THE WATER RELATIONS AND IRRIGATION REQUIREMENTS OF AVOCADO (Persea americana Mill.): A REVIEW

Abstract

The results of research on the water relations and irrigation need of avocado are collated and reviewed in an attempt to link fundamental studies on crop physiology to irrigation practices. Background information is given on the centre of origin (Mexico and Central America) and the three distinct ecological areas where avocados are grown commercially: (1) Cool, semi-arid climates with winter-dominant rainfall (e.g. Southern California, Chile, Israel); (2) Humid, subtropical climates with summer-dominant rainfall (e.g. eastern Australia, Mexico, South Africa); and (3) Tropical or semi-tropical climates also with summerdominant rainfall (e.g. Brazil, Florida and Indonesia). Most of the research reported has been done in Australia, California, Israel and South Africa. There are three ecological races that are given varietal status within the species: Persea americana var. drymifolia (Mexican race), P. americana var. guatemalensis (Guatemalan race) and P. americana var. americana (Antillean, West Indian or Lowland race). Interracial crossing has taken place. This paper summarises the effects of water deficits on the development processes of the crop and then reviews plant-water relations, crop water requirements, water productivity and irrigation systems. Shoot growth in mature trees is synchronised into flushes. Flower initiation occurs in the autumn, with flowering in late winter and spring. Flowers form on the ends of the branches. A large heavily flowering tree may have over a million flowers, but only produce 200-300 fruits. Fruit load adjustment occurs by shedding during the first three to four weeks after fruit set and again in early summer. Water deficits during critical stages of fruit ontogeny have been linked to fruit disorders such as ring-neck. Reproductive growth is very resistant to water stress (compared with vegetative growth). Avocado is conventionally considered to be shallow rooted, although roots extend to depths greater than 1.5 m. The majority of feeder roots are found in the top 0.60 m of soil and root extension can continue throughout the year. Leaves develop a waxy cuticle on both surfaces, which is interrupted by stomata on the abaxial surface (350-510 mm −2 ), many of which are blocked by wax. Stomata are also present on the sepals and petals at low densities (and on young fruit). During flowering, the canopy surface area available for water loss is considerably increased. Stomatal closure is an early indicator of water stress, which together with associated changes in leaf anatomy, restricts CO 2 diffusion. There have only been a few attempts to measure the actual water use of avocado trees. In Mediterranean-type climates, peak rates of water use (in summer) appear to be between 3 and 5 mm d −1 . For mature trees, the crop coefficient (K c ) is usually within the range 0.4-0.6. The best estimate of water productivity is between 1 and 2 kg fruit m −3 . Soil flooding and the resultant reduction in oxygen level can damage roots even in the absence of root rot. Avocado is particularly sensiti...

Factors associated with postharvest ripening heterogeneity of ‘Hass’ avocados (Persea americanaMill)

“…This load adjustment is characteristic of this species and there are two periods of fruit drop, during the third and fourth weeks post-fruit set and in summer [19]. This is due to the strong competition between young fruits and the vegetative growth.…”

“…Avocado is quite unique compared with other fruits in that cell division in the mesocarp is not only restricted to the initial period of growth, but it continues during fruit development and in mature fruit attached to the tree [18]. This fruit presents a very extensive flowering period of 2 to 3 months, and it does not ripen on the tree [19]. Fruit can hang on a tree for more than 12 months, long after reaching physiological maturity.…”

“…The initiation and time of flowering is affected by different factors including temperature, water stress and application of gibberellins [19]. Temperature is the main factor involved in the transition from the vegetative to the reproductive phase, since subtropical avocados produce buds if subjected to temperate temperatures.…”

“…Water stress does not induce blooming when temperatures are high, but increases the induction of blooming when temperatures are temperate. However, blooming can be delayed up to a month after water stress is finished [19]. It is important to emphasize that floral organs can increase the potential water loss surface of the tree by 90% [48].…”

Water status and productivity of 'Hass' avocado trees in response to supplemental irrigation during winter

“…production in the Southeastern region of Brazil -which includes the states of São Paulo, Minas Gerais, Rio de Janeiro, and Espírito Santo -occurs mainly under rainfed conditions, with no supplemental irrigation during the winter dry period. This practice, however, is common in some regions of Mexico, Guatemala, Honduras, Dominican Republic, Cuba, Colombia, and Venezuela, which are characterized by very different edaphoclimatic conditions (Carr, 2013). Therefore, more information is necessary on the proper managements for rainfed avocado production, in order to validate the cultural practices applied in other countries, aiming to optimize avocado yield in Brazil.…”