H ORTS CIENCE 27(9):981-982. 1992. High Night Temperatures Do Not Cause Poor Lateral Branching of Chrysanthemum James E. Faust 1 and Royal D. Heins 2 Department of Horticulture, Michigan State University, East Lansing, MI 48824-1325 Additional index words. axillary bud, Dendranthema ×grandiflorum, lateral shoot Abstract. Dendranthema ×grandiflorm (Ramat.) Kitamura ‘Powerhouse’ plants were pinched to five nodes and grown in growth chambers at 35C day temperature (DT) and 14,17,21,24, or 27C night temperature (NT) to determine if NT influenced lateral shoot development on plants exposed to high DT. Vegetative cuttings were removed from two successive flushes of lateral shoots and evaluated for lateral shoot development after rooting and subsequent apex removal. Lateral shoot development was determined on a third flush of shoots that developed on the stock plants. The percentage of nodes that developed lateral shoots on stock plants or vegetative cuttings was not related to NT. The percentage of first-order, second-order, and third-order axillary nodes that developed a lateral shoot on the stock plants, averaged over all NT, was 76, 65, and 12, respectively. The percentage of nodes that developed lateral shoots on the first- order and second-order cuttings was 29 and 19, respectively. We concluded that cool NT were ineffective in preventing a decrease in lateral branching on plants grown under high (35C) DT conditions. Successful commercial production of harvested from stock plants are rooted and flowering, container-grown plants often de- pinched so that five to eight nodes remain to pends on the development of axillary buds produce lateral shoots (Crater, 1980; Han- after stem apex removal (pinching). In chry- zel, 1989). Plants with an insufficient num- santhemun production, vegetative cuttings ber of lateral shoots are not marketable. Most chrysanthemum cuttings produced in Received for publication 18 Oct. 1991. Accepted the United States originate from stock plants for publication 3 Apr. 1992. We acknowledge the grown in Florida and California. The cut- support of the Michigan Agricultural Experiment tings are harvested from field-grown plants Station and Yoder Brothers, Inc., Barberton, Ohio. and shipped to growers for forcing. Some The cost of publishing this paper was defrayed in cultivars (e.g., ‘Powerhouse’) grown during part by the payment of page charges. Under postal the summer near Fort Myers, Fla., have many regulations, this paper therefore must be hereby nodes that do not develop lateral shoots after marked advertisement solely to indicate this fact. pinching (C. Scharfenberg, personal com- 1 Graduate Research Assistant. 2 Professor of Horticulture. munication). Failure to branch is related to incomplete axillary bud development. Nor- H ORTS CIENCE, VOL. 27(9), SEPTEMBER 1992 mally branched plants are produced when cuttings are harvested from these cultivars during the winter near Fort Myers or during summer and winter near Salinas, Calif. The major environmental difference between seasons and the two locations is temperature. The daily mean minima and maxima during January are 20/27C near Fort Myers (M. Hackmann, personal communication) and 4/ 16C in Salinas (U.S. Dept. of Commerce, 1960); during July, they are 22/34C and 17/ 22C, respectively. High temperatures, dur- ing either the day, night, or both, were there- fore hypothesized to be the cause of the poor lateral-shoot development on summer-grown cuttings produced in Florida. The objective of this experiment was to determine the effect of night temperature (NT) on lateral-shoot development of chrysanthe- mums grown at high day temperature (DT). Rooted cuttings received from Salinas were potted in 1271-cm 3 pots and placed in growth chambers set at 25C DT and 20C NT. After 1 week, the plants were pinched to five nodes. Cuttings were selected that possessed an axillary bud at each node. Twenty-five plants were placed into each of five growth chambers with NT of 14, 17, 21, 24, or 27C. DT was programmed to increase from 27 to 35C during the first 4 h of the photoperiod, to remain at 35C for 8 h, and to decrease to 27C during the last 4 h of the photoperiod. The photosyn- thetic photon flux density at canopy height was maintained at 600 to 700 μmol·m -2 ·s -1 from cool-white fluorescent lamps for 16 h·day -1 (35 to 40 mol·m -2 ·day -l ). Lateral shoots developed from the first- order axillary buds and cuttings were taken 27 days after pinching (Fig. 1). A cutting 5 to 7 cm long with about seven unfolded leaves was taken from each lateral shoot, and two nodes were left on each of the stock plants’ lateral shoots. These cuttings were sent over- night to Yoder Brothers, Inc., Alva, Fla., 981