The Purpose and Goal of Phenolgy
By Mary Lu Arpaia
Development of a phenology model for avocados could greatly enhance a grower's ability to plan management practices in relation to the events occurring within the tree. Knowledge of the time of root and shoot growth, flowering and fruit set, and the relationships between these events and carbohydrate utilization within the tree will allow for application of irrigation, fertilization, and other cultural practices at optimum times. Another goal is to define the changes in carbohydrate partitioning in relation to phenological events in cv. Hass avocado trees in an attempt to understand the factors involved in alternate bearing. The California avocado industry exists on the interface of the urban agriculture complex of southern California. In order to maintain the viability of the avocado industry in such a challenging environment, growers must develop the necessary tools to maximize productivity while minimizing inputs into the agricultural system. Characterization of the changes in the avocado in relationship to the cyclical bearing habit of the tree will also aid in developing cultural management strategies that will minimize alternate bearing.
It has been proposed that productivity management in avocado trees is dependent on the management of carbohydrate in the tree (Wolstenholme, 1986; Whiley and Wolstenholme, 1990). The management of carbohydrate in this context refers to the accumulation and mobilization of assimilates to ensure that the flowering event is successful in terms of consistent annual fruit set, fruit retention is at an acceptable level, vegetative growth is balanced without becoming highly competitive with fruit yield, root growth is maintained and the crop matures with acceptable quality. Based on significant correlations between reserve carbohydrate (starch) prior to flowering and fruit set in citrus and avocado (Goldschmidt and Golomb, 1982; Scholefield et al., 1985), Whiley and Wolstenholme (1990) proposed integrating a starch curve with phenological growth models developed in Australia and South Africa to be used as a quantitative index for predicting potential yield. Using data from work by Scholefield et al. (1985) and Whiley and Wolstenholme (1990) an annual starch curve for avocado trees has been developed. When related to the phenological cycle, starch reserves in the wood of trunks are at their highest during the prolonged winter rest period when growth demands are lowest. Starch reserves fall rapidly during flowering and fruit set, and reach their lowest concentration during the summer fruit drop period before increasing to their winter maximum (Whiley and Wolstenholme, 1990).
Whiley and Schaffer (1994) note that world production of avocados falls into 3 main climatic zones: cool, semi-arid climates with winter rainfall (characteristic of California); humid, subtropical with summer rainfall (e.g. South Africa) and tropical or semitropical with summer rainfall (e.g. Florida). The phenological model proposed by Whiley and Wolstenholme is based on trees growing in subtropical Australia and South Africa and was developed using grafted trees on seedling rootstocks. The growing conditions in these humid subtropical areas is quite different from California conditions. Clonal rootstocks are used in California orchards because of their resistance to Phytophthora cinnamomi Rands. and may affect the phenology of the tree by influencing scion behavior.
We are interested in emerging results from Australia and South Africa (Whiley and Wolstenholme, 1990) which implement a starch curve with other phenological events as a quantitative index for predicting potential yield and yield alternation. However, using a starch curve in this manner is a simplistic approach to a complex issue. We have developed a preliminary phenological model for cv. Hass avocado grown in California conditions (Robinson et al., 1994), and have also just completed a 2 year study of carbohydrate from the same trees. We have found that while starch serves as an important non-structural carbohydrate reserve in avocados, there may be an overlooked group of other non-structural carbohydrates which are in higher concentrations than starch. These are D-mannoheptulose and the alcohol sugars (polyols) dulcitol, perseitol (D-glycero-F-galacto-heptitol), and volemitol (D-glycero-D-mannohepitol) (Richtmyer, 1970a, 1970b). Of the polyols detected, perseitol is clearly the dominant polyol found in avocado tissue, however, its precursor, D-mannoheptulose, is also found in high concentration. The other major polyols typically associated with other plant species, sorbitol and mannitol, are not found in appreciable amounts in avocado tissue. The proportion of D-mannoheptulose and polyols to total soluble sugars was high, with 50 - 90% being in this group. Common soluble sugars such as glucose, fructose, and sucrose were generally below 1% in dry samples. Starch concentrations in these same samples ranged from 0.8% to 9.2% indicating that starch is a major non-structural carbohydrate in avocado, although it may not play as important of a role in regulation carbohydrate reserves as previously believed. The energy balance and carbohydrate budget of trees is thought to play a major part in alternate bearing and we believe this study will elucidate some of the factors involved in alternate bearing of avocado trees.
Our preliminary results suggest that avocado trees have unusually high concentrations of D-mannoheptulose and polyols compared to common soluble sugars and starch (Sakai, 1966; Rasmussen and Henry, 1990). The carbohydrate budget and overall energy balance of the tree is likely to be greatly influenced by these sugars. According to the literature, little is known about this group of sugars and their quantitative analysis. This is probably due to their minor importance in other horticultural trees (Sakai, 1966; Rasmussen and Henry, 1990). It is interesting to note that recently D-mannoheptulose has been found to have anti-tumor growth function and is considered a potential treatment for cancer patients (Board et al, 1995). This study may play an important role in furthering information on the potential health benefits of consuming avocados.
As production costs continue to increase and market competition increases for the California industry we need to look at ways to minimize production costs while enhancing tree productivity. Canopy management can play a pivotal role in achieving this goal. Another critical components of orchard productivity can be related to tree spacing. Little work has recently been conducted except in South Africa pertaining to tree spacing (Kohne, 19 ). It is highly possible that with the selection of upright trees such as the 'Lamb Hass' that high density plantings may be economically feasible. Tree pruning and trunk girdling are also cultural practices which have not been routinely practiced in California, although elsewhere (such as Israel) these practices are utilized and tools to control vegetative growth while enhancing productivity. Cutting (19 ) and others (Kohne, Whiley ) have reported on results of vegetative growth management strategies. The majority of this work, however, has been conducted in humid semi-tropical regions such as Australia and South Africa. The data we have collected so far in this project on tree phenology will aid us in California to develop canopy management practices which are designed for California conditions and therefore assist us in maximizing production.