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DIF AND HEIGHT GROWTH CONTROL OF CONIFER SEEDLINGS. HUMIDITY INFLUENCES?
Eric van Steenis |
| Abstract - In the last several years DIF (Differential Between
Day/Night Temperature) has been shown to impart operationally significant
control over internodal growth in a variety of flowering and foliage plants.
DIF in oC = day temperature - night temperature.
Conifer seedling growers using the "cool morning pulse" method of reducing DIF report favourable results, ie. reduced height growth rate or "stretching". In 1993 a preliminary trial was undertaken to see how much control could be achieved using a cool morning pulse as a -ve DIF regime vs a standard +ve DIF or regular, growing regime. 24 hour average temperature was kept constant between treatments. The trial was conducted in glasshouse compartments using underbench hot water heat, roof and side ventilation, and Argus computer control. Species utilized were Engelmann spruce and interior Douglas-fir. Reduced DIF (using the cool morning pulse) was instated during the rapid growth phase which occurred throughout the month of June for these particular 1-0 stocktypes. In this particular experiment significant differences were not observed. One reason being that a substantial DIF was not achievable on many days due to warm ambient night temperatures. Perhaps the most interesting aspect of the trial was the humidity and vapour pressure deficit data which indicated a greatly reduced transpirational demand placed on the -ve DIF plants relative to the +ve DIF plants. It is speculated that "stretching" due to reduced transpirational demand negated the control of internodal extension imparted by the more -ve DIF regime. To institute this form of DIF reduction requires cool outside night and early morning temperatures and adequate ventilation capacity. The former is not generally available in coastal B.C. during the month of June. It would be easier to subject summer ship crops to the treatment since their rapid growth phase occurs earlier in the year when ambient outside temperatures are cooler. Northern nurseries could make use of the concept throughout more of the calendar year. Dehumidification may be warranted in addition to temperature reduction, depending on crop type and humidity range employed. |
Research on the DIF concept in flowering and foliage plants has found the following principles to generally apply.
- Overall plant growth and speed of development (# of leaves/plant/unit time) is dependent on the average temperature (24 hour average).
- Stem elongation rate (internodal distance) is DIF dependent, with day temperature apparently being more influential than night temperature.
- On a daily basis, stem elongation rate is at a maximum at sunrise and shortly thereafter. Hence a drop in temperature at sunrise (2 to 4 hours) should effect a reasonable degree of height growth control without compromising the 24 hour average temperature and associated overall plant development rate to a great extent.
- Reducing temperature during this period may effect up to 80% of the control that can be achieved. Maintaining a reduced temperature for the rest of the day might only effect another 20%.
- Reducing DIF from a +ve value to zero effects a greater degree of height control than going from a zero DIF to a -ve DIF.
- Plants respond most rapidly and to the greatest extent during their rapid growth phase.
- Negative DIF leads to shorter internodal distance and pedicel length, and increases numbers of lateral shoots/branches, and flowers. It also leads to shorter, wider leaves. (These are termed thermomorphogenetic responses).
- Positive DIF results in a response similar to applying gibberellic acid [GA], (growth promoting hormone).
- Positive DIF effects can be overcome by applying [GA] synthesis blocking compounds such as ancymidol.
- Negative DIF effects can be overcome by applying [GA].
- Plant response to DIF varies with daylength. - plants are shorter under a -ve DIF when days are 8 hours long than when they are 12 hours long.
- under a +ve DIF plants are taller under 16 hour days than 10 hour days.
- DIF may have a greater effect under lower irradiance levels earlier during the growing season ( winter sow, summer ship crops). This may indicate that carbohydrate levels are not a major factor responsible for stem elongation and that perhaps DIF influences hormonal balance to create the desired effects.
- Implementation depends on outside weather conditions, ie. warm temperatures around sunrise will not allow effective greenhouse cooling.
