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Published in Agron J 100:711-719 (2008)
DOI: 10.2134/agronj2007.0188
© 2008 American Society of Agronomy
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POTATO

Interactive Effects of Carbon Dioxide and Water Stress on Potato Canopy Growth and Development

David H. Fleisher*, Dennis J. Timlin and Vangimalla R. Reddy

USDA-ARS Crop Systems and Global Change Lab., 10300 Baltimore Ave., Beltsville, MD 20705. Mention of a trademark or proprietary product does not constitute a guarantee or warranty of the product by the USDA and does not imply the exclusion of other available products

* Corresponding author (david.fleisher{at}ars.usda.gov).


   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Reductions in potato (Solanum tuberosum L.) canopy growth are observed with mild water stress. Potato growth is enhanced by elevated atmospheric carbon dioxide ([CO2]), but interactions of [CO2] and water stress on canopy formation and dry matter partitioning have not been studied. Two soil-plant-atmosphere research (SPAR) experiments were conducted at 370 or 740 µmol mol–1 [CO2] and six different irrigations from 10 to 100% of the daily water uptake of the control. Increases in plant length from 23 to 111 cm at 60 d after emergence (DAE), leaf appearance duration from 38 to 71 d, leaf appearance rate from 0.5 to 0.93 leaves d–1, individual leaf area from 50 to 175 cm2, and lateral branch elongation were observed as irrigation increased. Values were generally smaller for elevated [CO2] plants under water stress. Biomass increased with irrigation from 73 to 346 g plant–2. The percentage allocated to the canopy increased with irrigation from 50 to 80% in ambient and 30 to 80% in elevated [CO2]. Despite decreased canopy size, elevated [CO2] plants produced similar total biomass, but higher yield, at most irrigations. Reduced canopy mass in elevated [CO2] plants was attributed to suppressed lateral branch development due to an interactive effect of [CO2] and water stress on tuber sink strength. These results indicate that water stress predicted by climate change models will be mediated somewhat under [CO2] enrichment.

Abbreviations: AS, apical lateral branch • DAE, d after emergence • L, compound leaf length • MS, main stem • SPAR, soil-plant-atmosphere research • W, width of the terminal leaflet


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher.

Received for publication June 3, 2007.
   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
POTATO IS DROUGHT SENSITIVE with even mild levels of water stress limiting canopy formation (van Loon, 1981; Jefferies and MacKerron, 1987; Gregory and Simmonds, 1992; Jefferies, 1993; Lahlou et al., 2003) and thus, net assimilation rate, over the course of the season (Susnoschi and Shimshi, 1985). As with other crops having the C3 biochemical pathway, potato seasonal net assimilation and total biomass production increases with [CO2] enrichment (Wheeler et al., 1991; Miglietta et al., 1998; Sicher and Bunce, 1999; Wheeler et al., 1999; Schapendonk et al., 2000; Conn and Cochran, 2006; Fleisher et al., 2007). Elevated [CO2] can partially alleviate long-term whole plant responses to water stress (Bhattacharya et al., 1990; Eamus, 1991; Baker et al., 1997; Wall et al., 2006; Fleisher et al., 2007). However, the manner in which potato canopy formation is influenced by the interaction of [CO2] and water stress, and how those dynamics are correlated with whole plant growth, have not been studied. Such interactions may play an important role in studying agricultural management and production under various scenarios of global climate change (Chaves and Oliveira, 2004).

