Introduction

Heating costs can be a major greenhouse operational expense, especially in the northern regions of British Columbia. In spring, there may be a large temperature differential between the outdoor air temperature and greenhouse air temperature. When this temperature differential is large, fuel costs will be substantial. The application of minimum DIF (day temperature minus night temperature) regimes may be one method to reduce heating costs, and at the same time provide height control.

Research indicates the smaller the DIF value, the greater the degree of height control of horticultural crops (Erwin et al. 1988; Heins and Erwin 1990; Heins et al. 1988). Also, low temperature during sunrise, when maximum rate of internode elongation is greatest, was the most crucial time to have low temperatures (Heins et al. 1988). A two hour temperature drop at sunrise was nearly as effective in controlling height as cooling all day. This research suggests that the conventional high day and low night temperature regimes used to grow conifer seedlings may encourage stem elongation. Perhaps conifer seedlings should be grown with the smallest DIF value possible so that height control may be achieved.

Based on the above experiments with DIF, the easiest and most effective time to apply low temperatures in conifer container production would be at sunrise. This is when the temperature differential between inside and outside the greenhouse is the greatest. Quick temperature drops in the morning, as described by Heins et al. (1988), may be effective in controlling conifer seedling height. In addition, minimizing DIF at sunrise can reduce greenhouse operational costs in two ways. The heater will not be on at sunrise when the temperature differential is the greatest between outside and inside the greenhouse. Heat normally provided by the greenhouse heater at sunrise will not be exhausted outside a few hours later when the sun warms the greenhouse. In the absence of heating at sunrise, cooling should be activated for shorter periods of time.

Application of low temperature regimes at sunrise has the potential of offering several advantages in the production of conifer seedlings. This experiment will determine if low temperature regimes at sunrise is an effective method of controlling conifer height (ht) without any undesirable effects on root collar diameter (RCD), root dry weight (RDW), stem dry weight (SDW), and shoot/root dry weight ratio (S/R).

Materials and Methods

In 1992, Sitka x white spruce (Picea sitchensis x P. glauca (Bong.) Carr.) seedlots 29160 and 6381 were sown in styroblock containers (PSB 313b, 65 ml volume cavity; Beaver Plastics, Edmonton, Alberta) filled with a peat based medium on March 9. After sowing, 28 styroblocks of each seedlot were placed in a double polyethylene covered greenhouse at Red Rock Research Station in Prince George, British Columbia. Seedlings were cultured as described by Eng (1991), except when the following four treatments were applied (7 styroblocks per treatment):

a. POLY - Seedlings were placed in a double polyethylene covered greenhouse with heating/cooling regimes as described by Eng (1991). Greenhouse sidewalls were opened when necessary to reduce excess heat buildup.

b. OUT - Seedlings were placed in a double polyethylene covered greenhouse with heating/cooling regimes as described by Eng (1991), except from July 6 to August 27. From July 6 to August 27, seedlings were placed outside (no polyethylene cover, no temperature control, limited control over nutritional and moisture levels).

c. B/O - Seedlings were placed in a double polyethylene covered greenhouse with heating/cooling regimes as described by Eng (1991). Blackout was imposed 2100 h to 0700 h from July 6 to July 16.

d. DIF - Seedlings were placed in a double polyethylene covered greenhouse as described by Eng (1991), except from July 6 to August 27. From July 6 to August 27, the following conditions were imposed to minimize heating costs and to encourage warm nights and cool days: heaters were turned off, sidewalls were kept opened, blackout curtains were closed from sunset to sunrise, and exhaust fans were turned on for 30 minutes when the blackout curtains opened at sunrise.

The experiment was repeated in 1993. Due to concurrent experiments being conducted by other scientists, the following changes were made. In 1993, Engelmann spruce (Picea engelmannii (Parry) Engelm.) seedlot 30664 was sown on February 16 as described above. Treatments were applied to 20 styroblocks (5 styroblocks per treatment) in a similar manner as above, except for the following changes:

a. POLY - As previously described.

b. OUT - Treatment was applied from June 11 to September 7.

c. B/O - Treatment was applied 1830 h to 0730 h from June 3 to June 17.

d. DIF - Treatment was applied from June 11 to September 7. Blackout curtains were not used. Sidewalls and doors were opened during working hours from 0730 h to 1600 h. From 1600 h until sunset, exhaust fans were on to keep the greenhouse cool. From sunset to sunrise, the exhaust fans were off to keep in the warm air. At sunrise until the doors and sidewalls opened at 0730, the exhaust fans were on to cool the greenhouse.

