**5. Climate, overwintering and defoliation**

In period of tough climatic conditions some plants and animals can no longer neither resist nor adapt to rude conditions of autumn and winter. Therefore, they adopt a specific strategy to survive. This is possible by pausing growth and development, which can occur in different organs like seeds and buds. This is known as dormancy which is controlled both by genetic and environmental factors. As the most studied dormancy, seed dormancy is an important adaptive trait in wild plants. The plant hormone abscisic acid plays a crucial role in the establishment and maintenance of dormancy, whereas gibberellins promote germination. The abscisic hormone (ABA) is specific to plants, and plays many roles for plant responses to stresses such as drought, salinity, cold and freezing tolerance, heat stress and heavy metal ion tolerance. Dormancy is a main determinant period in the plant life cycle. It has strong variation between species [7].

According to the predictability of climate, the dormancy may be preventive or consequential. Predictive dormancy is when plants can predict the onset of winter through the short photoperiod and the decrease of temperature, however when climate is unpredictable and has a sudden changes, the organism enter directly in consequential dormancy after adverse conditions. This last may cause a high rate of mortality before entering in consequential dormancy which is a protection strategy. Furthermore, the biological clock in many spices determine autonomously the period of the year for every phenophase.

In soil, seeds dormancy is continually adjusted by a set of environmental signals (**Figure 1**). The time of the year is determined by signals related to the slow seasonal change and this may indicate how sensitive the plant sensors mainly in seeds are. The figure illustrates the range of environmental signals and how they can potentially inform the seed of the time [8]. As buried and incorporated into soil, seeds responds to a wide range of edaphic and physical conditions to inform about the time of year and its appropriateness to the germination as illustrated in the **Figure 1**. The nitrate is commonly known to have a very important role in informing plants about the surrounding environmental conditions.

For the evergreen plants like some trees such as conifers, the dormancy consisted of the sustained light quenching for the whole winter period by the

**33**

filter [11].

**7. Climate and bud burst**

*Climate as the Major Factor Controlling Phenology DOI: http://dx.doi.org/10.5772/intechopen.95893*

adverse conditions and the plant needs.

not include the seedling growth [11].

model, the hydro-time model of germination [11].

**6. Climate, germination and dormancy break**

xanthophyll-mediated non-photochemical antenna. This is a form of a protection for the evergreen foliage from photo-oxidative damage when photosynthesis is restricted or prevented by low temperatures during the winter. The molecular mechanisms of this cold acclimation are still unknown, it implies alterations in the photosystem II antenna. Photosystem I is also involved via its support of cyclic electron transport at low temperatures, and also by non-photochemical quenching of absorbed light irrespective of temperature. Processes like chloro-respiration and cyclic electron transport may also be important for maintaining the functional integrity of the photosynthetic apparatus of overwintering evergreens both during periods of thawing in winter and during recovery from winter stress in spring [9]. Defoliation (removal of leaves), on the other hand and for the deciduous plants, is the strategy to minimize or stop the photosynthesis in the overwintering period. Defoliation accelerates sink metabolism and hence remobilizes carbon and nitrogen reserves, leading to improved source-sink relations. Through removing lower and senescing leaves, plant can assure a greatest capacity of photosynthesis and carbon and nitrogen metabolism in despite of adverse conditions [10]. Hence, the defoliation consists of a balancing between the minimum of photosynthesis supplies in

Whenever conditions are favorable, dormancy and overwintering become useless. Plants and seeds start anew their active lives. This starting is accomplished through germination in seeds characterized by the emergence of embryo from seed enclosing covers the endosperm, perisperm, testa, or pericarp. This metabolism is mainly activated by the seeds imbibition by water. This would incite the respiration metabolism to provide the necessary energy for the expansion of the embryo and after that the radicle through the covering tissues of the seed. The emergence of the radicle out of the seed indicates the germination completed and hence called the visible germination which ends up with the seeds germinated. Germination does

In fact, not only water is the climatic factor inducing germination, there is also temperature and sunlight. However, the most essential environmental factor required for seed germination is water. Water availability acts following a specific

