The confluence of tree and reproductive plant water relations during floral and fruit development in macadamia

The Australian macadamia industry faces two significant challenges; high levels of fruit abscission during early fruit development, and declining average yields from orchards. The decline in average yield is occurring despite the maturing of an industry where production and plant husbandry methods have been considerably refined since the early days of macadamia farming in Australia. In terms of the high early fruit drop, it has previously been estimated that approximately 1 to 3 percent of the initial fruit set reaches maturity. Plant reproduction imposes significant resource costs to trees; these include water, carbohydrates and nutrients. The potential causes of the declining yield or the high fruit drop are likely to be complex and involve many factors. The research presented in this thesis is based on a physiological understanding of reproductive water relations and within tree interactions. The primary aim of the research was to identify factors other than soil water content that influence reproductive water relations under field conditions, and whether these factors affect reproductive success and yield in macadamia. Research was undertaken at two locations in the Bundaberg region; a commercial macadamia orchard and research blocks of macadamia trees (DAFF Research Station) managed in the same way as a commercial orchard. Tree water status was assessed through water potential and transpiration measurements. Trees and branches were manipulated to investigate reproductive and vegetative interactions, reproductive water relations, and vascular hydration pathways. A small number of detached stem experiments were also undertaken to investigate water relations and reproductive interactions. The interaction discovered between leaves and racemes in macadamia is perhaps the most significant in terms of future research and the potential to increase commercial macadamia production. The interaction identified means that as floral or fruit load increases, leaf stomatal conductance in macadamia declines. This is in contrast to many other tree crops, which increase leaf stomatal conductance and photosynthesis in response to fruit presence and load. The potential effect in macadamia is to limit photosynthesis and carbohydrate production, which imposes a significant limit on reproductive development and therefore fruit set. Results from this study provide a mechanism that explains why macadamia have such a high level of early fruit drop. This reproductive load response in macadamia could be related to the maintenance of plant water relations and conservation of limited soil water under natural forest conditions and seasonally dry periods. Another important interaction was the source-sink ratio, that is, changing the ratio in favour of the sink (flowers and fruit) lowered the leaf stomatal conductance rate. This response is counter-intuitive as many plants typically increase leaf stomatal conductance and photosynthesis as the number of fruit increase. However, macadamia appear to be unusual and the reverse occurs as fruit load increases. Results from this part of the study showed that fruit drop will increase and yield will be significantly lower when the source-sink ratio favours fruit. Branch angle in macadamia trees also appears to be important, with the angle of branches influencing floral load and density, leaf stomatal conductance, and leaf water potential. Horizontal branches had the most positive influence on floral load in macadamia. An important aspect of branch angle was the influence of flushing and the potential for vertical branches to dominate the canopy. Flushing resulted in a significantly lower leaf stomatal conductance rate on horizontal branches compared with vertical branches. This is likely to lead to lower vegetative growth on horizontal branches and comparatively more on vertical branches. The other aspect of flushing was a lower leaf stomatal conductance (indicative of a lower photosynthesic rate in macadamia), which has already been shown to negatively impact on fruit set. This thesis also investigated reproductive hydration pathways by measuring water potential gradients, cincturing and histology studies. Xylem hydration of flowers and fruit appears important in macadamia, however results indicate that water entering flowers and fruit via the phloem ranges between 5 and 13 percent. Results also indicated that macadamia florets have a diurnal transpiration pattern. Floral transpiration varies throughout the day and appears to follow a similar pattern to leaves. The final area investigated in this thesis was the effect of vapour pressure deficit on fruit set under field conditions. The microclimate surrounding macadamia racemes appears to be important, with a small change in the mean daily vapour pressure deficit significantly decreasing the initial fruit set (1.6 kPa compared with1.3 kPa). The findings from this PhD project identified a number of important physiological interactions between leaves and racemes in macadamia. One of these interactions was a lower leaf stomatal conductance rate in response to reproductive load, which potentially explains the high level of early fruit drop in macadamia.