Macadamia responses to mild water stress at different phenological stages

Mild water stress was imposed on bearing macadamia trees in through-draining lysimeters at various phenological stages. Water was withheld until a xylem water potential (Psi(x)) of -1.5 to 2.0 MPa was reached, this being maintained by partial water replenishment at 2-day intervals. Flowering, nut set, nut development, yield, and quality responses were assessed to identify critical, stress-sensitive stages. Stress during nut maturation was particularly detrimental to both yield and quality, although stress during floral development and the premature nut drop also had adverse effects. Low yields were due to reduced nut number and smaller nuts. Lower photosynthesis (c. 2 mumol CO2/m(2).s) at -1.5 MPa would account for reduced yield and poorer quality during nut maturation when energy demands of active oil accumulation are high. Stress at floral initiation was generally not detrimental. Overall, stress restricted growth. The rate of girth growth was significantly lower when stress was applied during the dormant floral initiation stage. When stress coincided with normal periods of vegetative growth, flushing was delayed until after re-watering when greater foliage production was stimulated. Judiciously imposing mild stress may be used to manipulate macadamia phenology, although it may not necessarily conserve water. Further refinement would be needed to develop stress manipulation as a practical and reliable management tool to achieve higher yields. Water should be applied to alleviate stress during critical stages of nut development and maturation. Mild stress after the current crop is mature, however, is unlikely to be detrimental to macadamia yield or quality. It may, in fact, be beneficial through manipulation of flushing patterns that influence yield.