Water stress primarily reduces potato productivity by limiting canopy expansion (Vos and Groenwold, 1988; Jefferies, 1995; Lahlou et al., 2003) and can delay tuber initiation and bulking (Wright and Stark, 1990; Costa et al., 1997; Bélanger et al., 2001; Walworth and Carling, 2002). Canopy development effects include decreased leaf size (Jefferies and MacKerron, 1987; Jefferies, 1989; Jefferies, 1993; Deblonde and Ledent, 2001) and leaf expansion rate (Jefferies, 1993), increased senescence rate and limited formation of new leaves (van Loon, 1981), and decreased stem height (Deblonde and Ledent, 2001). Jefferies (1989) observed both an increase and decrease in leaf appearance rate in droughted plants and explained the increase as a likely result of a higher leaf temperature in water-stressed leaves. In a study of potato canopy architecture, Schittenhelm et al. (2006) measured large differences among cultivars in terms of shoot and leaf dry weight, stem length, and leaf number in response to rainfed versus irrigated production. They concluded that cultivars that form more aboveground biomass in response to drought can produce larger yields. Despite these studies, there is a surprising lack of information on other aspects of canopy growth responses to drought. Most potato varieties are indeterminate (Allen and Scott, 1992; Struik and Ewing, 1995) and canopy development is largely dependent on appearance of lateral branches and leaf appearance on those branches (Allen and Scott, 1992; Vos and Biemond, 1992; Vos, 1995). Most published studies have focused primarily on studies of leaf dry matter accumulation, area expansion over time, and appearance on the main stem. In fact, Deblonde and Ledent (2001) indicate that leaf growth on upper (apical) and lower (basal) lateral branches in response to drought can play a significant role in maintaining leaf ground coverage and needs to be studied.

Developmental responses to [CO2] enrichment have not been well documented for most crops (Baker and Allen, 1994; Miglietta et al., 1996). Earlier flowering in wheat and sunflower have been observed in response to elevated [CO2] (Goudriaan and de Ruiter, 1983; Marc and Gifford, 1984). Baker et al. (1990) observed accelerated time to anthesis, shortened growth duration, decreased main stem leaf number, and leaf appearance rate in rice. Miglietta et al. (1998) reported accelerated flowering and leaf senescence in potato between ambient and elevated [CO2] in a free-air [CO2] enrichment study but did not observe differences in plant height, leaf number, and mid-season leaf area index. Fleisher et al. (2006) observed increased potato leaf appearance rates with elevated [CO2] and average 24-h temperatures in excess of 28°C. Wheeler et al. (1991) found that morphological differences in potato (e.g., leaf, stem, plant height characteristics) grown at varying levels of light, photoperiod, and [CO2] were due primarily to light and photoperiod. Schapendonk et al. (2000) did not observe differences in canopy closure, but found that leaf growth and senescence varied with [CO2], cultivar type, and the light environment. Finally, Conn and Cochran (2006) did not find differences in leaf number, height, or width of well-watered potato plants grown at three different levels of [CO2].

The focus of this study was to evaluate the interaction of [CO2] and water stress on potato canopy growth and development. Objectives include evaluation of (i) the effect of water stress on leaf appearance and expansion, whole plant length and elongation rate, lateral branching patterns, the duration of canopy development, and biomass partitioning at harvest, and (ii) how these factors are modified by [CO2] enrichment. Research results will support studies involving potato production under global climate change scenarios.


   MATERIALS AND METHODS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
SPAR Chambers
Two experiments were conducted concurrently at 370 and 740 µmol mol–1 [CO2]. Each experiment used SPAR chambers at U.S. Department of Agriculture–Agricultural Research Service facilities in Beltsville, MD, in the summer of 2005. SPAR chambers consisted of transparent chamber tops, 2.2 m by 1.4 m by 2.5 m (length, width, height) constructed of 0.0127-m-thick Plexiglas and were similar to systems at the University of Florida (Pickering et al., 1994), Corvallis, OR (Tingey et al., 1996), and Mississippi State University (Reddy et al., 2001). SPAR chambers were located outside and transparent to natural sunlight. Each SPAR chamber top was mounted to a steel soilbin measuring 2.0 m by 0.5 m by 1.0m (length, width, depth). A dedicated Sun SPARC 5 work station (Sun Microsystems, Inc., Mountainview, CA) logged environmental data (air and soil-media temperatures, relative humidity, [CO2], and photosynthetically active radiation above and below the canopy) every 300 s. The SPAR chambers also measured canopy [CO2] flux as described in detail by Baker et al. (2004).

Soilbins were filled in layers (approx 0.15 m thick) with a 75% sand/25% vermiculite (Grace Construction Products, Cambridge, MA) soil mix (by volume) and saturated with tap water as each layer was added. Time-domain reflectometry waveguides (15 cm, 3 rod) were installed horizontally in each soilbin to calculate daily plant water uptake and irrigation amount required in each chamber (Timlin et al., 2007).

