1999. Revista Chapingo Serie Horticultura 5:89-94.

 

THE ANNUAL PRODUCTION AND UTILIZATION OF DRY MATTER OF AN AVOCADO (Persea americana Mill.) TREE

 

G. Adar

Gaaton, 25130 Israel. Fax. 972-4-9858413. E-mail: gadar@research.haifa.ac.il

 

SUMMARY

 

This work, which was a part of the M.Sc. studies in the Faculty of Agriculture in the Hebrew University of Jerusalem, was carried out under the supervision of Prof. E.E. Goldschmidt. Two basic processes which take place concurrently in the avocado tree during the yearly cycle of growth and reproduction are discussed in this essay: Dry matter production and its utilization.

 

Key Words: Tree physiology, reserves, productivity.

 

 

INTRODUCTION

 

The production of dry matter and its distribution among plant organs are basic processes on which other internal activities are dependent. Therefore it is important to study the carbon balance of the avocado tree.

 

The objective of this essay is to compile a dry matter budget for a fruiting avocado tree and to find its relations to the yearly avocado growth cycle.

 

Eventually, this dry matter budget may be part of a dynamic avocado productivity model, as developed for citrus by Harpaz et al. (1990) and for other crops. The data presented in the essay are based on photosynthetic measurements made in Israel under Israeli climatic conditions, but important data from worldwide sources are included.

 

The data, calculations and estimations presented in this paper, refer to an avocado tree which reaches a canopy surface area of 50 m² by the end of September. Such a tree has been defined a “standard” tree.

 

The main data which are presented in following sections are based on photosynthetic measurements made by Sharon & Bravdo (unpublished) on the south quadrant of a ‘Hass’ avocado tree, on the dry matter content of ‘Hass’ avocado tree tissues presented by Lovatt (1996), on the avocado fruit growth presented by Ofer (1986) and on the yearly growth of the perennial parts of an avocado tree which grows in a commercial orchard.

 

The basic figures

Four rounds of photosynthetic measurements, with an interval of two hours, conducted on September 21, 1977 (12 hours of daylight) on the external layer of the south quadrant of a ‘Hass’ avocado tree, revealed that the daily assimilation of a quadrate meter was 0.21 mol CO₂. This figure refers to the hours of direct sunlight as well as to indirect lighting during the day.

 

According to this figure, estimates of the assimilation of the east, west and north quadrants have been made. So, it follows that the daily assimilation rate of an avocado tree, with an external canopy surface of 50 m² is 5.18 of CO₂.

 

According to data on reduced light intensity which reaches leaves on the inner, shaded parts of the tree (Schaffer and Andersen, 1994), it was estimated that for the whole tree, those leaves assimilate 1.9 mol of CO₂ per day. So, the total daily assimilation of an avocado tree is 7.08 mol of CO₂, which are 311.5 gram of CO₂.

 

Factors which influences the assimilation rate

The impact of each of the following five factors on the monthly assimilation rate of an avocado tree was calculated: leaf age, day length, yearly canopy growth, eastern dry wind stress, and climatic conditions during winter.

 

Leaf age

1.1)           The life span of an avocado leaf is approximately one year.

1.2)           The flush of avocado leaves and their shedding occurs in an uneven rate all year long (mostly between April and July).

1.3)           The maximum assimilation efficiency of avocado leaves is attained between the 25^th and 70^th day after sprouting. Afterwards, their assimilation efficiency declines gradually towards senescence.

 

These are the physiological facts according to which the term “assimilation potential” has been defined.

 

“Assimilation potential” is the monthly assimilation yield, in mol CO₂, of an avocado leaf, calculated according to the order of the leaf flush and age. This factor refers to the internal capability of the leaf to assimilate CO₂.

 

Day length, eastern dry wind stress and climatic conditions during winter, are external factors that have an impact o the leaf’s assimilation rate.

 

The adjustments of the monthly assimilation potential

As mentioned earlier, our estimate of the assimilation of an avocado tree, 7 mol of CO₂ per day, or 210 mol of CO₂ per month, was calculated according to photosynthetic measurements made on September 21, when the hours of daylight equal those of darkness.

 

During spring, the number of light hours that extend by 5.5% per month while during autumn they are reduced at the same rate. Therefore, the monthly assimilation potential has been adjusted accordingly: the first adjustment.

 

It was estimated that the seasonal growth of the canopy’s outer layer area of an avocado tree in a commercial orchard reaches during summer, about 14%. Most of this yearly growth is vertical. The second adjustment of monthly assimilation has been done according to this estimation.

