The effect of night temperature on chrysanthemum
¯owering: heat-tolerant versus heat-sensitive cultivars
D.H. Willits
a,*, D.A. Bailey
ba
Department of Biological and Agricultural Engineering, North Carolina State University, Box 7625, Raleigh, NC 27695, USA
b
Department of Horticultural Science, North Carolina State University, Box 7625, Raleigh, NC 27695, USA
Accepted 22 July 1999
Abstract
The effect of night temperature on the ¯owering of heat-tolerant and heat-sensitive cultivars of potted chrysanthemum (Chrysanthemum xgrandi¯orum) was examined in four experiments over a period of 4 years. Temperature reductions were imposed only while the plants were under black cloth using a combination of air-conditioning and under-cloth ventilation. The two heat-sensitive cultivars tested were `Yellow Mandalay' and `Coral Charm' and the two heat-tolerant cultivars were `Iridon' and `Dark Bronze Charm'. Differences in time-to-¯ower (TTF) between heat tolerance classi®cations were less than anticipated. TTF was affected the most in `Iridon', a heat-tolerant cultivar, decreasing by an average of 4.2 days/8C as mean diurnal temperatures (MT) decreased from about 268C to about 238C. TTF was affected the least in `Coral Charm', a heat-sensitive cultivar, decreasing by an average of 2.8 days/8C over the same range. In¯orescence diameter, on the other hand, increased by as much as 9% in the two heat-sensitive cultivars but by only about 4% in the heat-tolerant cultivars. The results suggest that the heat-tolerant cultivars tested here may have been classi®ed based on consistency of ¯ower quality rather than TTF.#2000 Elsevier Science B.V. All rights reserved.
Keywords: Dendranthema xgrandi¯orium; Chrysanthemum morifolium; High temperatures; Temperature stress; Heat stress
1. Introduction
Heat delay can be a signi®cant problem for chrysanthemum producers in warm climates during the summer. For most of the year scheduling is very regular;
*Corresponding author.
however, high temperatures during the summer can delay ¯owering by several weeks or cause some buds not to open at all (Cockshull and Kofranek, 1994). Variations in timing and quality can result in varying amounts of plant material available for market at a given time.
Cathey (1957) suggested that chrysanthemum ¯owering was more sensitive to night temperature than either day temperature or mean temperature, but Cockshull and Kofranek (1994) showed that his conclusion was in error and that the data actually showed a dependence on MT instead of night temperature. Karlsson et al. (1989) suggested that ¯owering was dependent upon an interaction between day and night temperature, as well as the incident daily photosynthetic photon ¯ux. They also found that a night temperature between 178C and 188C yielded the largest ¯owers and the shortest TTF in `Bright Golden Anne', regardless of day temperature or irradiance level. Pearson et al. (1993) presented a model relating the reciprocal of ¯owering time to mean daily temperature and validated that model on several previously published datasets, including that of Karlsson et al. (1989). Cockshull and Kofranek (1994) suggested that above a mean daily temperature of about 228C the dependence of ¯owering time solely on mean daily temperature begins to break down.
There are some remedies to heat delay. The use of ventilation under the black cloth is a possibility, as are re¯ective and permeable cloths (Cockshull and Kofranek, 1994); however, these remedies cannot reduce temperatures below greenhouse levels, which rarely approach the optimum night temperature predicted by Karlsson et al. (1989) in the summer in warm climates. Ventilation and evaporative pad cooling set points can sometimes be reduced, but these adjustments are typically not made. An alternative which has the potential to address the above limitations, and one which is used commercially, is the use of heat-tolerant cultivars; however, the authors were unable to ®nd any studies in the literature comparing the temperature response of heat-tolerant cultivars to those that were classi®ed as heat-sensitive. This study will conduct such an examination to determine the extent to which heat-delay can be overcome using this approach.
2. Materials and methods
Rooted cuttings were placed into 12.7 cm diameter azalea pots ®lled with either Metro Mix 350 (The Scotts Co., Marysville, OH) in 1993±1995 or Fafard 4B (Fafard, Agawam, MA) in 1996, 1 per pot (on 30 June in 1993, 8 June in 1994, 6 June in 1995 and 11 June in 1996). The plants were periodically misted for 1 week while the roots expanded. Long days were imposed during this period (and the week following) by lighting from 23:00 to 01:00 hours using 60 W incandescent bulbs spaced 1.2 m apart, 1.5 m above the bench. At the end of long days, the plants were given pinched and moved into the greenhouse (on 19 July in 1993, 27 June in 1994, 22 June in 1995 and 27 June in 1996). The pots were placed on the ¯oor on 24 cm centers and short days were imposed by covering the plants with heavy black cotton cloths from about 17:00 to 07:30 hours each day. Only the apical in¯orescence on each shoot was allowed to mature. The plants were fertilized twice per week with a commercial formulation of 20 N±4.3P± 16.6K (The Scotts Co., Marysville, OH) at 200 ppm N.
