THE EFFECT OF MOISTURE CONTENT IN THE SUBSTRATE ON ROOTING OF SEEDLINGS IN PLUG TRAYS

Root growth, especially adventitious or basal, is important for both transplant quality and plug stability for mechanical transplanting. Two experiments were done on the effects of moisture level in the plug substrate on root morphology of six species grown in 288-plug trays. Two species per substrate moisture class [dry (verbena and zinnia), medium (coleus and petunia) and wet (begonia and impatiens)] were chosen based on the recommended moisture level. After radicle emergence, the moisture content of the substrate was maintained at three different moisture levels within each class (Experiment 1), or the plug substrate was allowed to dry out to one of three moisture levels before the trays were rewetted to the original moisture level for germination (Experiment 2). The number and the position of lateral and adventitious roots on the primary root, total root length and the length of the primary root were determined after the first leaf had fully expanded (3 weeks), while the number of lateral and adventitious roots and dry weights of the root and shoot were determined after 6 weeks. Two species showed a reduction in the number of adventitious roots (verbena, dry; and coleus, medium moisture), while total root length was reduced for coleus and the primary root length of impatiens (wet) decreased with increased moisture stress (Experiment 1). Total root length decreased for coleus (medium) and impatiens (wet) with increased moisture stress in Experiment 2. Root dry weight after 6 weeks was not affected by moisture stress in either experiment. An increased moisture deficit showed either no effect (zinnia, petunia and begonia) or resulted in a lower number and reduced total root length of both adventitious and lateral roots. Also the different moisture deficits did not affect the position of the lateral and adventitious roots on the primary root.