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Hannah Mathers, The Ohio State University, Horticulture and Crop Science |
Heat Shock Proteins (HSPs) form in response to many of the abiotic stresses outlined earlier, including high and low temperatures (Waters etal., 1996); osmotic or salt stress; arsenic; anaerobic conditions; high ABA concentrations; high ethylene levels; high auxin levels; and drought (Vierling, 1991). HSPs belong to a larger group of molecules called chaperones, which have a role in stabilizing other proteins. Low molecular weight HSPs are generally produced only in response to environmental stress and little was known about their function (Howarth and Ougham, 1993) until 1998.
Heckathorn etal. (1998) found that HSPs are involved in protecting Photosystem II during exposure to high temperatures. Heckathorn etal. (1998) showed that whole-chain electron transport was greater in pre-heat-stressed plants relative to controls at 117F which indicates that acclimation to high temperatures occurred in pre-heat-stressed plants. This acclimation appeared to be entirely the result of production of low molecular weight HSPs.
High root-zone temperatures have a profound effect on plant growth. Root growth is retarded at temperatures greater than 30C or 86F. Root growth in many woody species stops at temperatures exceeding 40C or 103F (Johnson and Ingram, 1984). Cessation of top growth and shoot necrosis also occurs at these temperatures. High root-zone temperatures can result in decreases in photosynthesis. HSPs are involved in protecting Photosystem II during heat stress (Heckathorn etal., 1998).
Different species within genera have different tolerances to heat (Ranney and Peet, 1994), and different seed provenances within taxa have different heat tolerances. Provenance refers to the geographic origin of the seed. Root zone temperatures of 50C or 121F occur in Florida containers (Ruter, 1989), and 122F is reported in Oregon (Svenson, personal communication, 2000). Conventional containers in South Carolina commonly reached highs of 90 to 95F and can reach 110F in the center (London etal., 1998). Temperatures as high as 137F have been recorded in southern states (Martin and Ingram, 1988, and Ruter, 1997a). Normal root functioning ceases when root zone temperatures exceed 96F for holly (Ruter and Ingram, 1992) and at even lower temperatures, approximately 90F for less heat tolerant plants (Levitt, 1979). Media highs of 138F are also reached in Ohio in the center of one-gallon containers on gravel beds (Struve, personal communications, 2001).
The importance of keeping container substrate temperatures below 100F is well documented; however, as mentioned earlier, high substrate temperatures in above-ground containers in Florida and other states are not uncommon. Thuja occidentalis, Euonymus alatus, and Hosta spp. are particularly sensitive to high root temperatures. Picea glauca 'Conica,' the dwarf Alberta spruce, can be sensitive to high root zone temperatures when they are first spaced out in the spring, due to the narrow pyramidal habit of the plant. An even browning of foliage over the crown is one way root burn is expressed in Alberta spruce. In above-ground containers, the roots in the western quadrant of the container are often injured or killed by high temperatures.
In Pot-In-Pot (PIP) systems, roots in the western quadrant were 23F cooler than in above-ground pots (Ruter, 1997b). In the PIP production system, a planted container is placed in a holder pot that has been permanently placed in the ground. PIP was first started in the southern states to protect roots from extreme summer temperatures but really caught on in northern states because of the advantages in winter protection. Plants grown in PIP had 20% more top growth and nearly twice the root mass compared to conventional container-grown plants due to the protection provided from high temperature extremes.
Recently Fuchigami and Cheng (1999) emphasized that plants must be able to photosynthesize and maintain optimum chlorophyll levels to ensure optimum growth and plant health. Plants that experienced high root-zone temperatures suffered loss of chlorophyll and protein production in shoots (Kuroyanagi and Paulsen, 1988). Research indicates this has a significant impact on overall plant health (Ruter and Ingram, 1992).