THE WATER RELATIONS AND IRRIGATION REQUIREMENTS OF MACADAMIA (MACADAMIA SPP.): A REVIEW

Abstract

SUMMARYThe macadamia nut is the only commercial food crop indigenous to Australia. It originated along the fringes of rainforests, only becoming an important internationally traded crop in the late 20th century. Australia is now the main centre of production, following a lead set by Hawai'i. The crop is also important in Central America and southern Africa. The majority of recent research reported in the literature on the physiology and water relations of macadamia has been undertaken in Australia. Macadamia is a tall evergreen tree in which vegetative growth occurs in a series of flushes. Mild water stress suppresses growth, but compensatory growth occurs on re-watering. Flowers form on hardened wood within the leaf canopy. Floral initiation occurs in late autumn, with flowering in the following spring. Flowering is profuse but only 5–10% of the flowers set fruit and less than 1% of the flowers reach maturity as fruits due largely to premature abscission. High temperatures (>30 °C) can contribute to the shedding of fruit. It takes about 12 months from floral initiation to the completion of harvest. Macadamia root systems are described as relatively shallow and spreading. The taproot can extend to depths of >1.2 m, with a dense matt of fibrous roots in the top 0.4 m. Proteoid rootlets are also present. Stomata are only found on the abaxial surface of the leaf. Macadamia has several attributes that help to explain its apparent tolerance to periodic dry conditions as found in its native habitat. These include leaves with xeromorphic adaptations, stomatal closure without concurrent changes in leaf water potential and an efficient water transport system. There has only been one serious attempt to measure the actual water use of macadamia. Most irrigation experiments have been poorly planned and/or the outcomes were inconclusive. Nut yields are cyclical and highly variable, making it difficult to establish cause and effect. It is not yet possible to say with evidence where and when irrigation is worthwhile.