Temperature treatments were imposed in two ways: (1) within each greenhouse, two sub-treatments were imposed; one in which greenhouse air was drawn under the black cloth and one in which it was not. Two houses were used in each test, with one being air-conditioned and one not. The non-air-conditioned house used only conventional cooling whereas the air-conditioned house used conventional cooling during the day and air-conditioning (208C set point) whenever the black cloth was in place. Set points for conventional cooling (day and night) in all houses were 258C, 278C and 288C (low vent, high vent and evaporative pad, respectively).
The houses were controlled via computer and environmental data (outside air temperature, solar radiation, greenhouse air temperature, and temperature under the black cloth) were recorded as 30 min averages of 1 min readings. Photosynthetically active radiation (PAR) at canopy level was estimated from outside solar radiation readings using calibration factors developed over several years (Willits and Peet, 1998).
When in¯orescence buds began to open, the number of buds showing color and those fully open on each plant were recorded daily. TTF was calculated as the number of days from the beginning of short days until 50% of the ¯owers had fully opened. At that time, the total number, number of aborted and number of partially-open in¯orescences were recorded. In¯orescence diameters were recorded as two measurements on perpendicular axes across the in¯orescence, then averaged. Average in¯orescence diameter for the whole plant was the mean of the all of the fully open ¯uorescence diameters.
3. Results and discussion
treatment and years. The regression equations are similar to those presented by Pearson et al.; however, attempts to predict an optimum temperature based on their equation (3) were unsuccessful, undoubtedly because our temperatures were supra-optimal. The slopes of the equations shown in Fig. 1 are different among cultivars (P0.0001) but the intercepts are not. Nevertheless, the equations are presented as independent because of the convenience of obtaining individualR2's. The equations suggest that TTF was most affected in `Iridon', a heat-tolerant cultivar, with an average rate of change of 4.2 days/8C over the observed range of 23±26.58C. The cultivar least affected was `Coral Charm', a heat-sensitive cultivar, with an average rate of change of 2.8 days/8C over the same temperature range. This suggests that the use of heat-tolerant cultivars, at least the ones tested in this study, will not improve heat delay in chrysanthemum.
The effect of temperature on average in¯orescence diameter is shown in Fig. 2. The data were normalized by dividing each in¯orescence diameter by the mean in¯orescence diameter for the warmest treatment for that cultivar in that year. Again, the data shown are averages over cultivar, treatment and year. Normalized diameters (ND) were plotted against mean night temperature (NT), rather than MT, because the R2's were higher. The data were tested using the model
NDabNTcCVdCVNT, where CV is a class variable represent-ing cultivar, CVNT is the cross product between cultivar and night temperature, and a, b, c,d are the regression parameters. The results suggested that `Yellow Mandalay' and `Coral Charm' did not respond differently (P0.05) nor did `Iridon' and `Dark Bronze Charm'. On the other hand, both slopes and intercepts were different between heat tolerance classi®cations (P0.0001) when the class variable TYPE was substituted for CV. The resulting equations suggest that ND was more responsive to NT in the heat-sensitive cultivars than in the heat-tolerant cultivars. ND for the heat-sensitive cultivars was approximately 9% larger at 218C than at 258C but only about 4% larger for the heat-tolerant cultivars over the same temperature range. The response of ND to NT may have been the basis for the heat classi®cation, since a more stable in¯orescence diameter would result in a more consistent quality over a wider temperature range.
It should be noted that in¯orescence diameter in our study was not as responsive to NT as was the case in Karlsson et al. (1989). The temperature differences experienced in our study would have predicted differences as large as 47% using their data. Differences between cultivars (they used `Bright Golden
Fig. 2. ND vs. NT. Symbols are the same as in Fig. 1.R2was 0.45 for the model NDabNT
cTYPEdTYPENT, where TYPE is either `sensitive' or `tolerant' depending upon heat tolerance classi®cation. TYPENT is the cross product between type and night temperature, and
Anne') and protocols (they allowed only 3 in¯orescences per plant versus 7±10 in our study) may have contributed to this discrepancy. Our data suggest no temperature optimum for in¯orescence diameter, probably because our mean night temperatures were several degrees higher than the 188C optimum predicted by Karlsson et al. (1989).
Acknowledgements
The authors wish to express appreciation for the ®nancial support provided by the Fred C. Gloeckner Foundation, Harrison, N.Y., and for the plant material donated by Yoder Brothers, Barberton, OH.
References
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