Plant Culture
Time release fertilizer (Osmocote 14–14–14, The Scotts Company, Cleveland, OH) was incorporated in the top 5 cm of the soil in each chamber at a rate of 60 g m–2 before planting. Certified potato (‘Kennebec’) seed tubers (54.9 ± 10.04 g mean fresh weight) were planted so that a minimum of 5 cm was maintained above each tuber in six rows (12 plants m–2). All SPAR chambers were maintained at a 16 h 23°C day/8 h 18°C night thermoperiod. Daytime [CO2] was controlled at either 370 or 740 µmol mol–1 for the six SPAR chambers in each [CO2] treatment. Night time [CO2] was uncontrolled and averaged 536 ± 55.7 µmol mol–1 for all chambers. Photoperiod averaged to 14.1 h during the course of the experiment. After emergence, potato plants were hand-pruned to a single main stem and transparent plastic film (4 mil thickness) was used to cover the top of the soil around each plant to minimize evaporation from the soil surface.

Irrigation was supplied in the form of 1/2 strength Hoagland's solution (Hewitt, 1952) and distributed by drippers arranged in three 2.0-m rows with 0.20-m spacing between rows in each chamber. The drippers were spaced at 0.10-m intervals. Irrigation treatments were initiated at 21 DAE, corresponding to main stem flowering. Varying amounts of irrigation were provided to each SPAR chamber on a daily basis according to 90, 75, 50, 25, and 10% of the daily water uptake measured from the control chamber (100%). Separate control chambers were used for the two [CO2] experiments. Harvest times were selected when canopy photosynthetic rates dropped to below 50% of their peak value. The 90%, elevated [CO2] treatment was harvested earlier than this point due to rapid canopy senescence as a result of verticillium wilt first noted on DAE 94. Harvest dates, irrigation, and average 24-h conditions over the course of the experiment are summarized in Table 1 .


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  		Table 1. Harvest date, irrigation, and average seasonal 24-h environmental conditions per irrigation treatment (IRR) at 370 µmol mol–1 (ambient [Amb.]) and 740 µmol mol–1 (elevated [Elv.]) [CO2] from planting through harvest. Standard deviations are indicated where appropriate.

 
Data Collection
Biweekly measurements of leaf appearance, stem elongation, and leaf expansion were made on main stem (MS) and two apical lateral branch (AS) stems for five plants per chamber. Leaf expansion was measured on the 5th, 10th, and 12th MS nodes (from the soil surface), and the 2nd and 5th node (from the insertion point on the MS) on both AS stems. The measurements consisted of compound leaf length (L; cm), defined as the distance from the stem to the tip of the compound leaf, and width of the terminal leaflet (W; cm) as in Fleisher and Timlin (2006). Leaf areas were measured with a LI-COR 3100 area meter (LI-COR, Inc., Lincoln, NE) on selected leaves as in Benoit et al. (1986) and Vos and van der Putten (1998). A linear relationship between L, W, and leaf area was obtained (predicted area = 0.744 LW; r2 = 0.96, n = 143) and used to convert the L and W measurements into leaf area estimates. An initial leaf length of 0.5 cm was used to indicate the start of leaf appearance for new leaves (Fleisher et al., 2006). Stem lengths were measured for the MS and AS stems by temporarily straightening the desired branch against a ruler. Total plant length was computed as MS length plus the longer of either AS stems.

At harvest, all senesced leaf material was gathered and recorded for each chamber. Root weights were estimated on a chamber basis from root cores that were taken at five different depths at four horizontal positions in each soil layer and included stolon weights. Plant organs were separated according to stem type (main, apical, or basal lateral stems), remaining green leaf, and tubers. Stems and leaves were divided based on their branching order (i.e., the number of branches between the current stem and the main stem plus one). All plant material was dried to constant weight in forced air ovens at 70°C.

