Internal conductances to CO2 transfer from the stomatal cavity to sites of carboxylation (gi) in hypostomatous sun‐and shade‐grown leaves of citrus, peach and Macadamia trees (Lloyd et al. 1992) were related to anatomical characteristics of mesophyll tissues. There was a consistent relationship between absorptance of photosynthetically active radiation and chlorophyll concentration (mmol m−2) for all leaves, including sclerophyllous Macadamia, whose transmittance was high despite its relatively thick leaves. In thin peach leaves, which had high gi, the chloro‐plast volume and mesophyll surface area exposed to intercellular air spaces (ias) per unit leaf area were similar to those in the thicker leaves of the evergreen species. Peach leaves, however, had the lowest leaf dry weight per area (D/a), the lowest tissue density (Td) and the highest chloro‐plast surface area (Sc) exposed to ias. There were negative correlations between gi and leaf thickness or D/a, but positive correlations between gi and Sc or Sc/Td. We developed a one‐dimensional diffusion model which partitioned gi into a gaseous diffusion conductance through the ias (gias) plus a liquid‐phase conductance through mesophyll cell walls (gcw). The model accounted for a significant amount of variation (r2=0.80) in measured gi by incorporating both components. The gias component was related to the one‐dimensional path‐length for diffusion across the mesophyll and so was greater in thinner peach leaves than in leaves of evergreen species. The gcw component was related to tissue density and to the degree of chloroplast exposure to the ias. Thus the negative correlations between gi and leaf thickness or D/a related to gias whereas positive correlations between gi and Sc or Sc/Td, related to gcw. The gcw was consistently lower than gias, and thus represented a greater constraint on CO2 diffusion in the mesophylls of these hypostomatous species.

Diurnal patterns of C0, and water vapour exchange were determined for Macadamia integrifolia and Litchi chinensis trees enclosed in whole-tree gas exchange chambers at Alstonville, New South Wales (28.5"s) during October and November 1991. Whole-tree gas exchange responses to photon irradiance (I), ambient partial pressure of CO, (C,) and vapour pressure deficit (D) were similar to those normally observed for individual leaves. Nevertheless, at a given I (above approximately 500 pmol quanta rn-, s-I) stomatal conductances (g,) and CO, assimilation rates (A) were higher under overcast, as opposed to clear sky conditions. This difference was maintained even when A and g, were examined as a function of sun angle. Combined with a simple light interception model, nested quadratic equations combining stomatal responses and biochemical characteristics of individual leaves were found to provide excellent descriptions of the gas exchange responses of the isolated trees. This indicates a close to optimal partitioning of photosynthetic machinery throughout the canopy. From the whole-tree gas exchange model it was found that higher A and g, under overcast conditions are attributable to both lower T, and D as well as to a more uniform distribution of irradiance across the canopy surface. References Batten, D., Lloyd, J., and McConchie, C. (1992). Seasonal variation in stomatal responses of two cultivars of lychee (Litchi chinensis Sonn.). Australian Journal of Plant Physiology 19,3 17-329. Brooks, A., and Farquhar, G. D. (1985). Effect of temperature on the C02/02 specificity of ribulose-l,5-bisphosphate carboxylasel oxygenase and the rate of respiration in the light. Planta 165, 397-406. Caemmerer, S. von, and Evans, J. R. (1991). Determination of the average partial pressure of C02 in chloroplasts from leaves of several C3 species.

Members of the genus Macadamia contain cyanogenic compounds, which release hydrogen cyanide (HCN) on hydrolysis. Concentrations of releasable cyanide were measured in tissues of mature nuts and seedlings of Macadamia integrifolia Maiden & Betche, M. tetraphylla L.A.S. Johnson and M. ternifolia F.Muell. Root, cotyledon and leaf samples were assayed at several developmental stages from germination to maturation of the first leaves. All samples contained detectable levels of cyanide. Concentrations were low (0.15 μmol g-1 fresh weight) in cotyledons of mature M. integrifolia and M. tetraphylla seeds, corresponding to the edibility of the seeds of these commercial species, and much higher (9.6 μmol g-1) in the inedible M. ternifolia seeds. Levels in cotyledons of all three species rose dramatically during germination. Root cyanide concentrations of 6-23 μmol g-1 were measured. The immature first leaf of the commercial species contained the highest concentrations (38-77 μmol g-1). Levels decreased with leaf maturity, correlating with toughening of the leaf and possibly a consequent diminished requirement for cyanide as a herbivory deterrent. The significance of the results with respect to plant-insect interactions is discussed.

Orchard crowding in the macadamia industry is common and there has been concern that it reduces yield and quality. To investigate this we monitored yield, nut quality, tree volume, and light interception in macadamia orchards (Macadamia integrifolia Maiden and Betche) that represented a range in crowding. The predicted rate of change in yield/ha with time remained positive, i.e. yield increased over time, for orchards with crowding levels up to a tree volume of 43 500 m3/ha and light interception of 94%. There was a trend for the rate of yield change to decrease with increasing tree volume/ha. For orchards with tree volume >43 500 m3/ha the rate was negative, indicating a slight decline in yield over time. There was no evidence of a detrimental effect of orchard crowding on percentage kernel recovery, unsound kernel, or grade 1 kernel. It appears that macadamia, unlike many other orchard crops, can maintain yield and quality up to a high level of orchard crowding. The implication of this is that timing of canopy management and optimum canopy dimensions will be dictated by management requirements for machinery access and effective spray coverage before yield decline becomes an issue.

Delayed or extended fruit abscission in many cultivars of macadamia, Macadamia integrifolia (Proteaceae), necessitates multiple harvests from the orchard floor. However, it is not known whether there is a link between fruit maturation and timing of abscission. In this study, relationships between kernel oil content, nut size, removal force and abscission were investigated in 2 commercial orchards, one in Queensland (24˚S) and one in New South Wales (29˚S). Abscission lasted 6 months (March–September) at both sites due to differences in timing of abscission between cultivars and extended abscission within cultivars. Abscission was consistently associated with declines in fruit removal force, from more than 2 kgf in early March towards 1 kgf at the peak of abscission. Later abscission in cvv. A16 and 246, compared with cvv. 344 and 741, was correlated with more gradual declines in removal force. Fruits containing small nuts and kernels tended to possess lower removal forces and these fruits were often heavily represented in the population of fruits that abscised earliest. There was little or no difference in kernel oil content between fruits of different removal force or between sample dates for any cultivar, either for samples taken from the tree or from the orchard floor. Attainment of maximal oil content and onset of fruit abscission are therefore independent processes in macadamia. If abscission can be accelerated, it may be possible to advance the harvest of late-abscising cultivars such as A16 and 246 which retain mature nuts on the tree up to several months after maximal oil accumulation.