In this presentation growth is simply defined as the product of cell division and expansion. It is further assumed that prior to lignification, plant cells are capable of expansion and do so primarily as a function of water uptake. Cell division and differentiation are biochemical processes requiring photosynthate. Biochemical reaction rates are temperature dependent hence heat sums or average temperatures can intuitively be assumed to have a large measure of control over this so-called "biochemical" growth.
Cell expansion involves water uptake hence is more of a physical process. Water, taken up by the roots, moves up through the shoot and out stomata and/or directly across membranes of succulent tissues. Transpirational draw, osmotic potential and root pressure drive water movement. Transpirational draw is created by the Vapour Pressure Deficit between the leaf and surrounding air which is a function of air/plant temperature and air mass moisture content. Root pressure at night is partly a function of the level of transpirational flow acquired during the previous day, which sets up a "flow" rate. Osmotic potential within cells, and root pressure in excess of transpirational draw outside cells, combine to determine the level to which a plant cell will "fill" with water. Cells filling with water results in their expansion and elongation or "physical" growth.
It is speculated that a large positive temperature DIF (higher day than night temperature) results in a relatively higher transpirational demand during the day and lower transpirational demand during the night. This sets up a daytime rate of transpirational flow which is partially maintained as root pressure at night. As transpirational draw is reduced during the course of the night root pressure may exceed transpirational draw. The excess water results in guttation, cell expansion if tissues are succulent and pliable, or both. Springtime in temperate climates are mostly positive DIF climates.
Perhaps reducing the day/night temperature differential (while maintaining the same 24 hour average temperature) reduces the amount of excess root pressure available at night and subsequent propensity to cell expansion and "stretching".
To visualize the concept, this presentation used a slinky (toy). Extending the slinky a set distance simulates daytime transpirational draw in excess of night time transpirational draw. Bringing the other end up the same distance simulates night time shoot elongation in response to this level of DIF. Large "slinky steps" simulate a large
positive DIF, smaller "slinky steps" simulating a less positive DIF.
It appears that there is more to controlling seedling or plant height growth than temperature. From this experiment it seems apparent that if temperature is influential it involves some indirect effects, and is perhaps mediated in part through associated changes in vapour pressure deficit and transpirational demand.
Applicability of the concept relies heavily on available ambient outside air temperatures, type of venting system, heating system heat capacity, as well as greenhouse covering. Radiative heat loss or cooling at night is reduced by the greenhouse roof hence differences might be noticed between glass vs poly coverings. The concept would probably be much easier to employ if a retractable roof system was available.
In this experiment the negative DIF treatment resulted in slightly taller plants, speculated to be due to the overriding effect of reduced VPD during the most crucial part of the day (dawn). The positive DIF regime developed more botrytis in late summer, speculated to be due to increased foliar wetness time (plant temperature below dewpoint temperature) during the night.
Adams, P. 1991. Effect of diurnal fluctuatuions in humidity on the accululation of nutrients in the leaves of tomato (Lycopersicon esculentum). Journal of Horticultural Science. 66 (5) 545-550.
Cuijpers, L. and J. Vogelezang. 1990. Geen effect luchtvochtigheid en wortel-temperatuur aangetoond. Vakblad voor de Bloemisteriu 47.
Heins, R. and Meriam Karlsson. 1990. Control Plant Growth with Temperature (4 articles).
Greenhouse Grower, Spring, 1990.
Hellmers, H. Genthe, M. K. and F. Ronco. 1970 Temperature Affects Growth and Development of Engelmann Spruce. Forest Science. Volume 16, #4. pp 447-541.
Hicklenton, Peter. 1992. DIF, Limited DIF and Stem Elongation Patterns in Pot Chrysanthemums. Canadian Florist, Greenhouse & Nursery. December 1992. pp18-19.
Moe, Roar. 1990. Effect of day and night temperature alternations and of plant growth regulators on stem elongation and flowering of the long-day plant Campanula isophylla Moretti. Scientia Horticulturae, 43. pp 291-305.
Thompson, S. 1989. Environnmental Control of Shoot growth in Scots pine, Sitka spruce and
Douglas fir seedlings. Forestry Supplement, Vol. 62.
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