Ornamental Plants Annual Reports and Research Reviews 2000

Special Circular 177-01

Effects of Water Availability on Carbon Assimilation and Structural Integrity of Potted Greenhouse Poinsettia (Euphorbia pulcherrima)

John E. Lloyd
Brian A. Kunkel

Introduction

Poinsettias (Euphorbia spp.) were introduced into the United States from Mexico in 1820 and were propagated for commercial use in the early 1900s (Ecke et al., 1990). The predominant ornamental species (Euphorbia pulcherrima) is the main commercially produced potted plant in the United States (Tayama and Hanniford, 1990).

Commercial production of poinsettias is driven by seasonal retail sales. From propagation to flowering and bract formation, poinsettias are provided with elevated levels of nutrients and water in amounts that sustain rapid plant growth (Hammer, 1990; Weiler, 1990; Peterson and Kramer, 1990). At different stages of plant development, poinsettias are treated with chemical growth regulators (paclobutrazol or uniconazole) to reduce apical growth and retain a branched structure (Williams, 1992). Poinsettias grown under this standard production regime tend to have stems and petioles that break easily during shipment and display (Nell et al., 1997).

Most production-based poinsettia research is focused on enhancing plant growth and aesthetic qualities. Many studies examine the impact different nutrient regimes and plant-growth regulators have on plant growth (Ruiz-Sifre et al., 1997; Whipker and Hammer, 1997). Little effort has been devoted to examining the effects of these practices on plant physiology and allocation strategies (Svenson et al., 1995; Schuch et al., 1996).

In the 1980s poinsettia growers experimented with water regimes in an attempt to comply with water-quality standards and reduce the need for numerous applications of plant-growth regulators. Reducing the amount of daily water resulted in reductions in apical growth without decreases in aesthetic plant qualities (Morvant et al., 1998; Schuch et al., 1996; Röber, 1995).

The application of "drought stress" instead of growth regulators during production is now a common practice among progressive growers. This treatment reduces the need for growth regulators and appears to reduce the incidence of plant breakage during shipment and display (Bradshaw, 1999). The impact that drought has on the physiology of the plant to produce these beneficial postproduction characteristics has not been investigated (Nell et al., 1997).

In resource-limited environments, trade-offs occur in plant allocation to apical growth, storage, reproduction, and defense (Cannell and Dewar, 1994; Chapin, 1991; Chapin et al., 1987). Poinsettias may acclimate to water limitation through a shift in resource allocation from apical above-ground growth to root growth, storage, and/or accumulation of non-structural carbohydrates. This reallocation should occur if water stress is severe enough to reduce apical growth, but not at the expense of photosynthesis. Increases in structural integrity could be related to enhanced carbon reserves available from decreased allocation to growth and a reduction in cell elongation in new growth (Bradford and Hsiao, 1982). The carbon produced could be allocated for cell-wall thickening and the formation of secondary metabolites (tannin and lignin) that enhance structural integrity (Lambers et al., 1998; Rhoades, 1985; Halsam, 1988).

The objective of our study was to examine the impact of drought stress on the carbon assimilation of poinsettia and its impact on structural integrity. The authors examined the effect of drought on the physiological parameters of photosynthesis, stomatal conductance, plant water potential, root and shoot biomass, specific leaf area, and petiole strength.

We predicted that a reduction in growth without a severe reduction in photosynthesis would increase structural integrity. If this prediction is true, plants under drought-stress treatments would exhibit symptoms of drought, such as decreases in water potential and stomatal conductance. Photosynthesis would be less impacted by drought, as photosynthetic water-use efficiency increases (amount of carbon gain per unit water loss) in the drought treatments through acclimation of stomata to water limitation.

An overall reduction in above-ground biomass would occur with a concomitant decrease in specific leaf area and an increase in the tension required to remove petioles from stems (Dijkstra, 1989; Lambers and Poorter, 1992).

Materials and Methods

Twenty-four four-month-old poinsettias, Euphorbia pulcherrima var. 'Festival Red,' in six-inch pots of the same standard color, size, and shape were selected for the study from a population of plants grown under standard commercial production practices. Plants were grouped into four blocks of six plants and were blocked for differences in direct incident light from greenhouse lights, as measured by a LiCor quantum sensor after dark (Figure 1).

All plants were provided nutrients according to the initial production protocol, to reduce the potential for confounding nutrient effects. Nutrient applications were applied daily in 60-mL doses on the following schedule.

Day 1: 250 PPM 20-10-20 NPK Fertilizer + Molybdenum

Day 2: 250 PPM 15-0-15 NPK Fertilizer + Molybdenum

Day 3: 250 PPM 20-10-20 NPK Fertilizer + Molybdenum

Day 4: Water leach of salts (200 mL).

Drought and water treatments were initiated on October 17, 1999. Treatments were randomly assigned using a random number table. Twelve plants, three per block, received the daily water treatment of 200 mL. The remaining plants only received 200 mL of water on Day 4 with the water leach and at leaf wilt.

