Evaluation of a Substrate-applied Humectant to Mitigate Drought Stress in Young, Container-grown Plants

Significance to the Horticulture Industry

We evaluated a commercially-available humectant designed to mitigate the effects of drought stress. For young, container-grown Zinnia elegans Jacq. ‘Thumbelina’, foliar wilt was delayed by substrate applications of the product at the manufacturer's recommended rate of 1.6%, and also at 3.2%. While the time period for extending drought-free symptoms was relatively short (3 to 4 days), the number of days to wilting was greater for plants grown in humectant-treated substrate than for similar plants grown in untreated substrate. At the concentrations used in this study, humectant treatment had a positive impact on root growth as evidenced by increases in the root:shoot ratio and in the length of small diameter water-absorbing roots. Assuming economic feasibility of the product, humectant treatment may offer a cultural alternative for improving irrigation efficiency as well as suppressing drought stress symptoms during early stages of the production cycle for bedding plants such as Z. elegans.

Introduction

Over the last half-century, the demand for agricultural water use has increased significantly (Hutson et al. 2004), and water management issues have become a topic of major concern for the horticultural industry. This is especially true in nursery and greenhouse production, where more than 50% of the plants are now grown in containers (U.S. Dept. Agric. 2007). This shift to container production has resulted in greater numbers of plants being grown per unit area and, concomitantly, has raised concerns about irrigation scheduling and the future availability of groundwater resources (Majsztrik et al. 2011). The potential for limited water availability has led, among other things, to a search for alternative cultural practices that conserve water and increase water-use efficiency in container crop production. One such alternative involves the use of soil amendments possessing substantial hygroscopic properties. These substances, referred to collectively as humectants, contain several hydrophilic groups, often hydroxyl groups, which form hydrogen bonds with molecules of water. When applied to growing media, aqueous solutions of these substances attract water vapor from the rhizosphere where it condenses, thereby becoming available for absorption by plant roots (Spindler 2013).

While humectants have gained widespread acceptance in food, pharmaceutical and personal care products, their potential use in horticulture as hygroscopic soil amendments has not been widely investigated. In a 1991 study, Barrett reported finding that a media-applied humectant could improve drought resistance in geranium (Pelargonium hortorum L.H. Bailey), impatiens (Impatiens walleriana Hook. f.) and vinca (Catharanthus roseus L). Working with woody plants, Arena (2001) reported that the caliper growth of container-grown, humectant-treated live oaks (Quercus virginiana Mill.) exceeded that of similar untreated oaks when both were grown under a reduced irrigation regime. More recently, Roberts and Linder (2010) found that humectant treatment could delay the onset of foliar wilt in one-year-old, container-grown seedlings of red maple (Acer rubrum L.), red oak (Quercus rubra L.) and yellow-poplar (Liriodendron tulipifera L.).

In greenhouse production, irrigation is especially important during plant establishment, a time when young plantings/transplants are extending new roots into the substrate (Kessler 2004) and, consequently, a time when plants are very susceptible to stress, particularly drought stress (Watson 1997). In the present investigation, the objective was to evaluate the efficacy of a commercial humectant that could be used as a substrate amendment to mitigate drought stress symptoms in container-grown Zinnia elegans. If successful, this technology offers the possibility for use by greenhouse managers as an option for improving drought tolerance in young transplanted seedlings while, at the same time, decreasing irrigation frequency and improving survival rates during the critical stages of plant establishment.

Materials and Methods

Seeds of ‘Thumbelina’ zinnia (Livingston Seed Co., Columbus, OH) were sown in moist paper towels in the dark at 23 C (73 F) until radicle emergence (week 1) prior to transplanting three germinated seeds into each of thirty-five 350 cm³ (21.4 in³; #SP3 trade) square plastic pots containing soilless substrate (Metromix 360, Sun Gro Horticulture, Agawan, MA). All 35 pots were sub-irrigated with water overnight to saturate the substrate and then placed beneath fluorescent lights (80 μmol·m⁻²·s⁻¹; 12 h photoperiod) at 25 C (77 F) for one week (week 2). Measurements of substrate volumetric water content (VMC) were taken in each pot on alternate days using a time-domain sensor and capacitance probe [(SM200); Delta-T Devices, Dynamax, Houston, TX]. In addition, gravimetric measurements of actual evapotranspiration (ET_A) were taken for each container every other day. Based on VMC and ET_A measurements, the substrate in each pot was hand-watered as needed to maintain the moisture content at or near container capacity. During week 3, the substrate in all pots was sub-irrigated twice with a water-soluble fertilizer [20N-2.6P-18.3K (JR Peters, Inc. Allentown, PA)] at a N rate of 200 mg·L⁻¹ (0.01 oz·1.10 qt⁻¹). After thinning the number of plants per pot to one, treatments were started at the end of week 4.

