Inconsistent internal fruit quality in avocados, caused mainly by rots and physiological disorders is a significant cause of consumer dissatisfaction, and poses a serious problem for the growing avocado industry in Australia. This thesis examines the role of minerals and rootstocks in fruit quality of 'Hass' avocado. It quantifies the tree to tree variation in fruit quality and fruit minerals, and the rootstock effects on fruit quality. It also examines whether these rootstock effects can be mediated through fruit minerals. In addition, it investigates whether rootstocks can reduce tree to tree variability in fruit quality and fruit minerals, and the possible mechanisms for these effects.
Tree to tree variation in fruit quality and fruit minerals was investigated in three commercial orchards in 1998, and one orchard in 1999. Trees had a very significant effect on 'Hass' avocado fruit quality across all locations in 1998 and 1999, including severity of fruit body rots (mainly anthracnose) without or after cold storage, and the internal disorders diffuse discolouration and vascular browning (after cold storage), days to eating soft (DBS), percentage dry matter (% DM), fruit mass, and skin area with purple-black colour at ripe. Trees also had a significant effect on flesh Ca, Mg, K, B, and Zn concentrations, and the (Mg+K)/Ca ratio. Significant correlations were observed between average fruit minerals concentrations for each tree (particularly Ca, Mg, and K) and fruit quality attributes (especially body rots severity, % DM, and fruit mass). As the trees were on seedling rootstocks, the results supported suggestions that rootstock may be a major contributing factor to the high free to tree variability in avocado fruit quality and fruit minerals. However, the inconsistency of the correlations across sites and years suggested that other factors can also influence the relation between fruit minerals and fruit quality.
To quantify the effects of rootstock on 'Hass' avocado fruit quality, fruit were obtained from trees grown on either seedling 'Velvick' (SV), clonal 'Velvick' (CV), or clonal 'Duke 7' (CD) rootstocks in a commercial orchard. Fruit were harvested over two seasons, ripened at 20 °C immediately after harvest (non-stored) or following storage at 5 °C for 4 weeks (stored), and fruit quality and minerals concentrations determined. In 2000, non-stored CV fruit had significantly (P<0.001) lower body rots severity (5% of flesh volume affected) than CD fruit (20%), while body rots severity in stored CV fruit was 20% of flesh volume affected compared to 38% in stored CD fruit. Stored CV fruit had significantly (P<0.05) lower diffuse discolouration severity (3% and 9% in 1999 and 2000, respectively) compared with CD fruit (7% and 20%, respectively). Stored CV fruit also had lower vascular browning severity (19% ) than CD fruit (33%) in 2000. Incidence (calculated as the percentage of the fruit with any level of flesh volume affected) of body rots, diffuse discolouration, and vascular browning followed a similar pattern as severity. Non-stored CD fruit ripened slightly but significantly faster (9.1 days) than SV (9.8 days) or CV (9.9 days) fruit in 2000. There were no rootstock effects on fruit yield, canopy volume, measured length of non-suberised roots, fruit mass, fruit length to width ratio, skin thickness, % DM, skin area with purple-black colour at ripe, or seed to fruit mass ratio.
In both seasons, CV fruit had 15-19% higher flesh Ca concentrations, 17-22% higher flesh B concentrations, 14-15% lower flesh N concentrations, and a 25-34% lower N/Ca ratio in the skin and 23-26% lower ratio in the flesh, than CD fruit (all significant at P < 0.05). In 2000, CV fruit also had 27% more skin Ca and 14% less skin N than CD fruit. Rootstocks also significantly affected leaf N, Ca, B, and Zn, rootstock wood Ca, and scion wood Ca in at least one of the years. In addition, correlation analysis using the means for each datum tree indicated significant relationships between DBS, and severity of body rots and diffuse discolouration) and fruit flesh, fruit skin, and leaf minerals concentrations (particularly N, Ca, and K).