Figure 1. Experimental design for poinsettia drought study. Randomized complete block with 4 blocks. Drought and Water Treatments
Block 1			Block 2			Block 3			Block 4
1	1	2	2	1	2	1	1	1	1	1	2
2	1	2	1	2	1	2	2	2	1	2	2
Standard water = 1; Drought = 2

Photosynthesis, stomatal conductance, and plant-water potential were measured on two randomly selected leaves of the same age and size per plant on November 7, 11 and 21, 1999. Photosynthesis and stomatal conductance were measured with a LiCor 6200 photosynthesis system using a closed 1/2 Liter chamber. From the LiCor data we were able to determine photosynthetic water-use efficiency (WUE) and the internal leaf CO2 to ambient CO2 (Ci/Ca) ratio. All measurements were made between 900 and 1,400 hours on each sampling date.

Plant water potential was measured on two excised leaves per plant, of the same size and age, with a Model 600 PMS Instruments Pressure Bomb. Pressure bomb readings were taken at mid-day when water stress would be highest. Specific leaf area was measured on two fully expanded leaves randomly selected from the same age and size class of each plant on November 7 and 21. Specific leaf area (SLA) was calculated from area measurements of fresh leaves, made with a CIAS digital area meter, and the leaf dry weight after 48 hours at 60ºC.

Structural integrity of the plants was assessed by inverting a Chatillon DFM2 portable penetrometer to measure the weight (tension) required to remove a petiole from the stem. Our leaf sampling entailed selecting leaves that were not fully expanded, to ensure that they had not fully developed prior to the implementation of treatments. However, time limitations of the project necessitated selecting leaves initiated prior to treatment application. Newly formed leaves were too flexible to remove using the penetrometer. The two median measurements from a total of four measurements per plant were used for the analysis.

Root and shoot weights were measured on six plants, three of each treatment, at the conclusion of the study. Weights were measured after the roots and above-ground biomass were washed and placed in a drying oven for 48 hours at 60ºC. Temperature records for the duration of the study were collected with an Onset Computer Corporation HOBO Pro Series Temperature Logger.

Photosynthesis, stomatal conductance, water-use efficiency, Ci/Ca, water potential, SLA, petiole tension, and shoot and root data were analyzed as a two-way analysis of variance (ANOVA) to determine treatment differences using Minitab" for Windows ver. 12.1 statistical software. All data met the conditions of normality and homogeneity of variance.

Results

Plant water potential, photosynthesis, and stomatal conductance were significantly different between the treatments on all sampling dates. Drought treatments required significantly more pressure to push water from the xylem than the standard water treatments, indicating low water tension (Figure 2).

Drought treatments also had significantly lower photosynthesis, and stomatal conductance than the standard water treatments (Figure 3).

Photosynthetic water-use efficiency was initially lower in the drought stress treatment, but increased significantly on the middle sampling date and remained higher than the water treatment on the last sampling date (Figure 4).

The Ci/Ca ratio of each treatment also shifted during the study. On the first sampling date the drought treatments had a significantly higher Ci/Ca ratio indicative of reduced photosynthesis due to stomatal closure. This trend reversed at the second sampling date. By the close of the study the Ci/Ca ratios between the watered and drought stressed plants did not differ (Figure 5).

Specific leaf area was much higher on the poinsettias under the stress treatments than on the plants in the standard watering regime. The fully expanded leaves on plants under drought stress either lost mass or gained leaf area as compared to non-water stressed plants. This trend was consistent through the study (Figure 6).

Poinsettias in the standard water treatments had significantly more above ground biomass (leaf and stem tissue) than did the drought stressed plants. Root mass appears to have been unaffected by the drought treatments (Figure 7).

Petiole strength appears to be higher in the drought stress treated poinsettias, but the differences apparent in our study were not statistically significant (Figure 8).

Figure 2. Effects of drought stress on leaf water potential of poinsettia. The data reflect the pressure to push water into the xylem vessels. Actual tension measurements would be negative. Data are expressed as mean ± standard error. Means within a date with different letters are significantly different (p < 0.05).	Figure 3. Effects of drought stress on photosynthesis and stomatal conductance of poinsettia. Data are expressed as mean ± standard error. Means within a date with different letters are significantly different (p < 0.05).
Figure 4. Effects of drought stress on poinsettia photosynthetic water-use efficiency. Data are expressed as mean ± standard error. Means within a date with different letters are significantly different (p < 0.05).	Figure 5. Effects of drought stress on poinsettia Ci/Ca ratio. Data are expressed as mean ± standard error. Means within a date with different letters are significantly different (p < 0.05).
Figure 6. Effects of drought stress on specific leaf area of poinsettia. Data are expressed as mean ± standard error. Means within a date with different letters are significantly different (p < 0.05).	Figure 7. Effects of drought stress on poinsettia above- and below-ground biomass. Data are expressed as mean ± standard error. Means within a date with different letters are significantly different (p < 0.05).
Figure 8. Effects of drought stress on petiole strength of poinsettia. Data are expressed as mean ± standard error. Means within a date with different letters are significantly different (p < 0.05).	Figure 9. High and low temperatrues in the greenhouse during the poinsettia study.