Data Analysis
Observational data were grouped into categories for leaf appearance, stem elongation, leaf expansion, and end of season branch characteristics and dry matter. Leaf appearance rates were estimated with the slope from regression equations relating leaf number versus days after emergence. Separate rates were evaluated for MS and the total plant (MS+AS). Leaf appearance duration was observed as the time from plant emergence to the last leaf to appear on the apical branches. A modified form of the Gompertz growth equation (Eq. [1]) (Thornley and Johnson, 1990, p. 78–82) was fit to stem elongation and leaf area data versus time for each of five plants per chamber:

Formula 1[1]
where X0 = initial leaf area or stem length at appearance (0.05 cm2 or 0.05 cm); Xf = final leaf area or stem length (cm2 or cm); X = leaf or stem length at current time increment (cm2 or cm); D = decay in specific expansion rate (d–1); DAA = days after leaf or stem appearance (d).

The NLIN procedure in SAS (The SAS system for Windows, 9.01, SAS Institute, Inc., Cary, NC) was used to obtain parameter values using the Gauss-Newton nonlinear least squares iterative method. Estimated values for final leaf area or stem length (Xf) and the time from appearance to 95% of Xf (Txf-95), or growth duration, were obtained from the fitted Gompertz curves. Rapid leaf expansion, or stem elongation, rate, Rmax, was estimated based on the average slope of the linear portion of the Gompertz curves. For leaves, this linear portion was defined as the time between leaf appearance and 80% Xf (Kirk and Marshall, 1992). For stems, the linear portion of the curve fell within 20 and 80% Xf. Values for Xf, Txf-95, D, and Rmax were averaged on a per chamber basis.

Two-way analysis of variance was performed with the SAS procedure GLM (The SAS System for Windows, 9.01, SAS Institute, Inc., Cary, NC). Least significant differences (P = 0.05) for comparing treatment means were calculated only for variables that had significant F-tests for both main effects (irrigation and [CO2]). Contrasts were used to compare [CO2] means across individual irrigation amounts when interaction terms were significant at P < 0.1. Regression analysis was used to analyze the response of some of the dependent variables versus end of season irrigation amount and [CO2] level. GLM was used to calculate the coefficients of the regression and compare linear and nonlinear regression lines between the [CO2] treatments where appropriate. Regression lines that were significantly different from one another were noted in the appropriate tables and figures.


   RESULTS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Plant Length and Stem Elongation
The average measured total plant lengths at 60 DAE were 111, 108, 87, 76, 63, and 57 cm for ambient [CO2] at 100, 95, 75, 50, 25, and 10% irrigation, respectively, and 110, 103, 74, 66, 37, and 23 cm for elevated [CO2] (Fig. 1 ). These lengths were significantly reduced by water stress within each [CO2] level (Table 2 ), with similar patterns in end of season total plant length (not shown). Differences in total plant length between [CO2] treatments were also apparent as irrigation levels dropped below 75%, as the irrigation x [CO2] interaction was significant at P < 0.1. Final stem length was 33, 53, and 61% larger for ambient versus elevated [CO2] at 50, 25, and 10% irrigation levels, respectively. Total plant stem growth duration also increased with irrigation amount (Table 2). A significant interaction between [CO2] and irrigation amount was observed where elevated [CO2] plants took less time to reach their final lengths as irrigation decreased below 75%. Elevated [CO2] also reduced apical stem growth duration at lower irrigation amounts. Thus, elevated [CO2] only decreased stem length and elongation duration as water stress increased. No significant differences were observed between well-watered plants grown at different [CO2] levels as in Conn and Cochran (2006) and Miglietta et al. (1998).


Figure 1
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  		Fig. 1. Total plant length versus days after emergence for potato plants grown at ambient (370 µmol mol–1) and elevated (740 µmol mol–1) [CO2] at six different levels of water stress (expressed as % of control irrigation). Lines are the fit of the Gompertz equation (Table 2). Data points are averages of measured length of five plants (standard deviations shown).

 

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  		Table 2. Gompertz equation{dagger} predictions for total plant, apical stem, and main stem length, growth duration, and rapid elongation rate for potato grown at 370 µmol mol–1 (ambient [Amb.]) and 740 µmol mol–1 (elevated [Elv.]) [CO2] at different irrigation levels (IRR). Values are averages of five plants.

