Photosynthetica 7, 1-7. SMITh M. W. and AGER P. L. (1988) Effects of soil flooding on leafgas exchange of seedling pecan trees. HortScience 23, 370-372. WAZm F. K., SMITH M. W. and AKERS S. W. (1988) Effects of flooding and soil phosphorus levels on pecan seedlings. HortScience 23, 595- 597. WONG S., COWAN I. R. and Farquhar G. D. (1985) Leaf conductance in relation to rate of CO 2 assimilation. III. Influences of water stress and photoinhibition. Pl. Physiol. 78, 830 834.
0098 8472]90 $3.00 + 0.00 ~ 1990 Pergamon Press pie
Printed in Great Britain.
I N F L U E N C E OF F L O O D - P R E C O N D I T I O N I N G A N D D R O U G H T ON LEAF GAS E X C H A N G E A N D P L A N T W A T E R R E L A T I O N S IN SEEDLINGS OF PECAN M I C H A E L W. S M I T H a n d S U S A N M . H U S L I G
Department of Horticulture and Landscape Architecture, Oklahoma State University, Stillwater, OK 74078, U.S.A.
(Received 16 October 1989; acceptedin revisedform 20 March 1990) SMITH M. W. and HUSLIGS. M. In[tuenceofflood-preconditioning and drought on leaf gas exchange and plant water relations in seedlingsof pecan. ENVIRONMENTAL ANn EXPERIMENTAL BOTANY30, 489495, 1990. Four-month old seedlings of pecan [Carya illinoensis (Wangenh) C. Koch] were either not
flooded or flooded for 14 days, then transferred to well-drained conditions for 23 days. Water was withheld from one-half of the trees for 6 days, then trees were rewatered, and water was withheld from all trees. Leaf expansion, leaf dry weight, and the number of new leaves that developed were reduced by flooding, but not trunk or root dry weights. Evapotranspiration rates of flooded trees after transfer to well-drained conditions were initially higher than those of unflooded trees, but decreased to rates of control trees after 12 days. Flooding had no effect on photosynthesis after trees had been transferred to well-drained conditions for 23 days. Droughtstressed trees with leaf water potentials as low as - 1.93 MPa had lower leaf photosynthetic CO2 assimilation rate (A), transpiration (E), and leaf conductance to CO2 (gL) as compared to wellwatered trees. Leaf internal CO2 concentration (c~) was reduced only by the most severe waterstress treatment. Water use rates and relative water content were lower at the permanent wilting point during a second drought stress when trees had been exposed previously to drought stress.
INTRODUCTION
SOIL flooding c o m m o n l y occurs in native pecan orchards a n d results in m a n y deleterious effects. These include reduced leaf expansion a n d form a t i o n of new leaves, decreased absorption a n d translocation of some elements, smaller root mass and lower A. (7'9'1°) Flooding usually occurs in the spring in O k l a h o m a a n d m a y be followed by extended periods of little or no rainfall. Flooding may predispose trees to d r o u g h t stress since root mass is reduced./9'1°) T h e objectives of this study were to determine (1) if soil flooding affected the tree's response to
subsequent d r o u g h t stress, a n d (2) the effects of d r o u g h t stress on leaf gas exchange (A, E, gL, ci), a n d water relations (RWC, ~kL, ~0~, ~p) of pecan. T h e flooding d u r a t i o n chosen was 14 days which we considered a moderate duration. Trees were then transferrred to well-drained conditions with n o r m a l watering for 23 days before d r o u g h t stress was imposed by withholding water.
