If you examine the literature on the effects of heat on plants you will find a range of effects in fruit and flower crops such as poor fruit set, smaller fruit and flowers, less plant growth, and a decrease anthocyanin development in fruit and flowers.
Over the last couple of years we have been working with a graduate student from South Africa, Ockert Greyvenstein, as well as with our colleagues who specialize in plant physiology and ornamental rose production (Drs. Terri Starman, Brent Pemberton, and Genhua Niu) on a project to better understand the response of roses to heat stress. Our ultimate objective is to develop roses that flower as well in the midst of a Texas summer as they do in the cool spring and fall seasons.
The work has gone into several directions: a series of growth chamber and greenhouse experiments, the heat beds where 14 cultivars were planted in a replicated trial (plants donated by the Antique Rose Emporium), and finally screening done of the Moore Rose collection planted in our experimental field in Texas (collaboration with Greenheart Farms and Seville Farms).
Our first discovery came from the growth chamber studies which revealed that roses are most sensitive to heat stress about 2 weeks before they flower. The heat treatment caused all roses to develop smaller and paler flowers as well as several roses that exhibited an increased amount of flower abortion (Figure 1).
Subsequent work in a heat stress chamber has concentrated on determining a way to consistently evaluate potted roses for their heat tolerance. It appears that if we shock the plants with high heat (111 F or 44°C) for 3 hours at the stage at which the new flower buds become visible, we can replicate their behavior in the field. Ockert, our graduate student, has also developed a physiological test that looks at cell electrolyte leakage in response to heat stress to distinguish between heat tolerant and heat susceptible roses. The idea here is to have an efficient screening technique that does not depend on growing the plants in the field for several years to determine their heat tolerance. With these techniques we should be able to eliminate seedlings not tolerant to heat before planting into the field – thus reducing the size of the field that is needed.
The other ongoing work is with the Ralph Moore rose collection that was initially grown by Greenheart Farms in Arroyo Grande, CA and then shipped to Texas (January 2010) and planted in a plot maintained by Seville Farms in Mansfield, Texas. This past year we had an excellent opportunity to evaluate the heat tolerance of these materials as Texas had record temperatures during July and August. In August the average temperature in the Mansfield plot over 90°F (32ºC) with all but one of the days breaking 100°F (38ºC). The approach to get an Figure 2. Temperature in Mansfield in 2011. Note that evaluations were done in May (mean temperature 22.7°C/72.9°F ), August (mean temperature 34.1°C/ 93.4°F) and October (mean temperature 20.1°C/ 68.2°F )idea of their heat tolerance, was to assess their flowering intensity on a 0-9 scale in the cool seasons (May and October) as compared to the hot season (August) (Figure 2). In our rating system 0 was no flowers, 5 was 50-60% of the plant covered with flowers, and 9 was more than 90% of the plant covered with flowers. This rating system takes into account flower number, flower size and plant size. I expected that we would see greater flower intensity (more and larger flowers) in the cooler seasons versus the hot season and that there would be obvious differences among roses.
Although I did see a small decrease in flower intensity in the hot versus the cool seasons among the roses in this collection, it was much smaller than I expected (Figure 3). Nevertheless the roses differed tremendously in their ability to produce abundant flowers under Texas conditions (Figure 4) with about 15% (ratings 2 or lower) with poor flowering intensity and 6% (>5 flower intensity) with excellent flower intensity throughout the year.
When I looked at the data in groups by their mean flower intensity several patterns emerged. Among the roses that flowered poorly, these usually flowered best in the spring and generally decreased from there. Among those that had average flowering (2.5 to 4.5), there were quite a few that flowered steadily throughout the year. This surprised me. These I would consider heat tolerant but not productive. Among this average flowering group, there were also a few that were very sensitive to the heat stress such as ‘Sweet Chariot’ which bloomed profusely in the spring but in the summer had few blooms and the plant grew poorly. In the fall, it recovered fully and bloomed profusely again. This I would call very heat sensitive.
Finally among the most productive bloomers (flower intensity mean ratings of 5 or greater), there were heat tolerant roses that also produced abundant blooms throughout the growing season. Examples of these would be ‘Nurse Donna’, and ‘Robi’.
The above field observations are based on one year of data and in many cases only one replication so any conclusions are preliminary. Nevertheless in this diverse rose collection the roses differ dramatically in both flower production and heat tolerance as measured by flower intensity ratings. Roses do have high levels of heat tolerance and flower productivity with which to breed with. This is good but it needs to be noted that we still need to factor in the flower quality (consistent flower size, color, and overall appearance) under heat stress which is our next step in this research