VIEWPOINT Viewpoints and Letters to the Editor are published in Hort- comments on matters of concern to horticulturists. These are Science to provide members of the American Society for Hor- not statements of official Society policy nor do they necessar- ticultural Science an opportunity to share their experiences and ily reflect the views of a majority of the Society’s members. Developmental Indices of Peach: An Anatomical Framework James Gage and Gary Stutte Department of Horticulture, University of Maryland, College Park MD 20742 Many factors affect the growth and de- velopment of peach fruit. Unlike the more easily controlled environments of the labo- ratory and greenhouse, field researchers are limited in their ability to control the micro- climate, soil, and cultural conditions that af- fect peach growth and development. As a result, variability within a peach population is often greater than variability among ex- perimental treatments. The date of full bloom in peach can vary substantially from year to year. In a 10-year study by Weinberger (1948), full bloom of Georgia-grown ‘Elberta’ varied between 1 Mar. and 4 Apr. This, in part, resulted from growing seasons that ranged from 104 to 124 days. In our studies covering 5 years at Beltsville, Md. (1985–1989), full bloom was spread over 9 days, from 31 Mar. to 8 Apr. Full bloom, the time of maximum blos- som opening, is something of a misnomer. Populations of blossoms may open and be- come fertilized over several days; therefore, precise assessment of full bloom is difficult (Ragland, 1934). Fischer (1962) noted that the determination of full bloom is subject to personal bias that introduces an inherent var- iability into the chronological description of subsequent developmental events. Growth of peach fruit is temperature de- pendent. Efforts to quantify the temperature requirements for fruit maturation have been made by several workers (Baker and Brooks, 1944; Brown, 1952; Fischer, 1962; Munoz et al., 1986). Batjer and Martin (1965) showed that nighttime low temperatures delay ma- turity throughout the growing season. Mod- erate nighttime temperatures accelerate fruit growth primarily during Stages I and II of development when ambient temperatures are low. This phenomenon could explain differ- ences in length of season between growing regions, and even within different areas of the same orchard. Topp and Sherman (1989) used mean monthly temperatures to explain variability in ripening period at 13 locations in Australia. Regression analysis of mean fruit Received for publication 19 Oct. 1989. Scientific article no. A-5032, contribution no. 8080 of the Maryland Agricultural Experiment Station, Col- lege Park. The cost of publishing this paper was defrayed in part by the payment of page charges. Under postal regulations, this paper therefore must be hereby marked advertisement solely to indicate this fact. growing season temperatures with fruit de- velopment period showed a 5-day delay in ripening period for each 1C reduction in temperature. Because of the variability in early fruit growth, classification of distinct stages of growth and development is especially diffi- cult in peach. However, the responses to dif- ferent experimental treatments do seem dependent on developmental stage of the fruit. Lombard and Mitchell (1962) found that sensitivity of peach fruit abscission to naph- thalene-acetic acid (NAA) decreased with time. They attributed this difference to em- bryo development and hormone content of the seed. Stutte and Gage (1990) found a differential response between gibberellic acid (GA) treatments of embryo aborted fruit at 30, 40, and 50 days after full bloom (DAFB). Fruit from the 30-day treatment developed a pit, underwent final swell, and ripened nor- mally, whereas fruit treated at 40 and 50 DAFB abscised. We attributed these find- ings to a differential sensitivity to GA be- tween early and late Stage I of development. These examples emphasize the value of a developmental index for determining ana- tomical events. However, the precision of the indices in predicting anatomical events is not known. The object of this report is to describe the available indices for the growth and devel- opment of peach fruit, to develop a contin- uum of early fruit development that can function as a tool for experimental design and evaluation, and suggest a means of ex- trapolating developmental processes among different growing regions. Indices of peach fruit growth There are four major types of indices used to describe the growth and development of peach fruit: 1) chronological; 2) morpholog- ical; 3) environmental; and 4) physiological. The advantages and limitations of each are described below. A chronological index can be based on the Julian date, the calendar date, the days after a specific morphological event (usually full bloom or anthesis), or the days after a spe- cific treatment. The Julian date is useful as it can be matched a posteriori to other data (rainfall, temperature, etc.); it does not, however, describe the development of the target fruit. The calendar date has the ad- vantage of being a universally recognized system that makes relative comparisons pos- sible. It is more difficult to integrate into a model than a Julian date because the number of days in a month is not constant. Days after full bloom is a graduated index based on an important morphological event. The varia- bility in bloom and the variability of envi- ronmental effects decrease its precision. Days after a specific treatment could minimize the importance of the anatomical stage on the success/failure of the treatment, focusing only on the results of the treatment. Fruit morphological indices have also been used to classify the growth stages of fruit. The three-stage double sigmoidal curve (Connors, 1919) is commonly used to de- scribe the development of peach fruit by di- ameter and weight. Stage I is characterized by cell division and rapid development of the endosperm, Stage II by endocarp sclerifica- tion (pit hardening), and Stage III by rapid expansion of mesocarp cells (final swell). Changes in measured rates of growth have led researchers to hypothesize that compe- tition during fruit development is responsible for the growth stages. Various hypotheses have been proposed to describe competition within an individual fruit, competition be- tween reproductive and nonreproductive sinks, and endogenous control of these events. Ex- ternal measures of fruit diameter or volume are indicators of pericarp activity, and, be- cause of variability of fruit, environmental factors, and differentiation of the tissues, do not define developmental events (Zucconi, 1987). One example is the hypothesis that the developing seed controls the onset of Stage II: The work of Crane (1963) with parthen- ocarpic fruit and Stutte and Gage (1990) with GA-treated, embryo-aborted fruit both show that normal fruit size and pit development are possible without the presence of a viable ovule. An environmental index is an attractive framework for a developmental chronology since it provides an objective, readily mea- sured parameter that has quantitative effects on fruit development. Fischer (1962) was one of first to use heat unit accumulation to ex- plain site specific differences in growing season. More recently, Topp and Sherman (1989) determined the effect of average daily temperatures on the ripening period of sev- eral peach cultivars. Munoz et al. (1986) have determined cultivar-specific differences in the perception of heat units. DeJong and Goud- HORTS CIENCE, VOL. 26(5), MAY 1991 459