MATERIALS AND METHODS
Stratified " D o d d " pecan seeds were germ i n a t e d in aerated water, and planted on 19
Abbreviations: Leaf net photosynthetic rate (A), transpiration (E), leaf conductance to CO2 (gi~), leaf internal CO2 concentration (ci), relative water content (RWC), leaf water potential (0IJ), leaf osmotic potential (0s), leaf turgor potential (Op). 489
490
M.W. SMITH and S. M. HUSLIG
February 1988 in 11.3-1 containers filled with a fire-hardened calcite clay (Turface, Wyandotte, MI). The media were amended with (in g/m 3) 3530 18 N-2.6 P-10 K (Osmocote slow-release fertilizer, Sierra Chemical Co., Milpitas, CA), 4694 dolomite, 882 triple phosphate (20% P), 480 FeSO4 (25% Fe), 92 MnSO4 (27% Mn), 21 CuSO4 (25.4'~'o Cu), 3.5 NaBO2 (20.5'}(~ B), 0.5 NaMoO4 (39% Mo), and 39 ZnSO4 (36% Zn). Trees were grown in a greenhouse with temperature controls set at 21°C night and 26°C day. On 3 June 1988, trees were transferred to a growth chamber at 2°C and manually detoliated. Trees were returned to the greenhouse on 8 July 1988. (a) Flooding On 9 July 1988, one-half of the trees were flooded by submerging the pots in containers of water about 2 cm above the soil surface. Dissolved oxygen, measured with an oxygen analyzer (Cole Parmer model POM1A), decreased ti'om 8 ppm when trees were initially flooded to 5 ppm by the fourth day, then remained constant. Water temperature, measured using copper-constantan thermocouples, was near ambient temperature (27 and 21°C mean day and night temperatures, respectively). After 14 days, flooded trees were transferred to well-drained conditions with normal watering (irrigated every second or third day until water exited the container bottom). Normal watering was maintained on the unflooded, control trees. Five trees per treatment were harvested 3 and 21 days after transfer to drained conditions. Leaf number was counted and leaf area measured using a L I - C O R model 3100 area meter (Lincoln, NE). The medium was shaken from the roots, then roots were washed to remove adhering medium particles. Leaves, trunk, tap root and fine roots were dried at 80°C until constant weight, then weighed. Evapotranspiration was determined alter trees were transferred to well-drained conditions. Containers were watered, excess water drained and containers weighed, then weighed prior to the next watering. Data were analyzed using the Student's t test. (b) First drought stress After 23 days in well-drained conditions, onehalf of the flooded and control trees were exposed
to drought stress by withholding water until the permanent wilting point was reached (trees remained wilted at dawn). Each treatment combination (flooding stress by drought stress) contained five single-tree replications in a randomized complete block design. Gas exchange parameters, which included A, E, g~, and cl, were measured 2, 4 and 6 days after water was withheld, using a flow-through system/gi with 350 #l/1 CO2, 1200 #tool m z see l photosynthetic active radiation (saturating light) at the leaf surface, and 1.3 1.8 kPa vapor pressure gradient between the leaf and air. The flowthrough system used a 14 x 14.5 x 20 cm plexiglass chamber with a water-jacketed aluminum base. Chamber temperature was controlled at 26_+0.5°C by circulating water from a refrigerated water bath through the aluminum base. The chamber was equipped with a circulating fan to minimize boundary layer resistance. The air source was regulated with mass flow controllers (Brooks Instrument Division model 5850 controller and model 5876 meter, Hatfield, PA), and humidified by passing the air stream over water in a controlled temperature bath. Carbon dioxide and water vapor exchange were measured with a differential Horiba PIR 2000 infrared gas analyzer (Irvine, CA) and an EG & G model 911 dew point hygrometer (Waltham, MA), respectively. The air stream was passed through an ice bath trap to remove excess water prior to CO2 measurement. Light was supplied from a 400-W General Electric metal halide lamp. Gas exchange parameters were calculated as described by FARQUHAR,SHARKEY, and VON
CAEMMERER.(3'4) Leaf water potential (~PL), osmotic potential (0~), and turgor potential (0p) were measured using leaf cutter psychrometers (J. R. D. Merrill, Logan, UT)./9) Three 0.24-cm 2 leaf disks were cut from one leaflet near the leaf used for gas exchange measurements and sealed in three psychrometers within 3 sec. Psychrometers were placed in a 30°C water bath for 3 hr, then wetbulb depressions were read in microvohs using a Wescor HP-115 water potential data system (Logan, UT). The microvoh readings were used with calibration equations derived for each psychrometer to calculate water potential values. Osmotic potential was determined by placing the
F L O O D - P R E C O N D I T I O N I N G AND D R O U G H T ON PECAN SEEDLINGS psychrometers containing the leaf tissue in a - 4 0 ° C freezer for 24 hr, then following the procedures outlined above. T u r g o r potential was calculated as the difference in @L and Cs. Leaf relative water content ( R W C ) was determined using three 5-mm diameter leaf disks per tree] 1) Leaf disks were cut and weighed immediately. T h e y were then floated in water for 4 hr, blotted dry, and reweighed. Disks were dried at 80°C for 24 hr and weighed. Relative water content was calculated as (fresh weight - dry weight)/ (turgid weight - d r y weight) x 100. D a t a were analyzed using an analysis of variance.