The tree to tree variability (as measured by the variances for each rootstock group) between rootstocks were significantly different for body rots severity in non-stored fruit in 2000 and in stored Suit in 1999, and for diffuse discolouration severity in stored fruit in 1999. Clonal 'Velvick' trees had the smallest variances (an indication of less variation between trees) for body rots and diffuse discolouration. The tree to tree variances between rootstocks were also significantly different for flesh Ca concentration in 2000, with CD trees having the lowest variance and SV the highest. These results indicate that clonal rootstocks can reduce the variability in fruit quality between trees.
To further investigate possible mechanisms associated with rootstock effects on tree minerals, young 'Hass' trees on seedling 'Velvick' (SV), clonal 'Velvick' (CV), seedling 'Duke 7' (SD), or clonal 'Duke 7' (CD) were grown in the glasshouse for eight months. Rootstocks significantly affected leaf Ca and Mg, branch Ca, B, and Zn, scion stem Mg, K, B, and Zn, and non-suberised root Mg, K, and B. There were no rootstock effects on leaf N (other plant tissues were not measured). Generally, CV plants had more rootstock stem Ca, scion stem and rootstock stem Mg, root K, branch B, and rootstock stem Zn than the other plants. Seedling 'Velvick' plants had more leaf and branch Ca, root B, and branch and scion stem Zn than the other treatments. Contrast analysis indicated that both the rootstock cultivar (either 'Velvick' or 'Duke 7') and the propagation method (either seedling or clonal), played a role in the treatment differences above. In addition, the dry mass of non-suberised roots relative to the dry mass of the total root system was significantly higher in CV plants (38%) than in SV (18%), SD (24%), and CD (27%) plants. There were no significant rootstock effects on dry mass of leaves, scion (branch plus stem), and total root system, or on total plant dry mass. Rootstocks had little effect on cumulative vegetative growth of the plants over 36 weeks (measured as plant height growth, growth of selected branches, growth of rootstock stem diameter and scion stem diameter, number of new branches and leaves).
The rootstock effects on leaf Ca observed in the glasshouse were also observed in mature frees in the field for SV, CV, and CD trees, but results were not consistent between both experiments for the same rootstocks. Under field conditions there were also rootstock effects on leaf N, K, B and Zn (in at least one of the years), which did not occur in the glasshouse. In contrast, in the glasshouse there were rootstock effects on scion wood Mg, K, B, and Zn, and on rootstock wood K and B, which did not occur in the field. These results suggest that the rootstock effects on free minerals may vary with plant age or growing conditions, but in general give no clear indications of how rootstocks may affect fruit quality and minerals.
Graft union effects on the translocation of minerals (particularly Ca and Mg) were observed in CV trees in the field and in the glasshouse, based on lower ratios of minerals concentrations in the scion stem to rootstock stem. However, although these results may suggest restrictions of minerals transport across the graft union in CV frees, this did not appear to affect minerals accumulation in the plant tissues beyond the graft union (i.e. branch and leaf in young frees or leaf and fruit in mature frees) or fruit quality. Thus, mature CV frees had relatively more Ca in the rootstock wood, but higher leaf and fruit Ca and better fruit quality. These results suggest that the CV roots could absorb and/or translocate Ca more efficiently out of the roots, possibly as a result of the higher non-suberised root dry mass (relative to the total root system dry mass) observed in the glasshouse. This would overcome any graft restrictions and allow higher leaf and fruit Ca, contributing to better fruit quality.
This thesis shows that rootstocks can affect 'Hass' avocado fruit quality, particularly its susceptibility to body rots (with or without cold storage) and internal disorders after cold storage. It also indicates that rootstocks can reduce the free to free variability in fruit quality and fruit minerals. Across both seasons, the generally better quality of CV fruit was associated with higher flesh Ca and B, higher skin Ca, and lower flesh and skin N. Since there were significant rootstock effects on fruit skin and flesh minerals, but not on plant vegetative growth, canopy volume, fruit yield, fruit maturity, or fruit physical characteristics (including mass and skin thickness), the rootstock effects are likely to be partly mediated through differing efficiencies in minerals absorption and/or distribution to the fruit. However, other areas such as the role of rootstocks on antifungal compounds in the fruit skin, and on enzymes in the scion that may affect fruit quality, warrant further investigation.