On the day of treatment, 28 pots were placed in 9 cm² (3.5 in²) plastic trays and 70 mL (2.4 oz) of a tap water (pH 8.8; EC 5.19 dS·m⁻¹) solution which contained 0.0 0.8, 1.6 (the manufacturer's recommended rate for potted or containerized plants) or 3.2% by volume of HydretainES [58% humectant (sugar alcohols, polysaccharides and neutral salts of alphahydroxyproprionic acid); 1.3% non-ionic surfactant; 0.7% inert ingredients], a proprietary product manufactured by Ecologel Solutions, Ocala, FL, was applied to the substrate in each of seven pots. Prior testing showed that 70 mL of liquid was sufficient to thoroughly wet the substrate volume, resulting in a small amount of drainage which was collected in each tray and quickly reabsorbed. An additional 7 pots (irrigated controls) received 70 mL of tap water. After treatment, all pots were placed back beneath the fluorescent light bank. Starting the following day, and continuing for the duration of the study, irrigation was discontinued for all humectant-treated substrates (28 pots), while the remaining seven pots containing untreated substrate continued to receive water based on measurements of VMC and ET_A taken every other day. Zinnia plants grown in humectant-treated substrate were harvested individually when the foliage of each plant showed signs of incipient wilt during the day and did not rehydrate overnight. For this reason, decisions on when to harvest were made at 0730 h each day, allowing adequate time for any rehydration to occur during the nighttime hours. Irrigated control plants were harvested at the end of the study.

At harvest, the number of days without irrigation was recorded for each seedling (days to wilt, DTW). Also at harvest, shoots and roots were separated at the base of the stem, and shoot water potential (ψ_x) was measured in a pressure chamber (SoilMoisture Equipment Corp., Santa Barbara, CA). Leaves from each plant were then removed, counted and the leaf area measured (LI-3100; LI-COR, Lincoln, NE) prior to oven-drying each shoot (leaves and stem) at 80 C (176 F) for 48 h to obtain aboveground dry biomass. After harvesting the shoots, the root ball in each pot was removed, placed inside a No. 18 U.S. Standard Sieve [1 mm (0.04 in) opening], and washed with a fine spray of water to remove most of the substrate debris. Washed root systems were then transferred to glass petri dishes containing water, and any remaining debris removed by hand using a bench-mounted magnifying lens and forceps. Cleaned root systems were placed into 15 mL (0.5 oz) capped centrifuge tubes containing 50% ethanol and stored at 8 C (46 F) until analyzed. Analysis included optical scanning and digitizing each root system [WinRHIZO Pro (Reagents Instruments, Victoria, BC, Canada)] to determine the following morphological characteristics: length, surface area, diameter and volume. Belowground dry biomass was recorded after oven-drying each root system at 80 C for 48 h. Above- and below-ground dry biomass were used to calculate the root:shoot ratio. Root morphological data were used to determine total root length (TRL), specific root length (SRL) and root length density (RLD). The experiment was a completely randomized design. Orthogonal polynomials were used to examine linear and quadratic trends in response to humectant treatment. Dunnett's test was used to compare differences between humectant-treated and untreated, irrigated (control) plants. All data were analyzed using statistical software [Statistix 10 (Analytical Software, Tallahassee, FL)].

Results and Discussion

Shoot growth (shoot dry weight and leaf area) in watered (control) plants was always higher than it was in droughted plants, regardless of the concentration of humectant applied (Table 1). There was, however, a linear increase in the shoot growth of treated plants as humectant concentration increased (e.g. shoot dry weight increased 24% and leaf area 55% as the concentration of humectant increased from 0.0 to 3.2%), suggesting that zinnias grown in soilless medium treated with higher concentrations of HydretainES (1.6 and 3.2%) were better able to tolerate the short-term impact of drought stress. As anticipated, xylem water potential (ψ_x) was substantially lower (less negative) in irrigated plants (controls) than in those receiving no irrigation. Although ψ_x tended to decline (become less negative) with increasing humectant concentration, no significant linear trends were observed.

Table 1. The effect of humectant treatment on shoot growth and plant water relationships of 4-week-old drought-stressed, container-grown Zinnia elegans ‘Thumbelina’^z.

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Substrate moisture levels, expressed here as volumetric water content (VMC), were lower and evapotranspiration (ET_A) higher in untreated, drought-stressed substrate than they were in humectant-treated, drought-stressed substrate (Table 1). A significant linear change in both VMC and ET_A was observed as humectant concentration increased from 0.0 to 3.2%, suggesting a slight improvement in substrate-water content and a concomitant reduction in evapotranspiration as a result of treatment. These observations are corroborated by the linear relationship observed between humectant application rate and days to wilt (DTW). DTW was extended 4 days (33%) as a result of a single pre-drought application of humectant at 3.2%. This delay in the onset of foliar wilt could be important, not only by reducing the need for more frequent irrigation, but by improving survivability during plant establishment. In earlier studies using a similar humectant product, Barrett (1991) reported that DTW could be delayed in container-grown vinca when a humectant was applied to the substrate at a dilution of 1:10.