,c go0 ~ 8a0 ~_
O CONTROL ~ FLOOOEO
7OO
-J ~o ~ 50~ 400 ~ 30o ~ 200
' ~
~jo--------o ~ . o j -
RESULTS A N D DISCUSSION
(a) Flooding Flooded trees had less leaf area and leaf dry weight than unflooded trees 3 days after being returned to well-drained conditions (Table 1). Flooded trees had fewer leaves than unflooded
~
'
~ 100 ~ 0 -DRYS RFTER FLOOOING
(c) Second drought stress Trees were watered when the permanent wilting point was reached, then water was withheld from all trees. Thus trees had either not been exposed to d r o u g h t previously, or were exposed to drought prior to the second d r o u g h t stress. Evapotranspiration, OL, ~ks, ~p, and R W C were determined at d a w n 2, 4, 5, 6 and 7 days after water was withheld. Each treatment combination contained five single-tree replications in a randomized complete block design.
491
Treatment
Leaves produced (No.)
Fro. 1. Influence of flooding on post-flooding evapotranspiration rates. Treatments contain five replications. Vertical bars indicate the standard error of the mean. trees 21 days after being returned to well-drained conditions, but leaf n u m b e r was not affected after 3 days of drained conditions. Trunk, tap root, and fine root dry weights were not affected by flooding. An earlier report indicated that root dry weight was reduced by 8 3 % when trees were flooded for 31 days] l°/ Apparently 14 days of flooding was not sufficient to cause root loss or inhibit root growth when returned to drained conditions since root dry weight was not affected. Trees that had been flooded had greater evapotranspiration rates than unflooded trees 3 and 7 days after transfer to well-drained conditions (Fig. 1). Evapotranspiration rates were not affected
531 ±55I" 253±39*
6.4±0.2 6.8±0.7
Dry weights (g) Leaves
Trunk
Fine roots
Tap root
3.8±0.2 3.8-t-0.1
2.9___0.3 2.5±0.1
8.8_+ 1.1 13.1±1.1
3.7±0.2 4.1_+0.2
15.54-0.5 15.14-0.4
3 days after transfer Control Flooded Control Flooded
1404±58 1317±76
1
TERMINRTEO
Table 1. Influence of rootflooding on leaf area, leaf number, and leaf, trunk,fine root and tap root dry weights of seedlings of pecan Leaf area (cm2)
,
2.5_+0.2 1.2±0.1"
21 days after transfer 10.04-0.3 8.7±0.3 4.64-0.'1 9.2±0.4* 8.4±0.5 4.7_+0.1
* Significantly different from the control, 5 % level. I" Mean of five replications _ the standard error of the mean.
492
M . W . SMITH and S. M. HUSLIG
Table 2. Influence offlooding for 14 daysfollowed by 23 days recovery, then drought stressed 2, 4, or 6 days on photosynthesis, transpiration, leaf conductance to C02, and leaf internal C02 concentration; analysis of variance of main effects and interactions Mean squares Source
A
Flood Drought Flood x drought Error
6.52 0.16 0.05 12.57
Water withheld 2 days 0.02 127.3 0.02 0.1 0.05 99.5 1.17 2209.1
783 839 214 716
Flood Drought Flood x drought Error
8.5 170.3" 13.1 17.9
Water withheld 4 days 2.8 4101" 7.2** 10,601"* 0.2 46 0.5 491
735 3054 244 l 1998
Flood Drought Flood x drought Error
2.0 387.6*** 7.9 22.3
Water withheld 6 days 1.2 1194 20.4*** 23,336*** 0.2 628 0.9 1128
4801 23,722* 494 3786
*
E
gL
ci
** *** Significant at 5°/; (*), 1 (~,Ji~(**), or 0.1% (***)
by flooding t r e a t m e n t from 12 to 20 days after flooding. W A z m et al. ~°) reported that stomatal resistance of pecan d u r i n g flooding was greater in flooded than unflooded trees t h r o u g h 22 days of flooding, and SMITH and AeER (9/ found that flooded pecan trees h a d higher OL a n d lower gL than those of unflooded trees. I n both earlier reports, (9'~°i and the present study, leaves did not wilt while trees were flooded. This indicates that while trees were flooded, s t o m a t a were at least p a r t i a l l y closed, and leaves were turgid. However, when trees were transferred to well-drained conditions, s t o m a t a a p p a r e n t l y opened, but were unable to regulate w a t e r loss for a few days, since e v a p o t r a n s p i r a t i o n rates of trees that h a d been flooded were over twice as great as those of trees not flooded. Root flooding did not affect the gas exchange p a r a m e t e r s 23 days after trees h a d been transferred to d r a i n e d conditions then exposed to d r o u g h t stress, except gL was lower in trees that had been flooded after 4 days of d r o u g h t stress (Table 2). T h e r e were no interactions between flooding and d r o u g h t stress. SMITh a n d AoEI~/9/ reported that flooding of pecan trees for 15 days