While water availability and imbibtion of seed are indispensable to launch germination, temperature is important as well for germination and for all physiological functions both for animal and plants. In fact, the regulator role of temperature is commonly recognized in physiology as well as in germination. Indeed, temperature determines the germinability of seeds by determining its rate. It removes primary and secondary dormancy and temperature also induces the second dormancy [11]. Light is primarily responsible for the effect inducing germination after turning soil. As little as one millisecond of exposure to full sunlight can cause many seeds to germinate and produce seedlings. This principle may be utilized to reduce the use of herbicides in weed management programs. Hence, soil plays the role of light

Break in dormancy in many plants is triggered by temperature [1]. A sufficiently

high temperature is indeed needed to make bud bursts. This would occur due

#### **Figure 1.**

*Environmental signals controlling seeds dormancy and germination. Source: Reference [8].*

*Climate as the Major Factor Controlling Phenology DOI: http://dx.doi.org/10.5772/intechopen.95893*

xanthophyll-mediated non-photochemical antenna. This is a form of a protection for the evergreen foliage from photo-oxidative damage when photosynthesis is restricted or prevented by low temperatures during the winter. The molecular mechanisms of this cold acclimation are still unknown, it implies alterations in the photosystem II antenna. Photosystem I is also involved via its support of cyclic electron transport at low temperatures, and also by non-photochemical quenching of absorbed light irrespective of temperature. Processes like chloro-respiration and cyclic electron transport may also be important for maintaining the functional integrity of the photosynthetic apparatus of overwintering evergreens both during periods of thawing in winter and during recovery from winter stress in spring [9].

Defoliation (removal of leaves), on the other hand and for the deciduous plants, is the strategy to minimize or stop the photosynthesis in the overwintering period. Defoliation accelerates sink metabolism and hence remobilizes carbon and nitrogen reserves, leading to improved source-sink relations. Through removing lower and senescing leaves, plant can assure a greatest capacity of photosynthesis and carbon and nitrogen metabolism in despite of adverse conditions [10]. Hence, the defoliation consists of a balancing between the minimum of photosynthesis supplies in adverse conditions and the plant needs.

#### **6. Climate, germination and dormancy break**

Whenever conditions are favorable, dormancy and overwintering become useless. Plants and seeds start anew their active lives. This starting is accomplished through germination in seeds characterized by the emergence of embryo from seed enclosing covers the endosperm, perisperm, testa, or pericarp. This metabolism is mainly activated by the seeds imbibition by water. This would incite the respiration metabolism to provide the necessary energy for the expansion of the embryo and after that the radicle through the covering tissues of the seed. The emergence of the radicle out of the seed indicates the germination completed and hence called the visible germination which ends up with the seeds germinated. Germination does not include the seedling growth [11].

In fact, not only water is the climatic factor inducing germination, there is also temperature and sunlight. However, the most essential environmental factor required for seed germination is water. Water availability acts following a specific model, the hydro-time model of germination [11].

While water availability and imbibtion of seed are indispensable to launch germination, temperature is important as well for germination and for all physiological functions both for animal and plants. In fact, the regulator role of temperature is commonly recognized in physiology as well as in germination. Indeed, temperature determines the germinability of seeds by determining its rate. It removes primary and secondary dormancy and temperature also induces the second dormancy [11].

Light is primarily responsible for the effect inducing germination after turning soil. As little as one millisecond of exposure to full sunlight can cause many seeds to germinate and produce seedlings. This principle may be utilized to reduce the use of herbicides in weed management programs. Hence, soil plays the role of light filter [11].

## **7. Climate and bud burst**

Break in dormancy in many plants is triggered by temperature [1]. A sufficiently high temperature is indeed needed to make bud bursts. This would occur due

#### *Agrometeorology*

the expansion of internodes and leaves formerly formed in previous season. This high temperatures is almost needed for the newly formed buds due to the apical meristem getting activity resumed. On the other hand, the burst of dormant buds is caused by the elongation of the internodes following to cell expansion [12]. The young buds are very sensitive to coldness and may be severely damaged if they burst early in winter. This probable damage of buds have an inevitable repercussion on the crop.

Although, almost factors controlling the phenology of bud burst are poorly understood, bud burst has a particular timing controlled by some climatic and non-climatic factors including:

