Drought Stress Can Produce Small but not Compact Marigolds

We examined the effectiveness of an elevated capillary mat system to maintain constant and different moisture levels in the growing medium and verify the potential of drought stress conditioning in producing small and compact bedding plants. To dif- ferentiate between plant height and compactness, we determined compactness as the leaf 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 fi lled with a soilless growing medium. A capillary mat was laid on top of a greenhouse bench which was raised by 15 cm on one side compared to the other side to create an elevation effect. Seedlings were 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 with elevation. The moisture content in the growing medium averaged from 26 to 294 mL/pot at different elevations. Regression analysis indicated that growth parameters including, shoot dry mass, leaf area, leaf number, and plant height decreased linearly with decreasing soil 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 growing medium. Plants in high moisture treatments had more dry mass and leaf area per unit length of the stem compared to those in low moisture treatments. Our results indicate that drought stress can produce small, but not truly compact bedding plants. Excessive elongation of bedding plants during greenhouse production is common and results in low-quality plants. Excessively tall plants generally are considered to be of low quality due to the increased shipping costs of these plants and their increased susceptibility to lodging. One method to limit elongation is to limit water uptake by plants. Drought stress conditioning involves exposing plants to non- lethal water defi cits (Latimer and Severson, 1997). Imposing drought stress not only con- trols plant growth, but also conditions plants for stresses during shipping, marketing, and after planting in the landscape (Latimer and Oetting, 1998). Drought stress is commonly used to condition bedding plants in greenhouse production (Brown et al., 1992; Latimer, 1992; Latimer and Severson, 1997; Latimer and Oetting, 1998). Generally, drought stress is imposed by withholding water from the plants until they wilt or a targeted, low water potential of the growing medium is reached (Brown et al., 1992). With both methods, the time interval between subsequent irrigations may vary, because plant water use and evapo- ration from the growing medium depend on the environmental conditions (Brown et al., 1992). Therefore, plants are not subjected to identical drought stresses during subsequent wetting 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 level may be diffi cult. By spreading a capillary mat, normally used for subirrigating greenhouse