reduced A by a b o u t 5 0 ° , and that A r e m a i n e d a b o u t 25~yo lower than that of unflooded trees 14 days after transfer to d r a i n e d conditions. T h e a d d i t i o n a l 9 days of well-drained conditions allowed complete recovery of A. (b) First drought stress L e a f R W C and gas exchange were not affected by 2 days of d r o u g h t (Table 3). However, 4 days of d r o u g h t reduced R W C by 4.2% (Table 3) and decreased ~L and ~Op, but Os was not affected (Table 4). Photosynthetic CO2 assimilation and E were 44 and 51 0/0 lower than that of control trees, respectively, after 4 days of drought. Stom a t a l c o n d u c t a n c e of stressed trees decreased 56 ~/o, but ci was not significantly different from control trees. W h e n water was withheld for 6 days, trees were n e a r the p e r m a n e n t wilting point. Relative w a t e r content was almost 10°/0 lower than that of control trees, and ~kL, ~ , and 0p were lower ( - 1 . 9 3 , - 2 . 0 6 and 0.13 M P a , respectively) than those of control trees ( - 1.43, - 1.73 and 0.30 M P a , respectively). Photosynthetic CO2 assimilation, E , gL, a n d cl of stressed trees were 63, 77, 76 and 4 2 % lower than those of control
F L O O D - P R E C O N D I T I O N I N G AND D R O U G H T ON PECAN SEEDLINGS
493
Table 3. Influence of&ought stress on photosynthesis transpiration, leaf conductance to CO2, and leaf internal CO2 concentration of seedlings of pecan
Treatment
A (pmol m -~ sec J)
E (mmol m 2 sec i)
gl. (mmol m -2 sec-i)
Water withheld 2 days 1114-9 1114-19
cl (pl/l)
R WC (0,~)
176+_9 163+13
92.5+l.2 93.14-1.4
Irrigated Not irrigated
16.02 4- 1.01 ~ 16.20+_1.25
2.99_+0.27 2.924-0.42
Irrigated Not irrigated
13.884- 1.32 7.824-0.97*
Water withheld 4 days 3.05 4-0.32 1064- 12 1.51 _+0.14"* 474-4**
197_ 11 1664- 12
92.5+ 1.2 88.34-0.9*
Irrigated Not irrigated
14.00+2.12 5.194-0.60"**
Water withheld 6 days 2.74+0.37 93_+13 0.64_+0.07*** 224-2***
172_+18 100_+ 16"
95.1+1.7 85.3_+1.6"
*, **, *** Significantly different from the control at 5°/i~ (*), 1 °/o/ (**), or 0.1 o~ (***). ~ Mean of 10 replications 4- the standard error of the mean. trees, respectively. These data indicate that moderate drought stress reduced A by limiting the mesophyll assimilation capacity since ci was not decreased significantly, but A decreased 6 3 0 . Further drought stress, near the permanent wilting point, decreased A, gL, and ci, indicating that both mesophyll assimilation capacity and CO2 diffusion into the leaf were limiting. MOLDAU/a/ concluded that reductions in A caused by drought stress were due to partial stomatal closure, since desiccation of bean leaves from - 0 . 5 to - 1 . 3 M P a did not affect mesophyll conductance. However, HUTMACHER and KRIEC,,(6) working with cotton, found that drought stress (up to - 3 . 1 MPa) caused greater reductions in A than gL, indicating that the mesophyll assimilation capacity was reduced. Similarly, drought-stressed Zea mays Ill' or Glycine max 15i had lower A and gL than those of control plants, but ci remained almost constant, indicating that the mesophyll assimilation capacity limited A more that leaf conductance to C O 2. (c) Second drought stress Trees that had been previously exposed to drought stress reached the permanent wilting point 2 days after those that had not been drought stressed (Table 3), although trees in both treatments used the same total a m o u n t of water during the drought stress period (1202 ml/tree vs 1227 ml/tree). Drought-stressed trees had lower evapo-