Discussion

Drought treatments had the expected effect of increasing plant water tension and reducing stomatal conductance and photosynthesis. The impact of the treatments was obvious at the first sampling date when the temperatures at the time of measurement were hovering around 80ºF (Figure 9). It appears that drought treatment plants experienced water stress, indicated by plant water potential readings, to the extent that they closed their stomata and reduced photosynthesis (Figures 2 and 3). This is supported by Ci/Ca readings that show a higher internal CO2 concentration in the drought-stressed plants.

At later sampling dates, photosynthesis and stomatal conductance were still lower in the drought treatments, but the plants had acclimated to the water limitation by increasing their photosynthetic water use efficiency (Figure 4). With an increase in WUE the drought-treatment plants were better able to utilize internal CO2 for photosynthesis resulting in a reduced Ci/Ca ratio (Figure 5). The decrease in water potential, photosynthesis, and stomatal conductance for the final measurement was attributable to the low day- and night-time temperatures that occurred after November 17 in the greenhouse (Figure 9). The optimal growing range for poinsettia is between 80º and 65ºF (Hall, 1992).

Above-ground growth was reduced in the drought treatments (Figure 7). Water limitation is predicted to impact growth since growth is more sensitive to stress than photosynthesis. (See the chapter on growth and allocation in Lambers et al., 1998). Our original hypothesis predicted a reduction in specific leaf area from reduced cell elongation and a shifting of carbon allocation from apical growth. In fact, the inverse was true. Leaves of the drought-treatment poinsettias had a larger specific leaf area than the standard water treatment plants on both sampling dates (Figure 6).

The leaves measured for specific leaf area were fully expanded at the time of collection. Their initial expansion occurred prior to the implementation of treatments. Only two leaves per plant were used for the analysis on each date, so we were unable to determine leaf mass ratio (LMR) or leaf area ratio (LAR). Despite these sample collection issues, there were obvious treatment differences. According to Lambers et al., 1998, SLA will change in a mature leaf, whereas cell size and number are more rigid. It is improbable that leaf area increased as growth was limited; therefore, the more likely scenario is that leaf mass (dry weight) was reduced. This may have occurred if nutrients and carbohydrates were reabsorbed from the leaves and reallocated to other sinks. In future studies, a more thorough destructive sampling regime to determine LMR, LAR, and root mass ratio (RMR) could identify if and where carbon was reallocated.

Tardieu et al., 1999, suggest an alternative explanation based upon the extreme variability of SLA under differing environmental conditions due to the independent processes that determine tissue expansion and cell division. He concludes that environmental conditions that depress leaf expansion rate more than photosynthesis will result in a decreased SLA, whereas conditions that have a more depressive effect on photosynthesis than elongation will cause an increase in SLA depending on the available light, plant water status, and time of day. It is possible that the water treatments were not at the extreme ends of the stress spectrum and that a non-linear effect of water stress could reduce photosynthesis without affecting leaf elongation.

Even within the time constraints of the study, the data illustrate a non-significant trend towards stronger petioles in the drought-stressed poinsettias (Figure 8). It would have been easier to detect treatment differences in petiole tension had we initiated treatments at the beginning of the production process, or had time to continue the study until leaf growth initiated after treatment implementation had fully expanded.

Conclusion

Drought treatments do impact the carbon assimilation of potted greenhouse poinsettias. After a period of acclimation, photosynthesis rebounds in drought-treated plants by an increased water-use efficiency. Although our study didn't illustrate a definite impact of water stress on carbon allocation to plant structural durability, the trends are worth further, more laborious, investigation. As a final note, when we disposed of the poinsettias by providing them free to people in the department, the drought-stress plants were the preferred choice by a 2:1 margin. When asked why they selected the drought-treated plant, the most common reply was "it just looked healthier." Although it is not a scientific assessment, drought stress doesn't appear to diminish plant aesthetic quality.

Acknowledgements

This project was designed to fulfill the laboratory requirements for Physiological Ecology of Plants (EEOB 674) in the Department of Ecology, Evolution, and Organismal Biology, Autumn Quarter 1999.

We would like to acknowledge the assistance of the people listed here, without whose help this project would not have been a success.

Peter S. Curtis and Michael H. Jones, Department of Ecology, Evolution, and Organismal Biology, The Ohio State University, instructors for the class Physiological Ecology of Plants.

Terry Moore, Millie Casey, and Richard Lindquist, OARDC Floriculture Insect Management Program, for providing plant materials and greenhouse space.

Dale Bradshaw, OSU-ATI Horticulture Technology Program, for insight into commercial poinsettia production practices.

Dan Herms, OARDC Woody Ornamental Insect Management Program, for technical assistance and the equipment used to measure the physiological parameters.

Roger Downer, OARDC LPCAT Laboratory for equipment.

Catherine Lloyd and Patty Rattlingourd, for assistance in treatment applications and plant maintenance

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