Agriculture Reference
In-Depth Information
survives temperatures above −4°C, although the fl owers are damaged and
fruit set can be very poor at 17/12°C (day/night).
Solar radiation and photoperiod
Avocado is a typical C
3
plant, with maximal CO
2
exchange occurring at
20-25°C. The light saturation point for photosynthesis of mature 'Hass'
avocado trees is 1110 μmol quanta/m
2
/s photosynthetic photon fl ux, about
half that at midday in the tropics. Leaves take about 40 days from bud-break
to when they become net exporters of photoassimilates. During these 40 days
leaves may compete with developing fruit for available photoassimilates, and
this competition infl uences fruit retention and fruit development. Day length
is apparently of little importance, as there have not been any published studies
on avocado responding to photoperiod.
Wind
The avocado tree is easily damaged by winds, due to its brittle branches.
Moderately high winds can cause severe damage. If orchards are not located
in naturally sheltered areas, windbreaks are advised.
GENERAL CHARACTERISTICS
Tree
The avocado tree is variable in shape, from tall, upright trees to widely
spreading forms with multiple branches. Trees can attain heights of 15-18 m,
with manageable height being controlled by pruning. Although classifi ed as
an evergreen, some cultivars shed leaves during fl owering, which are replaced
rapidly from terminal shoots. Others shed their leaves gradually, so that
they are never without leaves completely. The dark green leaves are spirally
arranged and variable in size, from 10 to 13 cm wide and from 20 to 25 cm
long, entire, elliptic or ovate to lanceolate (Fig. 7.2).
New growth occurs in fl ushes (Fig. 7.3). Flushes of shoot and root tend to
alternate on a 30-60-day cycle (Ploetz
et al.
,
1991). Root growth continues
throughout the year in subtropical areas, even at a low rate between the
fl ushes, while shoot growth may stop. The period between fl ushes varies
with location (Fig. 7.3) and cultivar. Growth fl ushes in summer tend to be
asynchronous, with only a portion of a tree canopy being involved. Trunk
starch concentration ranged between 4.5 and 6.3% during active tree and
fruit growth. In trees where the fruits are not harvested until full maturity
with 30% dry matter, a trunk starch concentration of 5.3% occurs prior to
fl owering, versus 7.7% in trees harvested earlier (Kaiser and Wolstenholme,
1994).