The root:shoot ratio of droughted zinnia seedlings grown in humectant-amended substrate was consistently greater than it was for seedlings grown in untreated substrate (Table 2). Increasing the level of humectant applied to the substrate concomitantly resulted in a linear increase in the root:shoot ratio. In other studies using drought-stressed zinnia (Z. elegans ‘Scarlet’), Sharp and Davies (1979) reported finding an increase in the root:shoot ratio as a result of drought. While it is known that both root and shoot growth are reduced by drought, shoot growth is frequently inhibited to a greater extent (Creelman et al. 1990, Pace et al. 1999). This was true in the present study where, at harvest, the shoot dry weight of untreated, non-irrigated zinnias decreased 61%, while root dry weight for the same plants declined only 15% (data not shown). For non-irrigated zinnias grown in humectant-treated substrate, a linear increase in the root:shoot ratio was observed as the application rate increased from 0.0 to 3.2%. This linear increase appeared to be the result of increases in both substrate VMC and in the growth of small diameter roots (Table 2). Although, as previously mentioned, the moisture content (VMC) of drought-stressed, untreated substrate was extremely low (1.37%; Table 1), the moisture content did increase linearly as the concentration of humectant increased. This increase in VMC, although small, may have been sufficient to initiate changes in biomass allocation and/or changes in the respective turnover rates within root and shoot tissues (Reich 2002), both of which could impact the root:shoot ratio. Research by Miller et al. (2013) reported that non-irrigated plants could have significantly lower yields than well-irrigated plants, yet root density could be very similar under both sets of conditions. This suggests that a greater fraction of assimilates may be partitioned to the roots when water becomes limiting, a situation which could increase the root:shoot ratio.

Table 2. The effect of humectant treatment on root system architecture of 4-week-old drought-stressed, container-grown Zinnia elegans ‘Thumbelina’^z.

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Root exploration within a substrate is largely dictated by root system architecture, and total root length (TRL) is fundamentally important in the acquisition of water and nutrients (Eissenstat 1991). Although somewhat confounded by differences in plant age at harvest (1 to 4 days), no differences in root length could be attributed specifically to humectant treatment in the current study. However, in comparing the length of small diameter (fine) roots [≤ 0.2 mm (0.08 in)] with the length of larger diameter (coarse) roots [≥ 0.6 mm (0.24 in)] on the same plant, a positive linear relationship was found between fine root length and humectant concentration — the length of fine roots increasing as the humectant concentration increased. This was not the case when comparing the growth of coarse roots, where no significant trends were noted. Because fine roots are known to conduct more water and nutrients per unit area than do coarse roots (Gambetta et al. 2013), it can be argued that one advantage of humectant treatment is that it affords treated plants the ability to resist and/or delay the onset of drought stress symptoms, thereby allowing additional time for fine roots to grow and explore the substrate in search of additional sources of moisture. Grossnickle (2005) has reported that, after transplanting into containers, young seedlings become established only when they develop a root system capable of coupling to available sources of soil moisture. Without this coupling process, symptoms of drought stress occur and plant establishment is either delayed or prevented altogether. This phenomenon could be of particular significance for young bedding plants during the establishment period following the transplanting of seedling plugs into marketable containers. It is during this important period of time that adequate substrate moisture is crucial for new root growth and development.

Specific root length (SRL), the ratio of root length to root dry mass, is also a frequently measured root parameter (Ostonen et al. 2007). In the current study a linear increase in SRL was observed with increasing humectant concentration, but no differences were found between drought-stressed and well-watered (control) plants. Although it is logical to expect higher SRLs under more favorable growing conditions, Tjoelker et al. (2005) reported finding higher SRL values for plant species growing in unproductive (nutrient-poor or dry) environments than for other species growing in more productive locations. Likewise, Ryser (1998) has shown that SRL may increase, decrease, or stay constant under nutrient stress. Based on their findings, along with results observed in the present study, SRL does not appear to be a reliable indicator for assessing the effect of drought stress on new root growth in container-grown zinnia, at least during the early stages of production.

Root length density (RLD), the amount of root tissue per unit volume of substrate, is also one of the important parameters required to understand plant performance (Pierret et al. 2000). We observed a 31% reduction in zinnia RLD as a result of drought stress (Table 2). These results agree with earlier published reports showing that drought can decrease not only root length, but root volume, thereby affecting the ability of water-stressed roots to maintain the turgor pressure necessary to absorb soil moisture (Davis and Bacon 2003, Manes et al. 2006). We also found that zinnia seedlings grown in drought-stressed, humectant-amended substrate exhibited a linear increase in RLD as the concentration of humectant increased from 0.0 to 3.2%. These findings corroborate earlier observations that the moisture content of drought-stressed plants grown in a peat-based medium trended upwards as the application rate of humectant increased. While we did not attempt to delineate the depth and extent of zinnia root system growth in the present study, it appeared that, during the root harvesting process, the effective root zone (the depth of substrate used by the main body of the root system for water and nutrient uptake) was within 2 to 3 cm (0.8 to 1.2 in) of the substrate surface. RLD distribution seemed to be greatest at this depth regardless of either drought treatment or humectant concentration. However, earlier research by Radersma and Ong (2004) indicates that it is not always possible to assume that RLD distribution in a soil profile is necessarily proportional to water extraction by roots within the same profile. Thus, additional research in needed to verify this relationship.