Proceedings of The World Avocado Congress III, 1995 181
- 184
EFFICACY STUDIES ON PREBLOOM CANOPY APPLICATIONS OF
BORON AND/OR UREA TO 'HASS'
AVOCADOS IN CALIFORNIA
I. Jaganath and C.J. Lovatt
Department
of Botany and Plant Sciences
University
of California
Results of four bloom studies
(two glasshouse and two field experiments) demonstrated the efficacy of
applying boron or urea sprays to 'Hass' avocado inflorescences during early
expansion (cauliflower stage) but prior to full panicle expansion and anthesis.
Anatomical analysis of the flowers provided evidence that the boron prebloom
spray increased the number of pollen tubes that reached the ovule (p<0.05)
and also increased ovule viability, but to a lesser degree than urea. The urea
prebloom spray increased ovule viability compared to boron- treated or
untreated flowers (p<0.05). Urea also increased the number of pollen tubes
that reached the ovule (p<0.05), but to a lesser degree than boron. However,
combining boron and urea resulted in double pistils (p<0.05), even when the
urea was applied 8 days after the boron.
The efficacy of using boron
and/or urea as a prebloom canopy spray to increase fruit set and yield is under
continued testing in commercial 'Hass' avocado orchards in southern and coastal
California with low, moderate, and high leaf boron concentrations, for years
when anthesis, pollination, fertilization and fruit set occur under optimal
versus less than optimal climatic conditions, and for both "on" and
"off' years of the alternate bearing cycle. The objective of the research
is to determine, on the basis of benefit to cost, under what conditions this
management strategy should or should not be utilized in California.
1. Introduction
The most successful fruit set
occurs at temperatures between 20 to 25ēC. At these temperatures, female and
male floral stages overlap for several hours (Sedgely, 1977). Warm temperatures
during flowering increase both ovule longevity and the growth rate of the
pollen tube, and thus increase fruit set by increasing the effective
pollination period (Williams, 1965). The longevity of the ovule minus the
length of time necessary for the pollen tube to reach the ovule to deliver the
sperm to the egg is the effective pollination period. Cool temperatures during
the flowering period decrease the viability of the ovule and increase the
length of time it takes for the pollen tube to grow from the stigma to the
ovule. Thus, the duration of the effective pollination period is significantly
shortened and fruit set is reduced.
It is well established that
boron is essential for pollen germination, for successful growth of the pollen
tube through the stigma, style, and ovary to the ovule, and for the mitotic
divisions necessary to produce the sperm (reviewed in Lovatt and Dugger, 1984).
Boron sprays applied either during fall or spring to trees not deficient in
boron (based on leaf analyses) have been effective in increasing fruit set in a
number of deciduous tree fruit and nut crops (Batjer and Thompson, 1949;
Degman, 1953; Bramlage and Thompson, 1962; Davison, 1971; Baron, 1973; Chaplin
et al., 1977; Callan et al., 1978; Crassweller et al., 1981; Yogaratnam and
Greenham, 1982; Hanson and Breen, 1985; Shrestha et al., 1987) and in avocado
(Robbertse et al., 1990, 1992). Boron sprays are more effective during seasons
when cool, overcast and wet weather predominate during bloom (Callan et al.,
1978; Hanson and Breen, 1985). Benefits from boron sprays are less likely when
conditions are optimal for fruit set (Degman, 1953).
Ovule longevity has been
improved in deciduous tree crops by a summer application of nitrogen (Williams,
1965).
On the basis of these
findings, we established and are continuing to conduct field experiments to
test the hypothesis that fruit set and yield of the 'Hass' avocado can be
increased by increasing the effective pollination period (i) by prolonging
ovule viability with a bloom application of nitrogen and (it) by accelerating
pollen tube growth rate with a bloom application of boron.