 
Total plant rapid elongation rates largely mirrored that of the apical stem. Both rates declined with irrigation amount (Table 2). Both total plant and apical stem rates were significantly smaller for the elevated [CO2] plants at the 25 and 10% irrigation levels. Main stem elongation rates were not correlated with irrigation but were slightly higher for ambient [CO2] plants (data not shown). When averaged across irrigation treatment, main stem elongation rates were significantly faster for ambient (1.0 ± 0.02 cm plant–1 d–1) versus elevated [CO2] (0.9 ± 0.02 cm plant–1 d–1) plants (P < 0.05). Overall, the shorter total plant lengths observed at the higher water-stress treatments between [CO2] treatments were attributed to smaller maximum elongation rates and durations due to interacting effects of [CO2] and irrigation on elongation rates and duration.

Leaf Appearance and Duration of New Leaves in the Canopy
An average of 18 d was observed from plant emergence to the appearance of leaves on AS for all [CO2] and irrigation treatments (data not shown). After this time, MS leaf appearance ceased, but new leaves continued to emerge on AS and other lateral branches. The length of time that leaves continued to form on the AS over the course of the season (leaf appearance duration) declined with irrigation at both [CO2] levels (Table 3 ). The ambient [CO2] duration was not affected by irrigation when the 10% irrigation treatment was removed from the analysis and averaged 70.5 ± 8.82 d. Elevated [CO2] plants exhibited a significant decline with irrigation (y = 0.06x + 29; r2 = 0.66). Durations were generally longer for ambient [CO2] plants at all irrigations, and this difference increased with water stress.


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  		Table 3. Leaf appearance duration, rate, and total number of leaves appearing over the course of the season for potato grown at 370 µmol mol–1 (ambient [Amb.]) and 740 µmol mol–1 (elevated [Elv.]) [CO2] at different irrigation levels (IRR). Values are averages of five plants.

 
Main stem leaf appearance rates were not significantly affected by irrigation or [CO2] and averaged 0.62 leaves plant–1 d–1 (r2 = 0.8; n = 352) for all chambers (data not shown). This rate was within the range of rates reported in Fleisher et al. (2006). After MS flowering, leaf appearance rates declined slightly with irrigation, but were not affected by [CO2] (Table 3).

The total number of leaves produced over the course of the season (Table 3) was less for elevated [CO2] plants. This result was due to the shorter canopy leaf appearance durations for the elevated [CO2] treatment. As with leaf appearance duration, elevated [CO2] linearly increased with irrigation (y = 0.07x + 22; r2 = 0.71) while ambient [CO2] leaf numbers were not correlated with irrigation when the 10% irrigation treatment was removed from analysis (62.1 ± 8.32 d).

Leaf Expansion
Final leaf areas (Xf) and area expansion rates (Rmax) increased with irrigation (Fig. 2 ). The Xf of MS leaves was smaller for potatoes grown under elevated [CO2] (Fig. 2A). Apical stem Xf showed an interaction effect with [CO2] and irrigation as indicated by the non-common slope (Fig. 2C). The Xf for both stem types followed the same relationships indicated by the leaf expansion rates (Fig. 2). Leaf expansion durations (Txf-95) were not significantly influenced by irrigation (data not shown). MS expansion durations were also not influenced by [CO2] and took an average 22 ± 2.2 d to reach final size. Expansion duration of AS leaves was 22 ± 4.3 d for ambient and 21 ± 5.2 d for elevated [CO2]. Thus, differences in final leaf sizes between [CO2] treatments were primarily due to variations in expansion rate.


Figure 2
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  		Fig. 2. Individual final leaf area [Xf, (A) and (C)] and leaf expansion rates [Rmax, (B) and (D)] versus irrigation amounts for mainstem and apical stem leaves on potato plants grown at 370 and 740 µmol mol–1 [CO2]. Error bars are standard deviations of five plants per observation. Lowercase letters indicate significantly different regression coefficients.

