Heat stress imposed on roots of container-grown plants is an important problem in the nursery industry. In a number of nursery-grown species, substrate temperatures over 30 °C may cause root growth to slow considerably (Johnson and Ingram, 1984). Furthermore, in a number of woody species, root growth stops completely at temperatures above 39 °C (Mathers, 2003). Substrate temperatures inside nursery containers can rise higher than 54 °C (Ingram et al., 1989; Martin et al., 1989; Mathers, 2000), resulting in crop loss or reduced crop quality. This is especially problematic to some woody species [e.g., Japanese holly (Ilex crenata Thunb. ‘Helleri’)] because their roots die when exposed to temperatures of 51 °C for merely 30 min (Martin et al., 1989).
The detrimental effects of heat stress on root growth affect the whole plant and thus also impacts aboveground production. For example, high root-zone temperatures in container-grown nursery plants may cause leaf wilting, chlorosis, and drop; reduce flower numbers and quality; cause abnormal branching; and interfere with normal physiological and biochemical processes (e.g., photosynthesis and respiration, water and nutrient uptake, hormone synthesis, and translocation processes) (Ingram et al., 1989). High root-zone temperatures may also increase the incidence of disease and cause plant injury or death (Ranney and Peet, 1994; Webber and Ross, 1995). Therefore, investigations into the effects of heat stress on root growth in container-grown plants would benefit by including evaluations of whole-plant responses.
It is possible that growing plants in containers made of colors lighter than standard black may improve root growth. Lighter colored containers have greater albedo than dark containers and thus reflect more solar radiation away from the container (i.e., less solar energy absorbed by the container) (Ham et al., 1993). Consequently, lighter colored containers may be a means to mitigate heat stress in nursery production.
Fretz (1971) reported that substrate temperatures were reduced by 5.6 °C in light- than in dark-colored containers. Whitcomb (1980, 1999, 2003) also found that substrate temperatures were reduced by 3 to 6 °C when standard black containers were covered with white laminated fabric sleeves (RootSkirts®; Rootmaker Products Co., Huntsville, AL). Similarly, substrate temperatures were decreased by 1 to 7 °C in containers made of an insulating black fabric that was coated on the outside with white polyethylene (Whitcomb and Whitcomb, 2006).
Ingram (1981) evaluated the effects of substrate temperature on root growth of flowering dogwood (Cornus florida L.), rhododendron (Rhododendron simsii Planch. ‘Formosa’), and japanese pittosporum (Pittosporum tobira Banks) grown in polyethylene bags with a white outer surface and conventional, rigid black containers. In their study, maximum daily temperatures were 6 °C higher in black containers than in white polyethylene bags. Root growth of plants in white polyethylene bags, with cooler substrate compared with black conventional containers, was three times greater in rhododendron and four times greater in flowering dogwood but unaffected in japanese pittosporum, indicating substantial differences in heat tolerance among species.
Although container-grown species in the nursery industry may vary in their susceptibility to heat stress, all species benefit from developing an extensive root system in propagation and early production (Davidson et al., 2000). If root growth is compromised during production, transplant survival and growth may later be negatively affected in the landscape (Richardson-Calfee et al., 2010). Therefore, investigations into potentially heat-mitigating practices such as the use of lighter colored containers on root growth and distribution may prove beneficial for a number of important nursery crops.
Red maple and eastern redbud are examples of important species in the nursery industry because they are native to the United States, tolerant of a wide range of environmental conditions, and have desirable ornamental characteristics (Dirr, 2009). Wilkins et al. (1995) evaluated several genotypes of red maple for tolerance to high root-zone temperatures and showed that some were relatively sensitive, whereas others demonstrated resistance to heat stress in the root zone. A study by Griffin et al. (2004) revealed that redbud is tolerant of high temperatures and drought.
In this study, we evaluated effects of container color on substrate temperatures, including spatial variability in substrate temperatures. Subsequent effects of container color and substrate temperature on root distribution and shoot development were also evaluated in container-grown maples and redbuds.
Davidson, H., Mecklenburg, R. & Peterson, C. 2000 Nursery management: Administration and culture 4th Ed Prentice Hall Upper Saddle River, NJ
Griffin, J.J., Ranney, T.G. & Pharr, D.M. 2004 Heat and drought influence photosynthesis, water relations, and soluble carbohydrates of two ecotypes of redbud (Cercis canadensis) J. Amer. Soc. Hort. Sci. 129 497 502
Ham, J.M., Kluitenberg, G.J. & Lamont, W.J. 1993 Optical properties of plastic mulches affect the field temperature regime J. Amer. Soc. Hort. Sci. 118 188 193
Ingram, D.L. 1981 Characterization of temperature fluctuations and woody plant growth in white poly bags and conventional black containers HortScience 16 762 763
Ingram, D.L., Martin, C. & Ruter, J. 1989 Effect of heat stress on container-grown plants Comb. Proc. Int. Plant Propagators Soc 39 348 353
Martin, C.A., Ingram, D.L. & Nell, T.A. 1989 Supraoptimal root-zone temperature alters growth and photosynthesis of holly and elm J. Arbor. 15 272 276
Mathers, H. 2003 Summary of temperature stress issues in nursery containers and current methods of protection HortTechnology 13 617 624
Omae, H., Kumar, A., Kashiwaba, K. & Shono, M. 2006 Influence of high temperature on morphological characters, biomass allocation, and yield components in snap bean (Phaseolus vulgaris L.) Plant Prod. Sci. 9 200 205
Petkova, V., Denev, I.D., Cholakov, D. & Porjazov, I. 2007 Field screening for heat tolerant common bean cultivars (Phaseolus vulgaris L.) by measuring of chlorophyll fluorescence induction parameters Sci. Hort. 111 101 106
Ranney, T.G. & Peet, M.M. 1994 Heat tolerance of five species of birch (Betula): Physiological responses to supraoptimal leaf temperatures J. Amer. Soc. Hort. Sci. 119 243 248
Richardson-Calfee, L.E., Harris, J.R., Jones, R.H. & Fanelli, J.K. 2010 Patterns of root production and mortality during transplant establishment of landscape-sized sugar maple J. Amer. Soc. Hort. Sci. 135 203 211
Webber, J.E. & Ross, S.D. 1995 Flower induction and pollen viability for western larch U.S. Dept. of Agriculture, For. Serv., Intermountain Research Station Ogden, UT
Whitcomb, C.E. & Whitcomb, A.C. 2006 Temperature control and water conservation in above-ground containers Proc. Int. Plant Propagators Soc 56 588 594
Wilkins, L.C., Graves, W.R. & Townsend, A.M. 1995 Responses to high root-zone temperature among cultivars of red maple and freeman maple J. Environ. Hort. 13 82 85