INTRODUCTION

Macadamia (Macadamia integrifolia Maiden and Betche) passes through several developmental stages in its annual cycle, including vegetative flushes, emergence of buds and racemes, anthesis (flowering), maximum fruit size, and fruit maturity (harvest). The timing of the occurrence of a specific stage and the passage from one stage to another are largely controlled by climatic and weather conditions, the most important being temperature. Models for accurately predicting the occurrence of stges of growth and development of perennial crops provide valuable planning tools for growers, processors, and researchers. For example, being able to estimate fruit maturity can help growers plan for harvest and packing house labor and can help buyers, shippers, and packing house suppliers plan their operations.

The objective of this study was to develop a model that predicted fruit growth of macadamia.

PROCEDURE

This study was conducted at the Waiakea Experiment Station in Hilo, HI. Trees of the cultivar 'Ikaika' were selected, and racemes at anthesis were tagged in January. At weekly intervals, 10 racemes were randomly harvested, and the diameter of all fruits on the racemes were measured. Data was collected from January through August.

Model development. The model was based on the assumption that air temperature was the most important factor affecting fruit growth. The optimum temperature and lower and upper temperature limits for fruit growth were estimated based on data for vegetative growth of macadamia. The data indicated that 25°C (77°F) was the optimum temperature for vegetative growth (stem diameter, leaf area, and plant dry weight); 10°C (50°F) was the lower temperature limit; and 35°C (95°F) was the upper temperature limit. Thus temperatures higher than 10°C and lower than 35°C were used in the model with 25°C being the optimum temperature.

Fruit growth equation. Because the pattern of fruit growth of 'Ikaika' is sigmoidal (S-shaped), the equation describing the rate of fruit growth was a sigmoidal rate equation. The growth rate is determined by a relative growth rate which is influenced by temperature (Table 1). The values for the relative growth rate were derived from the relationship between temperature and vegetative growth.

The rate equation was:

dY/dt = Yk(Ymax-Y)/Ymax

where dY/dt is the rate of fruit growth (mm/day); Y is the fruit diameter (mm); k is the relative growth rate (1/day); and Ymax is the maximum fruit size (mm).

In application of the model to field conditions, the daily maximum and minimum tempratures from Waiakea Experiment Station were used to calculate the daily average temperature. For each day, the average temperature was used to calculate the relative growth rate which was then used to calculate the growth rate. The model was written in the CSMP simulation language.

RESULTS AND DISCUSSION

The pattern of fruit growth macadamia 'Ikaika' followed a sigmoidal curve, reaching maximum fruit size in 111 days. Maximum fruit size was approximately 24.0 mm in diameter.

The model predicted fruit growth fairly accurately. Because information on the relationship between temperature and fruit growth was unavailable, the relationship of temperature and vegetative growth was used. It was assumed that this relationship also applied to fruit growth

The discrepancies between the observed and predicted fruit growth indicated that other environmental factors and other parameters may need to be investigated and incorporated into the model. Tissue temperature has been shown to be influenced by solar radiation, precipitation, cloud cover, and wind. Prediction errors may also have been caused by the use of daily average temperature which does not account for rapid temperature fluctuations and for tissue temperatures.