 
End of Season Branch Characteristics
Average lateral branch length, average leaf number per lateral branch, and branching order were influenced by irrigation (Table 4 ). Values for lateral branch number, length, and branching order were smaller for elevated [CO2] plants as compared with ambient [CO2] plants, particularly at lower levels of irrigation (50% and less) where differences between means were significant. Regression analysis indicated larger slopes, but smaller intercepts, of elevated versus ambient [CO2] treatment responses for average lateral length (elevated: y = 0.3x + 4; r2 = 0.9; ambient: y = 0.2x + 38; r2 = 0.7) and branching order (elevated: y = 0.01x + 1; r2 = 0.6; ambient: y = 0.001x + 3; r2 = 0.3) versus end of season irrigation amount. Average number of leaves per branch was unaffected by either [CO2] level. These results suggested that lateral branch formation was suppressed under elevated [CO2] and low irrigation amounts.


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  		Table 4. End of season average lateral branch number (lat num.), average lateral branch length (lat length), average leaf number per lateral branch (leaf num.), and degree of branching (order) for potato grown at 370 µmol mol–1 (ambient [Amb.]) and 740 µmol mol–1 (elevated [Elv.]) [CO2] at different irrigation levels (IRR). Values are averages of five plants.

 
Dry Matter Partitioning
End of season biomass increased with irrigation in both [CO2] treatments (Table 5 ). Total biomass was similar between [CO2] treatments at a given irrigation level. However, the ambient [CO2] treatments partitioned more dry mass to the canopy at most levels of irrigation than elevated [CO2]. The significant interaction between [CO2] and irrigation for aboveground dry mass indicated that proportionately more dry mass was partitioned to belowground organs for elevated [CO2] as irrigation decreased. In fact, partitioning between above- and belowground organs was clearly influenced by irrigation, with the ratio of belowground dry mass to total biomass increasing from about 0.2 to between 0.45 (for ambient [CO2]) and 0.65 (for elevated [CO2]) as irrigation decreased from the 100% to the 10% treatment (Table 5). Thus, in general, the relative amount of aboveground dry mass decreased, and belowground dry mass increased, in response to elevated [CO2] and increased water stress.


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  		Table 5. End of season total, above, and below ground biomass for potatoes grown at 370 µmol mol–1 (ambient [Amb.]) and 740 µmol mol–1 (elevated [Elv.]) [CO2] at different irrigation levels (IRR). Values are averages of 12 plants.

 
Consistent with the aboveground dry mass results (Table 5), stem and leaf mass increased with irrigation amount for both [CO2] treatments (Table 6 ). However, main and first order stem mass and total leaf mass, were smaller for elevated versus ambient [CO2] treatments (Table 6). As irrigation increased, leaf mass for ambient [CO2] plants increased to a greater extent than elevated [CO2]. The contribution of second and higher order stems was not significantly different between [CO2] treatments. Thus, the reduction in aboveground dry mass observed for elevated [CO2] plants was primarily a result of decreased leaf and first order stem dry mass.


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  		Table 6. Main and first order stem (stem 1), 2nd order stem (stem 2), and total leaf mass for potato grown at 370 µmol mol–1 (ambient [Amb.]) and 740 µmol mol–1 (elevated [Elv.]) [CO2] at different irrigation levels (IRR, % of control). Values are averages of 12 plants.

 

   DISCUSSION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
CONCLUSIONS
 REFERENCES
 
Stem growth and development played a major role in potato canopy development. The reduction in plant length and elongation rates with water stress has been observed in previous research (Trebejo and Midmore, 1990; Deblonde and Ledent, 2001). In our study, maximum main stem lengths ranged from 21 to 35 cm, less than half of the total plant length at 60 DAE for all treatments except the elevated [CO2] treatments at 10 and 25% irrigation (Fig. 1). Thus, the development of the apical stem played the largest role in determining total plant length. At increasing levels of water stress, apical branch elongation and duration were shortened (Table 2), particularly for elevated [CO2] plants. The development of other lateral branches exhibited similar patterns, as average lateral branch length and branching order were also reduced as irrigation decreased (Table 4). These patterns were reflected in the decrease in dry matter partitioned to the canopy and the relative contribution of secondary stem mass to the canopy as drought increased (Tables 5, 6).