transpiration rates than those of control trees 4 days after water was withheld. Relative water content was 85 °/o when trees that had not been exposed to drought reached the permanent wilting point, but previous exposure to drought lowered their R W C to 8 0 % at permanent wilting point. The decline in R W C of unstressed trees was linear, whereas the decline in R W C of previously stressed trees was curvilinear. Trees that had been exposed to drought stress had higher 0p and lower 0~ than unstressed trees when the second drought was initiated, but OL was not affected. Trees that had been exposed to drought maintained a positive pressure potential 2 days longer than did unstressed trees. Results of this study indicate that although flooding caused m a n y deleterious effects on pecans, trees recovered quickly if the flooding duration was not excessive. High evapotranspiration rates following flooding suggest that stomatal regulation was impaired. This could cause leaf damage by desiccation if the leaf-toair vapor pressure gradient were high, and root conductance to water were decreased] z) Pecan response to drought was similar to that of other plant species. (5'6'11/ Leaf photosynthetic C O 2 assimilation rate declined as the severity of drought stress increased. The decline in A during moderate stress was primarily due to a reduction in mesophyll assimilation capacity, then as the severity of stress increases A is limited by reduced
494
M.W.
SMITH
a n d S. M . H U S L I G
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FLOOD-PRECONDITIONING AND D R O U G H T ON PECAN SEEDLINGS mesophyll assimilation capacity a n d partial stomatal closure resulting in decreased CO2 diffusion into the leaf. Pecans which had been d r o u g h t stressed had lower rates of water loss t h a n those that were not exposed to d r o u g h t stress. Tissue h y d r a t i o n u p o n rewatering after d r o u g h t stress was not significantly different from that not exposed to drought.
Plant physiology 5.
6.
7. REFERENCES
1. BARR H. D. (1968) Determination of water deficits in plant tissues. Pages 235 267 in T. T. KOZLOWSKI, ed. Water deficits and plant growth. Academic Press, New York. 2. BRADFORDK.J. and Hsmo T. C. (1982) Stomatal behavior and water relations of waterlogged tomato plants. Pl. Physiol. 70, 1508 1513. 3. FARQUHARG. D. and SHARKEY T. D. (1982) Stomatal conductance and photosynthesis. A. Rev. PI. Physiol. 33, 317-345. 4. FARQUHARG. D. and VON CAEMMERERD. (1982) Modeling of photosynthetic response to environmental conditions. Pages 549-587 in O. L. FANGE, P. S. NOBEL, C. B. OSMONDand H. ZIEGLER, eds
8.
9.
10.
11.
495
New series, Vol. 12B. Springer, Berlin. HUBER S. C., ROGERS H. H. and MOWRY F. L. (1984) Effects of water stress on photosynthesis and carbon partitioning in soybean (Glycine max (L.) Merr.) plants grown in the field at different CO2 levels. Pl. Physiol. 76, 244-249. HUTMACHER R. B. and KmEo D. R. (1983) Photosynthetic rate control in cotton. P1. Physiol. 73, 658-661. LOUSTALOTA.J. (1945) Influence of soil moisture conditions on apparent photosynthesis and transpiration of pecan leaves. J. Agric. Res. 71, 519532. MOLDAU H. (1973) Effects of various water regimes on stomatal and mesophyll conductances of bean leaves. Photosynthetica 7, 1-7. SMITh M. W. and AGER P. L. (1988) Effects of soil flooding on leafgas exchange of seedling pecan trees. HortScience 23, 370-372. WAZm F. K., SMITH M. W. and AKERS S. W. (1988) Effects of flooding and soil phosphorus levels on pecan seedlings. HortScience 23, 595- 597. WONG S., COWAN I. R. and Farquhar G. D. (1985) Leaf conductance in relation to rate of CO 2 assimilation. III. Influences of water stress and photoinhibition. Pl. Physiol. 78, 830 834.