Many deciduous fruit species undergo a period of inactivity during the winter months. This period commences prior to leaf fall (July in the Northern hemisphere) and terminates in early Spring. Two distinctly different physiological processes are responsible for this stoppage of bud break and shoot growth. One, termed dormancy by Horticulturists,
refers to growth cessation due to an unfavorable environment (i.e. short daylengths, low temperature, drought and etc.). When the limiting
environmental condition(s) is/are provided, bud break occurs and shoot growth commences.
Although dormancy is of interest to Horticulturists, a second physiological phenomenon responsible for a blockage of bud break during the same period that dormancy is in force has received more research attention. This phenomenon is termed "rest" and is responsible for fruit crop losses in many areas of the world that experience warm winters. Unlike dormancy, "rest“ is controlled by an internal chemical imbalance within the plant. As of this writing, the identification of the chemical mechanism of "rest" is unknown. Buds under the rest influence can be forced to grow following exposure to low temperatures (4° to 10°C). The rest influence is found only in the buds of certain plants and these buds gradually progress into rest and complete 3 phases of rest before bud break occurs. The first phase is termed "early" rest and is a period under which the buds are lightly inhibited and can be induced to force by leaf stripping, growth promoting chemicals, high temperatures and etc. Each succeeding week that a bud is in "early" rest, the degree of inhibition within the bud increases and more drastic measures (i.e. greater concentrations of growth promoting chemicals) are required to induce bud break. By October (in most areas of the Northern hemisphere) very little can be done to force these buds. At this time, the buds are said to be in “deep" or "main" rest. The degree of inhibition at‘ which the buds are held during “deep“ rest vary with the length of the chilling requirement. Fruit cultivars (varieties) or bud types with short chilling requirements do not enter as deep a state of inhibition during "main“ rest as do those with long chilling requirements. As chilling progresses the buds exit main rest and enter "late" rest which is very similar to "early" rest except the degree of inhibition under which the buds are held becomes less as more chilling is received. It is during .this period that buds are responsive to rest breaking chemicals. The length of this period extends to bud break. The greater the number of hours of chilling temperatures received, the fewer heat units or growing degree hours (high temperatures) are required for bud break once the bud has been exposed to growing temperatures. It is not uncommon for buds on fruit trees stored for extended periods of time at low temperatures (4°C) to force and grow without exposure to growing temperatures, thus they progress through “late" rest and force when they are no longer under any rest influence. Thus buds gradually progress through rest while exposed to low temperatures (4° to 10°C). The length of exposure required to bring a bud to the point at which it can be forced following 2 weeks exposure to chilling temperatures (4° to 10°C) is termed the chilling requirement.
The chilling requirement is influenced by rootstock, the scion (variety), bud type, and temperature. Trees that receive insufficient chilling to complete their "rest" requirement can be forced to bloom and grow by the application of "rest" breaking chemicals. In other cases, the buds on trees growing in the field can be cooled to temperatures below that of the ambient air resulting in sufficient low temperatures to satisfy their chilling requirement even though insufficient chilling temperatures were received. My talk with you today will discuss each point in detail.
Individuals within a species (varieties) can also have chilling requirements of varying lengths. For example, the peach cultivar (variety) 'Florida Prince' has a chilling requirement of 150 hours while the cultivar 'Mayflower' requires 1200 hours of chilling temperatures to complete rest. Other peach cultivars have chilling requirements that are intermediate between the extremes cited. Apple cultivars such as 'Anna‘ have very low chilling requirements but 'Rome' apple requires approximately 1200 hours chilling.
From this discussion one can see that cultivar plays a very important role in determining the chilling requirement. Current peach and plum breeding programs in California, Florida, Louisiana, Texas, and Georgia, USA are developing cultivars with low chilling requirements.
Rootstock effects on the chilling requirement.