Leaf appearance rates declined with irrigation (Table 3). Potato leaf appearance rates were also influenced by water stress in studies by Jefferies (1989) and Struik and Ewing (1995). Despite slightly higher canopy leaf temperatures (0.8 to 0.9°C) for the elevated [CO2] plants for several irrigation treatments (presumably due to decreased stomatal conductance, data not shown), differences between [CO2] treatments were not detected. The duration of leaf appearance in the canopy decreased with water stress and was further suppressed under elevated [CO2] at 72, 25, and 10% irrigation levels (Table 3). Total leaf numbers were significantly less at harvest for elevated [CO2] plants at the low irrigation levels due to this decreased time. The decline in leaf duration with water stress was likely due to the decrease in lateral branch number and branching order, particularly for elevated [CO2] plants at low irrigation (Table 4). As a result, fewer sites for leaf formation were available, effectively decreasing the duration of leaf formation in the canopy and total leaf number at harvest.

Similar to Jefferies (1993), final leaf size closely mirrored expansion rate for leaves on all stems and was proportional to irrigation in this study (Fig. 2). The decrease in leaf area observed on main and apical stems for elevated versus ambient [CO2] plants at the lower irrigation levels was surprising. Fleisher and Timlin (2006) measured an increase in final leaf area in main stem leaves of potato plants grown at elevated [CO2] and Wheeler et al. (1991) also measured a positive response of [CO2] enrichment on potato leaf size. In the current study, curling of the tips of newly emerged main stem leaves in the elevated [CO2] treatments was observed, which may have limited some of the early leaf expansion in the elevated [CO2] treatments.

At both [CO2] levels, biomass increased with irrigation amount as in Trebejo and Midmore (1990), Costa et al. (1997), Deblonde and Ledent (2001), and Walworth and Carling (2002). We did not observe an increase in total dry mass with [CO2] enrichment (Table 5) as reported for potatoes grown in well-watered conditions (Miglietta et al., 1998; Schapendonk et al., 2000; Conn and Cochran, 2006; Wheeler, 2006). However, more dry mass was partitioned to aboveground organs for ambient [CO2] plants at most irrigation levels while tuber yields were larger for elevated [CO2] plants at the lower irrigation amounts. Our study indicated that these differences in canopy growth and development became significant between [CO2] treatments as irrigation dropped below 100% demand. The reductions in canopy growth and development with increasing water stress and elevated [CO2] were reflected in the distribution of stem and leaf mass at harvest (Table 6). Stem contributions to canopy mass were smaller for elevated [CO2] treatments except the 100% irrigation level (Table 6). Total apical stem mass was also consistently lower for elevated [CO2] (not shown). These responses supported the reduced lateral stem development reported in Table 4.

Root-to-shoot ratios generally increase with drought (Trebejo and Midmore, 1990; Jefferies, 1995; Bélanger et al., 2001) and elevated [CO2] (Bhattacharya et al., 1990; Farrar and Williams, 1991; Conn and Cochran, 2006). Our study followed the same pattern, with root-to-shoot ratios ranging from 0.04 to 0.5 for ambient, and 0.03 to 0.71 for elevated [CO2] plants at 100 and 10% irrigation levels. However, partitioning to tubers appears to be influenced by cultivar, environment, soil conditions, and timing and severity of drought (Jefferies and MacKerron, 1987; MacKerron and Jefferies, 1988; Dwyer and Boisvert, 1990; Trebejo and Midmore, 1990; Costa et al., 1997; Schittenhelm et al., 2006). In our studies, irrigation treatments did not begin until MS flowering. Thus, responses to water stress most likely occurred following tuber initiation, a point in time that may promote partitioning to tubers as opposed to the canopy (Jefferies, 1995). Decreased lateral branch development, plant elongation, and leaf appearance duration under water stress was likely a response to shifts in dry mass partitioning to tubers.

Our findings indicated that [CO2] enrichment interacted with water stress in a way that increased yield without an equivalent increase in canopy formation at the lower irrigation amounts. In fact, for several irrigation treatments, leaf dry mass was smaller for elevated [CO2] plants (Table 6). To sustain the same total dry mass production as ambient [CO2] plants, photosynthetic rates per unit leaf mass were likely higher for those plants (differences among end of season leaf area were not detected [data not shown]). No studies have been conducted for potato production under drought and elevated [CO2]; however, in a separate paper, Fleisher et al. (2007) measured increased seasonal canopy photosynthetic rate and radiation use efficiency in water-stressed potatoes grown at elevated [CO2] despite common amounts of seasonal light interception between [CO2] treatments. Thus, one reason for the larger amount of photosynthate partitioned to belowground organs could be increased photosynthetic efficiency of [CO2] enriched water-stressed leaves as compared to ambient [CO2].

