Effects of spatial and temporal regulation of drip emitters and tube configurations on water productivity of juvenile macadamia trees in the tropics

Surya P Bhattarai,David J Midmore,Lance Pendergast,Li Shuyan

doi:10.1017/s0014479722000011

Abstract

SummaryMacadamia in Australia is traditionally grown in semi-arid climates with hot and humid summers and cool winters supporting rainfed cultivation. Recent industry expansion into more northern, drier production areas of Queensland, Australia, requires supplementary irrigation for successful macadamia production. However, ever-increasing demand for irrigation water in these areas is both competitive and regulated. Limited information is available to optimize water use efficiency for field-grown macadamia trees. We trialled a technique that employs specially designed drip tubes with push-in emitter plugs to close emitters so that transplanting can start with emitters closed distant from tree bases and open next to the trunks of each tree. Additional emitters are then gradually opened (i.e., plugs are removed) as tree canopy size increases over subsequent years. This technique was tested on single and double in-line irrigation tube configurations per row of macadamia. Temporal regulation of emitter closure significantly reduced irrigation input by 75, 50 and 25% in the first, second and third year of treatment. Hence, irrigation over the three-year establishment period was reduced to one-half that of the non-regulated crop. These early reductions of irrigation in juvenile trees had no significant negative effects on plant growth (height, canopy spread, leaf chlorophyll and leaf photosynthetic rates), nor on nut counts. Control of emitter discharge between the plants along the row in the earlier stage (i.e., before complete within-row canopy cover) also reduced weed growth between the trees in the row. Notable growth advantages of the single in-line over the double in-line tube configuration were evident, with a non-significant but sizeable benefit on nut counts too. Effects of the temporal regulation of emitters and of in-line tube configurations must be validated on cultivars with differing water requirements and for the longer-term reproductive performance and nut quality.

Highlights

Macadamia (Macadamia integrifolia) is native to the coastal rainfall regions of southern Queensland and northern New South Wales of Australia (Turner, 1893)
Despite the implications of this regarding the absolute need for irrigation, there were a number of positive outcomes in our treatment comparisons relating to crop water management of interest for the macadamia industry
Trochoulias and Johns (1992) used a crop coefficient (Kc) of 0.88 for their eight-year plus macadamia crop with a canopy cover of less than 40%, whereas irrigation plus precipitation in the emitter regulation treatment in year one of the current trial was 78% of ETo, reducing to 46% in year two and 39% in year three as the grower customarily reduced water supply to the crop, during which canopy spread averaged less than 10% of the available plant area

Summary

Introduction

Macadamia (Macadamia integrifolia) is native to the coastal rainfall regions of southern Queensland and northern New South Wales of Australia (Turner, 1893). It has morphological adaptations to cope with dry sub-tropical environments. These traits include sclerophyllous leaves and root systems with ability to extract, transport and conserve water under low soil moisture conditions (Lloyd et al, 1991). Australia is one of the world’s largest macadamia producers (averaging 49 000 tonnes in shell production over the past five years (2015–2019) compared to that of the c. 160 000 tonnes produced globally), and macadamia constitutes the fourth largest Australian horticultural export crop in value. The Australian macadamia industry, valued at the farm gate at $293 million in 2020, contributes significantly to regional employment and rural economies (AMS, 2020). Australia has an estimated 10.2 million trees varying in age from newly planted to over 40 years old, covering an area of 33 000 ha with 25 000 ha currently bearing (AMS, 2020)

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