Agriculture Reference
In-Depth Information
and Northern China, Manchuria and Mongolia, and, in the Southern Hemisphere,
Southern South America, the tip of South Africa, Southern Australia, Tasmania
and New Zealand. Temperate fruit culture in marginal areas (Tropics and Sub-
Tropics regions) requires special horticultural practices and the use of specific
genotypes (cultivars with low chilling requirement). The temperate deciduous trees
or shrubs respond to seasonal changes in a number of ways depending on the extent
to which their internal physiology is affected by external environment. If the plant
is suited to a climate, each environmental change as light, temperature and gravity
is perceived and transduced to morpho-physiological change. The plant response
to environmental changes is mediated by internal signals such as the classical hor-
mones (IAA; GGAA; CK; Ethylene, ABA), peptides hormones, small interfering
peptides, small RNAs, sugars, etc.
The seasonal changes deeply affect deciduous trees behaviour determining a pre-
cise cycling of phases of intensive growth and rest. During the spring and the early
summer, the phenology of the tree opens with the bud-break and the shoot elonga-
tion. Shoot length, strongly affected by day-length (LD = long day), is made up by
pre-differentiated and newly formed phytomers. During the summertime, when a
shortening of the photoperiod occurs, shoot growth rate decreases due to a progres-
sive reduction of the internode length, while the morphogenetic activity of the meri-
stem continues. The progressive shortening of the internode determines the forma-
tion of the terminal bud, which is a good indication that the meristem and, actually,
the tree are entering in the rest phase, which is defined by reduced metabolic and
morphogenetic activity of organs (buds, tissues and cells). It is also conveniently
defined as a dormancy that from a functional point of view can be distinguished in
endo-, eco- and para-dormancy.
The endo-dormancy is induced by a shortening of the photoperiod occurring
during the middle-late summer. The shortening of the photoperiod is perceived by
light receptors (phytochromes and cryptochromes) that through specific signalling
pathways negatively affects GA biosynthesis and positively stimulates abscisic acid
(ABA) production. GAs and ABA are the two main actors regulating the establish-
ment of the endo-dormancy, although the role of other hormones cannot be ruled
out. The establishment within the tree of the physiological syndrome relates to dra-
matic morphological changes, particularly evident in the aerial part of the tree and
including formation of terminal buds, leaf senescence and abscission, structural cel-
lular changes leading to a block of the symplastic network.
The overcame of endo-dormancy is determined by chilling. The chilling require-
ment of a variety is an important characteristic since it defines the cultivation areas
of the temperate fruit species. It has been shown that it is a quantitative polygenic
trait controlled by specific QTLs. This notion allowed the constitution of low chill-
ing varieties and the introduction of some fruit species in marginal areas. When
the chilling requirement has been coped, the trees shifts from endo-dormancy to
eco-dormancy, a phase characterized by reduced metabolic and morpho-genetic
activity due to a lack or reduced accumulation of heat. All the main phenologi-
cal stages, such as bud-break, fruit bud development, and anthesis, are character-
ized by species/genotype-specific heat requirements. Heat accumulation might be