Source-sink relationships have also been identified as playing a large role in mediating the response of plants to [CO2] (Farrar and Williams, 1991) and may explain the interactive effects of [CO2] and drought on canopy development as well. In a study with elevated [CO2] and drought on sweet potato, Bhattacharya et al. (1990) observed an increase in partitioning to storage organs versus vegetative growth in plants grown with [CO2] enrichment. They suggested that [CO2] enrichment increased sink capacity for storage roots and delayed effects of severe water shortage on growth. In a study of oak seedling responses to elevated [CO2] and water stress, Anderson and Tomlinson (1998) and Tomlinson and Anderson (1998) speculated that the typical enhancement effects of [CO2] (including increased assimilation rate) were greatest during periods of high sink activity. They measured an increase in partitioning to older stems and roots at the expense of new shoot development with elevated [CO2]. Farrar and Williams (1991) noted that the capacity of sinks to continue to grow in size and strength under conditions that produce ample supply of sucrose in the leaves, as would be the case under elevated [CO2], can significantly impact whole plant partitioning and productivity. Work by Basu et al. (1999) indicated that the presence of tubers positively modified the photosynthetic response of water-stressed potato plants by maximizing sucrose assimilation and transport from leaves, thereby preventing a hypothesized drought-induced substrate feedback inhibition of photosynthesis. In our study, the timing of long-term drought coinciding with tuber bulking happens to be the point in time with the largest sink in potato (Allen and Scott, 1992; Basu et al., 1999). Thus, the larger tuber sink in water-stressed potato grown under elevated [CO2] helped to divert additional dry matter away from the canopy. In addition, as a result of increased assimilate production in response to [CO2] enrichment, the tuber sink continued to grow in strength during the course of the season. The result is the shift in partitioning between above- and belowground organs observed with decreased irrigation and elevated [CO2] (Table 5). In response to reduced dry mass available to the canopy, aspects of canopy development, particularly lateral branching and leaf appearance duration were reduced accordingly.

Care must be taken when extrapolating these results directly from a controlled environment study to field production. Nonetheless, the relative responses observed in the study are applicable for crop modeling and evaluating climate change scenarios. The responses indicated that [CO2] enrichment helped mediate negative effects of drought stress on potato productivity despite reductions in canopy growth and development. Global climate change scenarios frequently predict increased [CO2] along with reduced water availability in various crop production regions (Baker and Allen, 1994). Our results indicate that [CO2] interaction with water stress will compensate, to some extent, for anticipated decreased water supply available for potato production under future climate change scenarios.


   CONCLUSIONS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
REFERENCES
 
Potato productivity declines with water stress, primarily due to reduced canopy expansion over the course of the season. In this study, potato canopy growth and development was reduced in potatoes grown at 370 and 740 µmol mol–1 [CO2] in response to decreasing irrigation. General responses to water stress included decreased stem elongation and plant length, leaf appearance and expansion rates, duration of canopy formation, and lateral branch formation and development. Total biomass at the end of the season increased with irrigation amount, with a larger fraction of dry mass partitioned to belowground organs as irrigation decreased at both [CO2] levels. However, end of season total biomass was similar for elevated [CO2] versus ambient [CO2] plants, despite a decrease in partitioning of dry mass to the canopy in the elevated [CO2] treatments as irrigation decreased. We speculated that the presence of a larger tuber sink in the elevated [CO2] plants helped to mitigate water stress effects by minimizing potential drought-induced substrate feedback inhibition of photosynthesis and shifting carbohydrate partitioning away from new canopy development and into the storage organs. These results suggest that sink demand for assimilate supply plays a very large role in mediation of water stress under enriched [CO2] growth conditions. Thus, potential water deficiencies predicted by some global climate change models may be reduced to a certain extent due to [CO2] enrichment. These results can support models and simulations involved in identifying alternative management strategies and production scenarios for potato grown under various global climate change scenarios.

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   REFERENCES
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 





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