Bedding plants are widely used in landscaping or in containers. Two-thirds of the value of U.S. floriculture production in 2002 consisted of bedding plants.
 
Salvias have been one of the most common bedding plants used in landscaping and as a pot plant. The genus Salvia is one of the largest of all plant genera – approaching 1,000 species – and these plants are grouped into either annuals or perennials. Salvia splendens, or annual salvia, is the most common species used in floriculture, with many series providing a wide range of flower colors.
 
Two popular series (breeding selections which include a variety of flower colors) of salvia are Vista and Sizzler.

Though many bedding plants have been labeled heat-tolerant, such as salvias, they perform poorly in the heat of Louisiana and southern Gulf states. While some of these series or hybrids are heat-tolerant, many lack most of the characteristics that help a plant to withstand high temperatures. Despite the continued increase in popularity of herbaceous plants, the lack of available scientific data on the effects of heat stress on these plants and well-defined screening techniques for selecting heat-tolerant lines represent a major obstacle for cultivation and breeding.

We selected the red variety of each of Vista and Sizzler series of salvia to characterize their heat tolerance levels by studying their morphological (or structural) and physiological (or functional) responses under various higher-than-optimal temperature conditions. We conducted several experiments to determine the duration and extent of high temperature that most affected the Vista and Sizzler series of salvia.

For the first experiment, 3-week-old seedlings were subjected to several short-duration (3-hour), high-temperature treatments (86, 95, 104 and 113 degrees F) every three days, with 77 degrees F being the control or optimal temperature for growth. These treatments were applied until flowering. Exposure to 113 degrees F proved lethal to both series. Short-duration high temperatures of 86, 95 and 104 degrees F caused significant morphological changes in both series by the conclusion of the experiment. Vista had shortened internodes (the portion of stem between successive leaves), thicker stems, reduced plant height, no visual symp-toms of marginal leaf scorching and acceptable overall marketable quality (Figure 1). These morphological characteristics are similar to those that have been observed in other heat-tolerant plants.
 
The morphological characteristics of Sizzler were just opposite of Vista and proved to be less high-temperature tolerant.

A major effect of high-temperature stress is cellular membrane modification, causing what is termed a "leaky membrane." This is expressed as an increased permeability for ions and electrolytes, which can be readily measured by the outflow of electrolytes. Hence, membrane damage under heat stress can be determined by measuring membrane leakage of affected leaf tissue in an aqueous solution. We performed electrolyte leakage tests on leaf tissue of Vista and Sizzler to quantify the percentage of membrane damage. Vista had significantly lower membrane damage than Sizzler.

Heat Shock Proteins

Another signature physiological response of all living organisms to heat stress is decreased synthesis of normal proteins accompanied by an accelerated increase of a new set of proteins known as heat shock proteins (HSPs). Heat shock proteins are produced when plants are exposed to temperatures at least 5 to 10 degrees F above their optimal growing conditions. One unique feature of heat-tolerant plants is the abundance of HSPs during heat stress. They can be measured by their molecular weight using what is called a western blot analysis.

Vista and Sizzler plants were subjected to the same temperature treatments a second time, and plant protein was extracted to determine the expression of HSPs. As temperature increased, the levels of these proteins increased. The intensities of these bands were significantly higher at 95 and 104 degrees F in Vista than in Sizzler. This provided another indication of the ability of Vista to tolerate higher temperatures than Sizzler.

Finally, stomata (or pores) in plant leaves control the amount of water vapor exiting the leaf in the transpiration stream and the amount of carbon dioxide entering the leaf for photosynthesis. Stomatal conductance is defined as the measure of the amount of opening of the stomata. Studies with many agronomic crops prove that high-temperature-tolerant plants have higher stomatal conductance and transpiration rates. In Vista and Sizzler, both parameters increased as temperature increased up to 95 degrees F and decreased at 104 degrees F. Vista, however, had significantly higher stomatal conductance and transpiration rates.

The results from this research indicate that these types of experiments can be used to screen bedding plants for heat tolerance and in breeding plants with heat-tolerant traits. Based on the morphological traits, varieties with shorter internodes and thicker and erect leaves with short petioles (leaf stalks) are better survivors in extreme temperatures. Electrolyte leakage tests also can be easily performed for screening heat-tolerant plants. Furthermore, other physiological characteristics such as overexpression of heat shock proteins and higher stomatal conductance and transpiration rates are associated with heat tolerance. Hence, selection of bedding plants for heat tolerance should not only rely on field trials but also include an amalgamation of morphological and physiological characteristics that truly and more reliably define the ability of a plant to withstand high temperatures.