8/12/2019 Drought Stres Marigolds

1/4HORT SCIENCE VOL . 39(6) O CTOBER 20041298

Drought Stress Can Produce Smallbut not Compact MarigoldsMarc W. van Iersel 1 and Krishna S. Nemali

Department of Horticulture, University of Georgia, 1111 Miller Plant Science Building, Athens, GA 30602-7273

Additional index words. bedding plants, capillary mat, irrigation, height control,subirrigation, Tagetes erecta L.

Abstract . We examined the effectiveness of an elevated capillary mat system to maintainconstant and different moisture levels in the growing medium and verify the potentialof drought stress conditioning in producing small and compact bedding plants. To dif-ferentiate between plant height and compactness, we determined compactness as theleaf area or dry mass per unit stem length. Marigold Queen Sophia ( Tagetes erecta L.)seedlings were grown in square, 9-cm-wide, 10-cm-high containers lled with a soillessgrowing medium. A capillary mat was laid on top of a greenhouse bench which was raisedby 15 cm on one side compared to the other side to create an elevation effect. Seedlingswere subirrigated by immersing the low end of the capillary mat in a reservoir of water.The amount of water moving to the higher end of the mat progressively decreased withelevation. The moisture content in the growing medium averaged from 26 to 294 mL/potat different elevations. Regression analysis indicated that growth parameters including,shoot dry mass, leaf area, leaf number, and plant height decreased linearly with decreasingsoil moisture content in the growing medium. Of all the measured growth parameters,plant height was found to be least sensitive to decreasing moisture content in the growingmedium. Plants in high moisture treatments had more dry mass and leaf area per unitlength of the stem compared to those in low moisture treatments. Our results indicate thatdrought stress can produce small, but not truly compact bedding plants.

Excessive elongation of bedding plantsduring greenhouse production is common andresults in low-quality plants. Excessively tallplants generally are considered to be of lowquality due to the increased shipping costs ofthese plants and their increased susceptibilityto lodging. One method to limit elongation isto limit water uptake by plants. Drought stressconditioning involves exposing plants to non-

lethal water decits (Latimer and Severson,1997). Imposing drought stress not only con-trols plant growth, but also conditions plantsfor stresses during shipping, marketing, andafter planting in the landscape (Latimer andOetting, 1998). Drought stress is commonlyused to condition bedding plants in greenhouseproduction (Brown et al., 1992; Latimer,1992; Latimer and Severson, 1997; Latimerand Oetting, 1998). Generally, drought stressis imposed by withholding water from theplants until they wilt or a targeted, low waterpotential of the growing medium is reached(Brown et al., 1992). With both methods, thetime interval between subsequent irrigations

may vary, because plant water use and evapo-ration from the growing medium depend onthe environmental conditions (Brown et al.,1992). Therefore, plants are not subjected toidentical drought stresses during subsequentwetting and drying cycles.

Drought stress conditioning can also be

done by growing plants at a constant,but low,moisture content of the growing medium, al-though maintaining a constant moisture levelmay be difcult. By spreading a capillary mat,normally used for subirrigating greenhouse

plants (Dole, 1994; Morvant et al., 2001), ona sloped surface and immersing the lower endof the mat in water reservoir, the amount ofwater in the capillary mat will progressivelydecrease with increasing elevation from thesource of water. Thus, by growing plants ona sloped capillary mat, they can be exposedto different drought stresses, depending onthe height of the pot above the level of waterin the reservoir. Such a system can be usedto determine how water supply affects plant

growth and quality.When plants are exposed to drought stress,

stomates normally close to reduce water lossfrom the leaves (Chapman and Aug, 1994;Jones, 1998). Closure of stomates reducestranspiration, but at the cost of decreasedphotosynthesis (Jones, 1998). Compared tothe decline in leaf photosynthesis, leaf ex-pansion is more sensitive and ceases earlierin moisture-stressed plants (McCree, 1986).Although leaf expansion ceases, it is possiblethat stem elongation can continue at the samedrought stress level. Due to lower resistance towater movement in the xylem tissue in stemscompared to the water conducting veins in the

leaves (Steudle, 2000), it is possible for stemcells to maintain a higher turgor pressure thanleaf cells at a particular drought stress level.The higher turgor pressure may enable elon-gation of stem cells during periods of droughtstress, although not necessarily at a rate seenin well-watered plants. Since stem cell elonga-tion increases plant height, it is possible thatheight could be less sensitive to drought stressthan leaf area development. Thus, regulatingelongation by withholding water may result

Fig. 1. Diagram of the experimental setup. Pots were arranged on a sloped capillary mat in 7 columns and12 rows (indicated by the numbers on the pots). The highest pots were about 15 cm above the waterlevel in the reservoir.

HORT SCIENCE 39(6):12981301. 2004.

Received for publication 19 Aug. 2003. Acceptedfor publication 13 Dec. 2003. We thank KevenCalhoun and Larry Freeman for their technicalassistance, and George Elliott and an anonymousreviewer for their helpful comments on an earlierdraft of this manuscript.1To whom reprint requests should be addressed;e-mail address: mvanier@uga.edu.