2. Material and methods
The research was conducted at
two mature commercial orchards of 'Hass' avocado on Duke 7 rootstock. One
orchard was boron deficient at the start of the experiment with average leaf
analyses of 18 μg B per g dry weight leaf tissue. Subsequently, all trees
were brought into the adequate range (> 25 μg B per g dry weight leaf
tissue), because it was not the objective of the research to study the effect
of boron deficiency on avocado fruit set and yield. The second orchard had high
to excess leaf boron concentrations, > 180 μg B per g dry weight.
Treatments were applied to 16 individual tree replicates in a randomized block
design in 15 liters of H20 per tree to give full canopy coverage.
Boron was applied at a rate of 6.15 g B as Na2B4O7
ˇ 10 H2Oand nitrogen at 0.16 kg N as low-biuret urea prebloom, i.
e., during the cauliflower stage of inflorescence development, which occurred
about mid-March to mid-April depending upon the site and year.
The research was also
replicated in the glasshouse using 'Hass' avocado scions on Duke 7 rootstock
two years from budding. Treatments were applied at the same concentrations as
in the field to the point of run-off.
To quantify the number of
pollen tubes entering the ovule and the number of viable ovules, 20 to 30
flowers were collected for each analysis for each treatment 24 h after
open-pollination in the field or hand-pollination in the glasshouse. Flowers
were immediately fixed in 100% ethanol and concentrated glacial acetic acid (3:
1, v/v) for 10 min and then washed 3 times in 70% ethanol. Samples were
squashed in aniline blue to reveal pollen tubes; ovules were excised and
stained with aniline blue. Observations were made with a Zeiss fluorescence
microscope.
In the field experiments, all
fruit were harvested on each tree and weighed to determine total kg fruit per
tree. To determine packout, each of 200 randomly selected fruit per tree was
weighed.
3. Results
The number of pollen tubes
reaching the ovule averaged 0.77 for flowers collected in the field and
approximately 1.0 for flowers from the glasshouse-grown trees. The application of boron to the canopy when
the majority of the inflorescences were at the cauliflower stage of development
increased this number more than 2.5-fold for flowers from both field and
glasshouse trees (p<0.05). The number of pollen tubes reaching the ovule was
also significantly increased by a canopy application of low-biuret urea at the
cauliflower stage of inflorescence development to separate sets of trees. The
greatest effect was for flowers collected from trees in the field experiment
(approx. 2-fold, p < 0.05).
Viable ovules were found in approx. 70% of the flowers collected from both the field and glasshouse-grown trees. The canopy application of low-biuret urea at the cauliflower stage of inflorescence development significantly increased the number of viable ovules by 25% for flowers collected from both sets of trees (p<0.05). Only the field trees benefited from the foliar application of boron; ovule viability was increased 16 % (p < 0. 05).
In the orchard consisting of
trees with low to adequate leaf boron concentrations, the cumulative average
yield for the three years of the experiment at this site was 192 kg fruit per
tree. Applications of boron or nitrogen to the canopy when the majority of
inflorescences were at the cauliflower stage of inflorescence development
increased the cumulative average yield for the three years of the experiment 25
and 23%, respectively. The orchard characterized by excess leaf boron
concentrations consistently yielded better than the orchard with the low to
adequate leaf concentrations of boron. The cumulative average yield for only
two years at this site was 248 kg fruit per tree. In this orchard, yields tended
to be reduced as a result of the boron or low-biuret urea treatments. However,
the yield reductions were not statistically significant in any single year of
the experiment or for the cumulative average yield.
Combining boron and
low-biuret urea as a single treatment (despite the fact that low-biuret urea
was applied as long as eight days after the boron) resulted in a reduction in
yield in each year of the experiment and regardless of whether leaf boron
concentrations were low, adequate or in excess. Flowers collected from trees in
this treatment both in the field and in the glasshouse exhibited a
significantly greater incidence of double pistils, which resulted in greater
abscission of flowers and fruit.