Very little attention has been focused on rootstock as an influential factor on the chilling requirement of the scion: In all cases the rootstock is different genetically from the scion, thus it has the potential of having a different chilling requirement. It has also been known for I sometime that the rest influence can move across graft unions. Westwood and Chestnut found that when partially chilled 'Bartlett' (a long chilling pear cultivar) scions were grafted onto fully chilled P. calleryana (a short chilling pear species) rootstocks, the result was a reduction in the chilling requirement of the 'Bartlett' scions. Also Siberian C peach rootstock increased the chilling requirement of its scion. It was recently shown that the long chilling apple cultivar, Rome, growing in Vacaria, R.S., Brazil following a low chilling winter (400 hours) had fewer insufficient chilling symptoms when M26 and M7 were the rootstocks than when MM104 and MM106 were used. Both MM104 and MM106 are rootstocks with a long chilling requirement whereas M26 and M7 have shorter chilling requirements. This helps explain why long chilling cultivars growing in tropical areas, can be induced to flower by leaf stripping. Normally continued growth of long chilling species without allowing for the completion of the “rest“ requirement by chilling will result in growth stoppage and the induction of deep rest. In these areas a local rootstock selected from a “wild" seedling is used. Obviously the rootstock has a short chilling requirement since it originated in a tropical area and it transmits this reduced chilling characteristics to the scion.
The influence of rootstock on the chilling requirement of the scion is more noticeable under conditions of insufficient chilling. Differences in bud break due to rootstock were not evident in California or Germany where adequate chilling was obtained. However, it seems that a reduction in the scion chilling requirement of approximately 25% can be obtained through proper rootstock selection. This would be very important in areas that do not receive sufficient chilling. Studies are currently underway at the University of Georgia which should more clearly define the effects of rootstock on the scion chilling requirement.
The influence of bud type on "rest“.
The location of the bud on a shoot has an effect on the length of ‘the chilling requirement. Flower and/or vegetative buds are located terminally or laterally on a shoot. In the case of the apple the flower buds are located terminally while the vegetative buds are borne terminally or laterally. on the other hand, the peach, has terminally and laterally located vegetative buds but the flower buds are always located laterally. All terminally located buds have a shorter chilling requirement than those located laterally on the same shoot. This is true whether the buds are vegetative or flower. For example, the peach has a terminal vegetative bud that has a very short chilling requirement. Following winters during which there were an insufficient number of hours of chilling temperatures to satisfy the rest requirement of the lateral vegetative and flower buds, the terminal buds break and grow insuring survival of the tree. On the other hand the lateral flower and vegetative buds may not grow at all during that year. Also, the 2 lateral bud types in the peach may also have different chilling requirements. The lateral vegetative bud generally has the longest and the lateral flower bud is intermediate in chilling requirement between the terminal and lateral vegetative buds.
Since all apple flower buds are located terminally on long shoots or on spurs, this fruit species is better adapted (from a "rest“ standpoint) to warm climates than are many peach cultivars with intermediate and long chilling requirements. Following the low chilling winter of 1973-74 in Georgia, growers who grew both apples and peaches produced a normal apple crop, but few peaches. This was due to the short chilling requirement of the terminally borne apple flower bud which bloomed and set fruit while the longer chilling peach flower buds failed to produce fruit.
Therefore one can see that the location of the bud on the shoot plays a role in determining the chilling requirement. This variation in the chilling requirement due to bud type cannot be attributed only to correlative inhibition (apical dominance) since different lateral bud types can have different chilling requirements. Were it solely an apical dominance effect one would not expect this difference.
For years it has been accepted that chilling temperatures of 7°C (45°F) were optimum for satisfying "rest". Temperatures above 7°C were thought to be less effective and temperatures above 10°C (50°F) were felt to have no effect on "rest“ completion. It was also felt that "rest" completion resulted from exposure to continuous low temperatures and high temperatures fluctuated with chilling temperatures played no role unless the high temperature reached 24°C (75°F) then negation of some of the accumulated chilling occurred. However, the results of recent studies have changed our thinking relating to temperature effects on the completion of “rest" in deciduous plants.
We now know that continuous temperatures are sometimes not as effective as fluctuating temperatures and recent research has changed the effective range for chilling temperatures. Recent studies have shown that when unchilled plants are exposed to continuous temperatures, 8°C (45°F) is the most optimum for rest completion followed by 10°C (50°F) or 6°C (43°F) which are approximately 90% as effective as 8°C. Temperatures below 6°C have a reduced degree of efficiency so that 0°C (32°F) is only 20% as effective in satisfying chilling as 8°C. When temperatures are above 10°C, the chilling efficiency rapidly decreases as the temperature increases. Twelve degrees C (54°F) is 43% as effective as 8°C and 14°C (57°F) does not contribute towards satisfying the chilling requirement. These values are for continuous non-fluctuating temperatures, but we all realize that in nature fruit trees are rarely exposed to continuous temperatures. Generally, field temperatures fluctuate, rising during the day and becoming cooler at night. It has been found that fluctuating temperatures have a totally different effect on satisfying the rest requirement. When 6°C (43°F) is fluctuated in a cycle with 9°, 11°, 13°, or 15°C so that the low temperature 6°C is given for 16 hours and the high temperature for 8 hours, there is an enhancement effect in bud break. In other words, more efficient chilling occurs with the fluctuating temperature thus rest termination occurs with fewer hours of chilling. Also, fluctuating 0°C, 2°C, 4°C, or 6°C with 15°C in a diurnal cycle will result in an increase in the chilling efficiency of the low temperatures causing them to be as efficient in satisfying "rest” as 8° or 10°C applied either continuously or in a daily cycle in which the plants are held for 16 hours at the low temperature.
