Boron nutrition of hass avocado (Persea Americana Mill.)

Smith, Timothy Edward (2004). Boron nutrition of hass avocado (Persea Americana Mill.) PhD Thesis, School of Land, Crop and Food Sciences, The University of Queensland.

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Author Smith, Timothy Edward
Thesis Title Boron nutrition of hass avocado (Persea Americana Mill.)
School, Centre or Institute School of Land, Crop and Food Sciences
Institution The University of Queensland
Publication date 2004
Thesis type PhD Thesis
Supervisor Em Prof Colin Asher
Total pages 196
Collection year 2004
Language eng
Subjects L
300302 Plant Growth and Development
620401 Fresh fruit and vegetables (post harvest)
Formatted abstract Boron (B) deficiency of Hass avocados {Persea americana Mill, cv Hass) is recognised as a problem in many of the major avocado growing regions of the world. Boron deficiency was found to be a common problem also in the Queensland avocado industry with 67-79 % of commercial avocado farms sampled containing sub-optimal foliar B levels in their 1992 spring leaf flush, and the 1993 summer and 1995 summer leaf flushes.      

Soil texture had a major effect on the concentration of hot CaCl2 extractable soil B required to reach a foliar B concentration of 50 mg kg-1 in Hass avocados. Thus the critical values for red clay soils (Ferrosols1) of 7.2 - 11.2 mg kg-1 were at least 12 times higher than those for soils with sandy loam surface textures (Kurosols, Chromosols and Kandosols1) which ranged from 0.4 - 0.6 mg kg-1.      

Boron deficiency symptoms of Hass avocado trees grown in pots in a glasshouse included interveinal crinkling of leaves, misshapen leaf margins, shot-hole of leaves, nodal swelling of the shoots, prostrate growth of shoots, and loss of apical dominance of shoots and flower panicles. This appears to be the first time that loss of apical dominance of flower panicles has been reported as a symptom of B deficiency in Hass avocados.     

Soil boron applications increased pollen viability of Hass avocado trees grown in a glasshouse in B-deficient soil. Thus, pollen from trees grown in soil receiving 0.1-1.6 g B m-2 had 3 - 6 times higher germination in an artificial -B pollen tube growth medium than pollen from trees grown in soil not fertilized with B. No significant differences in pollen germination were found between B treatments when the pollen was germinated in a +B pollen tube growth medium.      

In a field experiment with 5-year-old Hass avocado trees of marginal B status (15-29 mg B kg-1 dry wt of summer flush leaves), spraying Solubor® onto fully developed panicles at the beginning of anthesis caused a 42 % increase in fruit set. However, this increase in fruit set was not accompanied by a significant increase in the number of fruit retained following the first fruit drop. Spraying panicles at earlier stages of development had no detectable effect on either fruit set or fruit retention.      

A further two field trials were established in commercial orchards (Gray's and Maywald's) on low-B Ferrosol soils having clay loam to clay textures, in the Maleny district of South East Queensland. On both properties, some trees had summer leaf flush B concentrations ≤20 mg kg-1.  Addition of B to the soil altered the B status of trees from deficient to adequate. This increase in B status did not result in significant increases in fruit number or total yield of fruit, but did produce a 13-18 % increase in mean weight per fruit (P<0.05). A linear increase in fruit size with increasing leaf B concentration was found between 19 - 43 mg kg-1 in summer flush leaves. The critical  summer flush leaf B concentration for fruit size of Hass avocados was calculated as 46 mg kg-1.     

In the Maywald field trial, fruit were harvested for post-harvest quality experiments. Fruit were either ripened at 22 °C until eating soft, stored at 7 °C for 4 weeks then ripened at 20 °C until eating soft, or stored at 2 °C for 7 weeks before ripening at 20 °C. In fruit ripened at 22 °C until eating soft, fruit from low-B trees reached peak ethylene production 4.4 days before fruit from trees supplied with adequate B, and produced 49 % more ethylene at the peak. The shelf life of low-B fruit was 4.3 days shorter at 22 °C without low temperature storage, and 2.4 days shorter at 20 °C following storage at 7 °C, compared to fruit with adequate B.  

Low B fruit stored at 7 °C also had more internal breakdown as indicated by a greyish/black discoloration of the endocarp. The blackening was concentrated near the top of the seed cavity (proximal to the stem) and extended to a depth of >10 mm into the pulp. This symptom, dubbed "endocarp black spot", was distinct from other internal breakdown symptoms, and evidence presented in this thesis suggests that it was specifically related to B deficiency in the fruit stored at 7 °C.      

Summer flush leaf B concentrations of trees in the Maywald trial were found to be positively correlated with B concentrations in all fruit tissues sampled, as was the B concentration in the fruit exocarp with other fruit tissues. Hence sampling and analysis of fruit at market may be an effective strategy for screening the B sufficiency of orchards within the industry.      

A mist culture trial was established in a glasshouse to assess the effects of B supply to roots on the growth and tissue B concentrations of avocado trees with various scion/rootstock combinations. The scion/rootstock combinations concentrated on the dominant scion variety Hass and the dominant rootstock varieties Velvick and Duke 7. It was found that trees with Velvick rootstocks were taller and had greater mass than trees with Duke 7 rootstocks and root growth was stunted in the Duke 7 rootstocks compared to the Velvick rootstocks in the low B supply treatments.      

A comparison of patterns of distribution of Ca, an element known to have very low phloem mobility, and of B, suggested appreciable mobility of B in the phloem of avocados. For example in the 19.9 µM treatment from the mist culture trial, the ranking of average (across all cultivars) vegetative tissue concentrations for Ca (%) were older leaves (1.85) > YFEL (0.47) ≥ immature leaves (0.37), whereas for B (mg kg-1) they were immature leaves (110) > YFEL (68) ≥ older leaves (54). Hass on Velvick and Hass on Duke 7 trees both had similar YFEL B concentrations, but trees with Duke 7 rootstocks appeared to have greater B mobility to immature leaves and stems. An example of fruit tissue concentrations was drawn from the 5.9 g B m-2 treatment of the Maywald trial, where rankings for Ca (%) were YFEL (0.81) > seed coat (0.14) > exocarp (0.043), whereas for B (mg kg-1) were seed coat (148) > exocarp (108) > YFEL (89). Further research on B mobility in avocados, using stable isotope techniques seems warranted.    

Results of the present study have confirmed the importance of B deficiency in the Australian avocado industry in terms of tree health, initial fruit set, fruit size, and storage characteristics of the fruit. In addition, apparently conflicting results sometimes obtained between leaf analysis and soil analysis tests for the same orchard have been shown to be due to a large effect of soil texture on the critical concentration of soil B. Other results obtained suggest a greater mobility of B in Hass avocados than of Ca, and further studies on B mobility seems warranted. 

1 Throughout this thesis, soils are named according to the Australian System of Soil Classification (Isbell, R.F.(1996). The Australian Soil Classification System.  Australian Soil and Land Survey Handbook No 4. CSIRO Publishing, Melbourne, Australia). Correlations with other soil classification systems shown in Appendix 1.

Keyword Avocado -- Varieties
Soils -- Boron content
Crops, Effect of boron on

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