Data were collected on November 2 in 1992 and November 7 in 1993.

Results

Periodic measurements of the air temperature (data not presented) indicated that seedlings receiving the OUT treatment were subjected to the lowest day and night temperature. Seedlings grown under the DIF treatment received the warmest night temperature. For the DIF treatment, at sunrise when cooling was in effect, there was a drop in air temperature. The temperature during the day for the DIF treatment was comparable to the OUT treatment. The POLY treatment had the highest day temperature.

Seedlot 29160 did not respond to the applied treatments (Table 1). In contrast, there were treatment effects on seedlot 6381 (Table 2). The blackout treatment produced the shortest seedlings. This was followed by the DIF and OUT treatment. The tallest seedlings were produced under the POLY treatment. Seedlings receiving the OUT treatment had the greatest RDW and SDW. The blackout seedlings had the least RDW and SDW. There was no treatment effect on RCD and S/R.

Similarly, for seedlot 30664, the B/O treatment produced the shortest seedlings, followed by the DIF and OUT treatment. Seedlings grown under the POLY treatment were the tallest. Treatment did not affect RCD. The DIF and OUT treatment produced seedlings with the largest RDW. Seedlings with the least RDW were grown under the POLY treatment. Blackout seedlings had the least SDW. Seedlings receiving the POLY treatment had the greatest S/R, indicating that these seedlings were more top heavy than the other seedlings.

Discussion

Plant response to the applied treatments varied, depending on seedlot. Seedlot 29160 did not respond whereas the other two seedlots did. For seedlot 6381 and 30664, the B/O treatment produced the shortest seedlings. This was followed by the DIF and OUT treatment.

In this experiment, applying DIF at sunrise produced seedlings that were comparable to those grown outside during the summer months. There may be several reasons why applying DIF at sunrise did not produce seedlings shorter than those grown under the OUT treatment. Firstly, the DIF treatment was applied late in the growing season. The seedlings may have already put on too much top growth before treatment was applied. Secondly, there may not have been enough of a drop in temperature at sunrise to effectively control height. Application of DIF at sunrise should work best in areas where the outside night temperatures are low. In areas with low night temperatures, it should be easier to cool the greenhouse at sunrise. Lastly, there is the possibility that the effects of DIF on conifer seedling growth is minimal.

Blackout was the most effective treatment in controlling height. In this experiment there was no adverse effect from opening the blackout curtains quickly. Thus, it would not hurt to keep the curtains closed all night to retain the warm air and to open them quickly to drop the temperature upon exposing the seedlings to light. Although there was no benefit from applying DIF at sunrise in this experiment, there is the possibility that DIF may provide height control under different conditions and at different geographic locations as cited in the literature.

Table 1. Seedlot 29160 response to treatment. Mean separation within columns using Duncan's Multiple Range Test (p<0.05).

Table 2. Seedlot 6381 response to treatment. Mean separation within columns using Duncan's Multiple Range Test (p<0.05).

Table 3. Seedlot 30664 response to treatment. Mean separation within columns using Duncan's Multiple Range Test (p<0.05).

Acknowledgements

The author is grateful to Bonnie Hooge, Chris Hawkins, and Edith Johnson for their technical assistance.

Literature Cited

Eng, R.Y.N. 1991. Temperature, nutrition, light and

moisture regimes used at Red Rock Research Station to produce container 1+0 spruce seedlings. B.C.M.F. RR91004-PG.

Erwin, J.E., R.D. Heins, and M.G. Karlsson. 1988. Thermomorphogensis in Lilium longiflorum. Amer.J.Bot. 76:47-52.

Heins, R.D. and J.E. Erwin. 1990. Understanding and

applying DIF. Greenhouse Grower 8:73-78.

Heins, R.D., J.E. Erwin, R. Berghage, M. Karlsson,

J. Biernbaum, and W. Carlson. 1988. Use temperature to control plant height. Greenhouse Grower 6:32.

¹Paper presented at the Forest Nursery Association of B.C., 15th Annual Meeting, September 18-20, 1995, Harrison

Hot Springs, B.C.

²Research Officer, Red Rock Research Station, B.C.

Ministry of Forests, RR7, RMD6, Prince George, B.C.

V2N 2J5.

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