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out gradually, in particular in pots at higherelevations. Averaged over the duration ofthe experiment, the moisture content in thegrowing medium was highest in the potsclosest to the water reservoir (row 12) andlowest in the pots at the highest elevation

(row 1) (Fig. 2B). The averagemoisture content throughout theexperiment ranged from 26 (row1) to 294 mL/pot (row 12) (or avolumetric water content of about5% to 59%). The relationshipbetween the moisture content inthe growing medium and eleva-tion was non-linear. Differencesin the moisture content of thegrowing medium in adjacent rowsincreased with increasing eleva-tion (or distance form the water

reservoir). This may be due to thenon-linear relationship betweensubstrate volumetric moisturecontent and matric potential(Karlovich and Fonteno, 1986)and/or evaporative water lossesfrom the capillary mat. Pots inrows 1 and 2 dried out almostcompletely, resulting in the deathof these plants.

Growth measurements . Plantgrowth, including shoot dry mass,leaf area, leaf number, and plantheight, decreased linearly withdecreasing moisture content in

the growing medium (Fig. 3) ( P < 0.001 for allmeasured parameters). The decrease in leaf areawas caused both by a reduction in the numberof leaves per plant, and by a linear decrease inaverage area per leaf with increasing drought(results not shown). Leaf expansion decreaseswith drought stress due to the inherent slowosmotic adjustment (ability to maintain turgor)and low membrane extensibility (looseningability) of leaf cells (Frensch,1997; Hsiaoand Xu, 2000). Turgor pressure is particularlyimportant for cell elongation, since it providesthe driving force for irreversible enlarge-ment of cells or organs (Hsiao and Xu, 2000;Lockhart, 1965). Thus, differences in turgor

pressure result in smaller leaf areas for plantsgrown under drought stress as compared towell-watered plants. A decrease in leaf areaand subsequent decline in photosynthesisdue to closure of stomates results in overalldecrease in growth of plants. Reduction in plantheight due to drought stress has been reportedearlier for tomato ( Lycopersicon esculentum Mill.), marigold ( Tagetes erecta L.) (Brown etal., 1992), New Guinea impatiens ( Impatienshybrida L.) (Latimer and Oetting, 1998), and

broccoli ( Brassica oleracea L.) (Latimer andSeverson, 1997). However, an important dif-ference between those studies and our work isthat in the previous studies, substrate moisturelevel uctuated. The growing medium dried outbetween irrigations, but was replenished at eachirrigation, thus resulting in episodic drought.In the current study, the substrate moisturecontent in the drier treatments declined gradu-ally throughout the experiment. Such a gradualimposition of drought stress may affect plantsdifferently from episodic drought.

Plant height was less sensitive to decreas-ing moisture content in the growing mediumthan leaf area, shoot dry mass, or the number

of leaves (Fig. 3). Relative to the plants grownat the highest moisture level, plant height atthe lowest moisture level decreased by 51%compared to 70%, 73%, and 92% for numberof leaves, shoot dry mass, and leaf area, respec-tively. Osrio et al. (1998) reported that leafexpansion of eucalyptus ( Eucalyptus globulus Labill.) seedlings under severe drought condi-tions was 1.5 to 2.0 times greater for leaveson the main stem axis compared to those onbranches. Apparently, under drought stressconditions, higher growth rates may be seenin stem and parts closely associated with stemcompared to those parts further away fromthe stem (e.g., leaves on branches). Results

from Osrio et al. (1998) and our experimentindicate that stem elongation, and thereforeplant height, are less sensitive to drought stressthan leaf expansion.

Compactness in plants . Drought stress didnot result in compact plants. Shoot dry massper unit plant height increased linearly withincreasing moisture content in the growingmedium (Fig. 4). This indicates that the plantsgrown at the lowest moisture level weighedless per unit stem length than those grown atthe highest moisture level. Similar to shootdry mass per unit plant height, leaf area perunit plant height also decreased linearly withdecreasing moisture content in the growing

medium (Fig. 4). Our results indicate droughtstress results in shorter, but not more compact,plants. Although regression analysis suggesteda linear relationship between moisture contentand plant growth and compactness (Figs. 3and 4), it is interesting to note that treatmenteffects were negligible in the range of moisturecontents from 240 to 280 mL/pot (48% to 56%).This likely is related to the hydraulic proper-ties of peat-based substrates. A large fractionof the water in peat-based media is held at awater potential between 0 and 5 kPa and thiswater is readily available to plants (Karlovichand Fonteno, 1986). Therefore, small changesin the volumetric water content of moist media

Fig. 3. Effect of different moisture levels in the growing medium on shoot dry mass, leaf area, leaf number,and plant height of marigolds at the end of the experiment. Data points are the average of three replica-tions. Shoot dry mass = 1.91 + 0.0232 moisture content ( r 2 = 0.71), plant height = 0.97 + 0.035 moisture content ( r 2 = 0.63), leaf number = 27.0 + 0.33 moisture content ( r 2 = 0.57), and leaf area

= 762 + 5.92 moisture content ( r 2

= 0.73), P < 0.001 for all regressions.

Fig. 4. Effect of different moisture levels in thegrowing medium on the ratio of shoot dry massto plant height and leaf area to plant height ofmarigolds at the end of the experiment. Data arethe average of three replications ( SE). Shoot drymass/plant height = 0.026 + 0.0015 moisturecontent ( r 2 = 0.58), leaf area/plant height = 0.65+ 0.0055 moisture content ( r 2 = 0.69), P