Postgermination drenches with PEG-8000 reduce growth of salvia and marigolds

We previously found that incorporation of PEG-8000 into the growing medium delayed germination and resulted in shorter seedlings. However, in that study, we were un- able to determine whether the reduced height was merely the effect of delayed germination or of reduced elongation after germination. To answer this question, we studied whether postgermination drenches with PEG-8000 can reduce seedling height. Annual salvia (Salvia splendens F. Sellow. ex Roem. & Shult. 'Bonfire') and French marigold (Tagetes patula L. 'Boy Orange') seedlings were treated with drenches of PEG-8000: 0, 15, 20, 30, 42, 50, 62, 72, or 83 g·L -1 . At least 20% of seedlings treated with 62 to 83 g·L -1 of PEG-8000 were dead 14 d after treatment. Salvia and marigolds treated with the remaining PEG-8000 concentrations were up to 34% and 14% shorter than untreated seedlings, respectively. Leaf water (Ψ w ) and turgor potential (Ψ p ) also decreased for salvia which were grown with greater concentrations of PEG-8000, one probable cause of the observed reduction in elongation. Since the PEG-8000 in this study was applied after germination, it is clear that PEG-8000 does not reduce elongation merely by delaying germination, but also by reducing the elongation rate. Thus, postgermination drenches with PEG-8000 can be used to produce shorter seedlings. Production of annual bedding plant seed- ling plugs is vital to commercial floriculture. Until the mid-1960s, most growers propagated seedlings in large trays without individual cells (Thomas, 1993). However, in the past 25 to 30 years, seedlings have been grown in trays containing small individual cells or plugs for each plant. Plug production is advanta- geous because root hairs are damaged less during transplant, and plugs may be held in greenhouses for longer periods than bare-root seedlings (Styer and Koranski, 1997). Plants grown from plugs also tend to be more uniform and survive transplanting better than bare-root seedlings (Thomas, 1993). Plug production has become so popular that by 1998, production value reached $107 million (National Agricul- tural Statistics Service, 1998). Despite the growing popularity of plugs, several production problems must be resolved. One problem with plug transplant production is that plants tend to out-grow their small substrate volumes quickly and become leggy (Styer and Koranski, 1997). Elongated transplants are more difficult to manage, more expensive to ship, and less aesthetically pleasing than compact transplants. Many commercial growers avoid using chemical plant growth retardants for seedlings, because chemical growth retardants applied according to the label restrictions (i.e., after the first true leaves have emerged) will not reduce hypocotyl height. Since hypocotyl height can account for 60% of marigold seedling height (Burnett, 2004), chemical growth retardants may not be the best option. Drought stress is also used to slow elongation since marigolds and annual salvia are shorter when irrigation is restricted (Eakes et al., 1991; van Iersel and Nemali, 2004). However, seedlings are grown in small volumes of growing media (8.5 mL for one cell in a 288 tray) which dry out rapidly in greenhouses. If the growing medium becomes too dry, seedlings photosynthesize less, and in extreme cases, may die (Mohr and Schopfer, 1995). Growers need a method of reducing soil water availability without these risks associated with decreased irrigation. One promising new option for growers is imposing controlled drought stress through the incorporation of osmotic compounds, such as polyethylene glycol or PEG-8000 to growing media. PEG-8000 forms hydrogen bonds with water (Kjellander and Florin, 1981) in the growing medium, making it unavailable for plant uptake. This osmotic compound has reduced growth of French marigolds and annual salvia when incorporated into growing media before seeding (Burnett, 2004). One advantage of PEG-8000 over other methods of reducing plant growth is that it also reduces seedling hypocotyl height (Burnett, 2004). When PEG-8000 was incorporated into the growing medium before seeding, height and hypocotyl height of salvia and marigold decreased with increasing PEG-8000 concen-