When the high temperature is increased to 18°C for 8 hours and cycled with 6°C for 16 hours, the enhancement effect is lost and the buds break at the same rate as when they were exposed to 6°C continuously. Increasing the high temperature to 19°, 20°, 21° or 22°C (8 hours) will result in chilling negation; High temperatures of 24°C for 8 hours will completely negate 16 hours of chilling temperatures. If the high temperature is reduced to 20°C (8 hours); only 13.2 hours of chilling is negated. Thus, the greater the temperature level, the greater the degree of negation.
The length of the exposure period also plays an important role in the degree of chilling negation. Although 8 hours of 24°C in a diurnal cycle with chilling temperatures will negate 16 hours of chilling, longer periods of exposure (3 to 6 days) following exposures to chilling temperatures will result in a maximum of approximately 24 hours of chilling negation. Thus the chilling effect seems to be fixed and is not likely to complete chilling negation. Lengths of exposure to high temperatures (24°C) for periods shorter than 8 hours in a diurnal cycle with chilling temperature will result decreasing degrees of chilling negation depending upon length of exposure. When the high temperature is 20°C applied in a diurnal cycle for 2 or 4 hours, an enhancement effect on bud break is noted. Longer exposure periods to this temperature results in chilling negation of varying degrees. Thus the degree of chilling negation by high temperatures will depend upon the duration and degree of the high temperature. Also, terminal buds are not as severely affected by high temperature chilling negation as are laterally borne buds.
Rest breaking treatments.
There has always been an interest in artificially, through chemical or other means, satisfying rest in insufficiently chilled buds. In all treatments that are successful, a large portion of the chilling requirement must be completed. Slightly chilled or unchilled buds do not respond to treatment and buds must be through “deep" rest and into late rest in order to respond to rest breaking chemicals. The current chemical treatment used to force partially chilled buds is a spray of 4 to 5% oil with 0.12% DNOC [dinitro-ortho-cresol]. Sometimes 2% thiourea is added to increase the vegetative bud response. The dormant trees are thoroughly covered with this material. The oil treatment covers the bud and excludes oxygen penetration into the bud which is known to release buds from rest while the DNOC is thought to initiate chemical reactions within the buds that help release them from "rest". However, care must be taken not to apply the oil when the temperatures are above 27° to 30°C (80° to 86°F) since bud injury will result. Trees treated in this manner will force and produce yields comparable to sufficiently chilled trees. Calcium cyanamide has been used to break dormancy in apple, grape, pear, peach, and raspberry. A 4% solution was found to be satisfactory.
Other methods used to release partially chilled buds from rest involve temperature reduction. Buds sprayed or covered with "whitewash" (builder's lime mixed with water) will have a lower temperature than untreated buds. Since buds are dark in color they absorb heat resulting in bud temperatures greater than that of the ambient air. The white covering provided by whitewash reflects the sun's rays, thus lowering bud temperatures. This treatment has been used effectively in combination with oil-DNOC in areas that experience insufficient chilling. Similar observations have been made where buds were shaded for extended periods of time by fog, cloud cover, or shade cloth.
The most recent method used for reducing bud temperature has been through evaporative cooling. Trees are misted with water supplied through solid set irrigation equipped with nozzles that emit low quantities of water. The irrigation is “on" only long enough to wet the buds and only when the ambient air temperature is above 10°C (50°F). The Water absorbs heat as it turns to-vapor thus the bud temperature is lowered. The maximum the bud temperature can be lowered is the difference between the "wet“ and “dry" bulb temperature. Thus a significantly greater degree of cooling can be obtained in dry areas than those that are humid. A major drawback to this method is the high initial investment for the irrigation system as well as the high energy cost necessary to operate it. In areas where the irrigation system is operated by gravity, the energy costs are not a problem.