4. Summary
While annual prebloom canopy
applications of boron, or urea, provided some benefit in increasing yield over
a period of several years in the orchard with low to adequate tree boron status
(please note that a similar analysis for N in each orchard is not yet available
and is equally important to evaluating the significance of these results), the
fact that yield reductions were observed each year in the orchard characterized
by excess leaf boron content suggests that strategy foliar applications of
boron or urea during the bloom period should not be used indiscriminately or
prophylactically. Further understanding
of the interactions between prevailing environmental conditions and tree
nutritional status during flowering and fruit set are required in order to know
when either nutrient element can be used to successfully effect a significant
increase in yield and profit.
Acknowledgement
- The
research was supported in part by a grant from the California Avocado
Development Organization, California Avocado Commission, California Department of
Food and Agriculture Fertilizer Research and Education Program and by the
Citrus Research Center and Agricultural Experiment Station of the University of
California.
References
Baron, L.C., 1973. The value
of boron sprays on filberts. Nut Growers Soc. Oregon, Washington, 58:22-28.
Batjer, L.P. and A.H.
Thompson, 1949 .The effects of boric
acid sprays applied during bloom upon the
Bramlage, W.J. and A.H.
Thompson, 1962. The effects of early season sprays of boron on fruit set,
color, finish and storage life of apples. Proc. Amer. Soc. Hort. Sci. 80:64-72.
Callan, N.W., M.W. Thompson,
M.H. Chaplin, R.L. Stebbins and M.N. Westwood, 1978. Fruit set of 'Italian'
prune following fall foliar and spring boron sprays. J. Amer. Soc. Hort. Sci.
103:253-257.
Chaplin, M.H., R.L. Stebbins
and M.N. Westwood, 1977. Effect of fall-applied boron sprays on fruit set and
yield of 'Italian' prune. HortScience 12:500-501.
Crassweller, R.M., D.C.
Ferree and E.J. Stang, 1981. Effects of overtree misting for bloom delay on pollination,
fruit set, and nutrient element concentration of 'Golden Delicious' apple tree.
J. Amer. Soc. Hort. Sci. 106:53-56.
Davison, R.M., 1971. Effect
of early season sprays of trace elements on fruit setting of apples. N.Z.J.
Agr. Res. 14:931-935.
Degman, E.S., 1953. Effect
of boron spray on fruit set and yield of Anjou pears. Proc. Amer. Soc. Hort.
Sci. 62:167-172.
Hanson, E. J and P.J. Breen,
1985. Effects of fall boron sprays and environmental factors on fruit set and
boron accumulation -in 'Italian' Prune flower buds. J. Amer. Soc. Hort. Sci.
110:389-392.
Lovatt, C.J. and W.M.
Dugger, 1984. Boron. pp. 389-421. In: E. Frieden (ed.), Biochemistry of the
Essential Ultratrace Elements, Plenum Pub. Corp.
Robbertse, P.J., L.A.
Coetzer, N.G.N. Swart and J.J. Bezuidenhout, 1990. The influence of boron on
fruit set in avocado. Acta Hort. 275:587-594.
Sedgley, M., 1977.
Physiology of pollination and fruit set and possibilities of manipulation. pp.
59-65. Proc. Avocado Res. Wrksp. South Queensland, Australia.
Shrestha, G.K., M.M.
Thompson and T.L. Righetti, 1987. Foliar-applied boron increases fruit set in
'Barcelona' hazelnut. J. Amer. Soc. Hort. Sci. 112:412- 416.
Thompson, A.H and L.P. Batjer, 1950. The effect of
boron in the germination
medium on pollen germination and pollen tube growth of
several deciduous tree fruits. Proc. Amer. Soc. Hort. Sci. 55:227-229.
Williams, R.R., 1965. The
effect of summer nitrogen applications on the quality of apple blossoms. J.
Hort. Sci. 40:31-4 1.
Yogaratnam, N. and D.W.P.
Greenham, 1982. The application of foliar sprays containing nitrogen,
magnesium, zinc and boron to apple trees. 1. Effects on fruit set and cropping.
J. Hort. Sci. 57:151-158.