 

The reduction of the monthly assimilation rate, caused by climatic conditions, such as spring east dry winds and the winter climate, was the third adjustment. The third adjustment has been done according to data provided by The Israeli Meteorological Service.

 

The figures of the calculated monthly assimilation of an avocado tree, after the three adjustments mentioned earlier, are presented in Table 1, line 2. According to the figures, the highest monthly assimilation of 216 mol of CO₂ is gained in August by the “standard” avocado ‘Hass’ tree, while the lowest monthly assimilation occur in February.

 

According to the figures, the calculated monthly assimilation of an avocado tree in January and in February are only about 29% of the potential monthly assimilation, while in August it is about 97% (Table 1, line 2/1). The monthly gap between the calculated (actual) assimilation of an avocado tree and its assimilation potential, demonstrates mostly the influence of climatic conditions and canopy growth, on the avocado’s assimilation performance.

 

 

The yearly growth of an avocado tree

In order to evaluate the yearly dry matter production, the following information was taken into account: Data presented by Cameron (1952) and Lovatt (1996) about avocado trees that were dismembered into organs. Their fresh and dry weights were reported; and growth rates of avocado shoots and roots measured in a rhizotron by Ploetz et al. (1992).

 

The following assumptions were made for the compilation of Table 2:

· A crop of 53.6 kg, F.W. per tree (1500 kg, F.W. per dunam).

· A yearly growth rate of 14% of the perennial organs.

· A flesh/seed ratio of 84/16 in the avocado fruit.

 

 

The yearly growth of every organ, presented in the Table 2, has been divided to months according to data from research reports, or to the author’s assumptions, when no data were available (Table 3).

 

 

 

The energy balance of the growing avocado fruit

The periodic accumulation of all energy comprising components in an avocado fruit were calculated, while the fruit’s own photosynthetic carbohydrate production was estimated for the same periods.

 

The calculations were based on the periodical measurements of net efflux of CO₂ of attached avocado fruit cv. Booth 7, which enabled Whiley et al. (1992) to calculate the fruit photosynthesis, and on data of fresh weight and oil content accumulation of growing ‘Fuerte’ fruits, presented by Ofer (1986).

 

The periodical dry matter weight of fruits, presented in both studies, has been used as “connection numbers”. That means, that the rate of assimilation, recorded in a ‘Booth 7’ fruit of a certain dry matter weight, were related to a ‘Fuerte’ fruit with the same dry matter weight.

 

The calculations have been made for fruits during the first period of 10 weeks after anthesis and afterwards for fruits during 8 periods, of two weeks each, until the 26th week after anthesis (Table 4).

 

 

The figures presented in Table 3, indicate that there are two main periods during fruit growth, connected to the fruit’s own photosynthetic activity.

 

1)     The first ten weeks after anthesis, when the fruit’s self assimilation provides about 20% of the total energy which accumulates in the fruit.

2)     The remaining growing period, from the eleventh week on, when the fruit’s self assimilation provides only about 10% of its accumulated energy.

 

This is the place to remind that in the spring season the assimilation efficiency of most of the previous year leaves are reduced towards shedding. At that time the new flushing leaves are still a sink for assimilates rather than a source. During this period a quick reduction in the starch reservoir level takes place in the avocado tree.

 

Those facts emphasize the importance of the self assimilation of the young fruits for their development and survival.

 

According to the figures in column (4) Table 4, the fruit’s self assimilation increases constantly during the period of fruit growth but its share, out of the total dry matter accumulated in the fruit, is reduced.

 

The yearly production and utilization of dry matter in an avocado tree

Since the monthly assimilation and the monthly increase in dry matter of an avocado tree have been presented, it is now possible to calculate the production and utilization of an avocado tree during a year.

 

The calculation of the dry matter production has been made by multiplying the calculated monthly assimilation, given in mol of CO₂, by 30 g (the weight of 1/6 mol glucose) (Table 5).

 

In order to estimate the maintenance respiration rate (Rm) of an avocado tree, the weight of an uprooted ‘Hass’ tree, presented by Lovatt (1996), has been used. The utilization of dry matter for maintenance respiration has been calculated as 3% of the phytomass of the growing tissues between May and August, 2% during March, April, September and October, and 1% between November and February. The total monthly utilization of fry matter of an avocado tree is obtained by adding the monthly Rm to the monthly growth (Table 6).