THE RELATION OF GROWTH TO SIZE IN CUCURBIT FRUITS

Abstract

AN UNDERSTANDING of the genetic basis of inherited differences among organisms must depend on a knowledge of how these differences arise during development. Characters of size, although notably difficult to analyze genetically, have a marked advantage for developmental studies since they can be examined throughout most of the period from inception to maturity, and since many developmental differences among them can be described in simple quantitative terms. It is possible, for example, to determine whether a given inherited size difference is due to a difference in rate or in duration of growth, and tlhus to state the problem of its genetic control more nearly in physiological terms. The fruits of the Cucurbitaceae and particularly of Cucurbita Pepo offer good material for such studies. Size differences among races of this species are very great, some of the largest pumpkins bearing fruits a thousand times the volume of the smallest gourds. The ovaries and fruits are easily accessible and their dimensions can be measured from a very early stage. They are determinate structures, each an organic entity with a developmental cvcle of its own which is comparable to that of an individual organism. In many cases pure lines, essentially homozygous, have been established by inbreeding. The purpose of the present paper is to compare fruit growth, from early ovary size to maturity, in a number of races differing markedly in inherited fruit size, and thus to provide evidence helpful in a developmental analysis of such differences. Relatively few studies have been made of the growth of fruits. Anderson (1895) brought a young and growing pumpkin fruit into the laboratory, still attached to its vine, and weighed it for a period of 47 davs. Growth showed a sigmoid curve and the data were later analyzed by Robertson (1923). Gustafson (1926) studied the growth of a number of cucurbit fruits, with results much like Anderson's. Growth curves have been determined for a few other fruits, such as tomatoes, apples, peaches, and orchids. The growth of leaves, also determinate plant structures, has been measured by a number of workers, notably by Weizsacker (1938) in tobacco. In all cases the basic growth program is that of a period of accelerating growth followed by a fall in rate until maturity, which produces, when plotted arithmetically against time, the familiar sigmoid curve. Little work seems to have been done, however, on the comparative growth behavior of large and small types in the same species. NMATERIALS AND METHODS.-A number of inbred lines of Cucurbita Pepo were studied. The five most thoroughly analyzed were SRC, a white spheroidal 'Received for publication February 6, 1945. The author wishes to express his thanks to Edmund W. Sinnott, Jr. and to Mildred Sinnott Hill for their efficient assistance during the course of this investigation. line with a fruit volume at maturity of about 50 cc.; TA, a white, ovoid type of about 40 cc.; SP, a green and yellow spoon gourd of about 70 cc.; CF, a large yellow pumpkin of about 7,000 cc.; and M35, another pumpkin tvpe of about the same volume. Also studied were lines MT, an elongate green pumpkin with a fruit volume of 4000 cc.; 0, a round orange gourd of about 175 cc., and P, a small sugar pumpkin of about 800 cc. These are all vine or runner types, but two bush races were also studied: 103, a nearly spherical white line, with a fruit volume of about 700 cc., and STN, a yellow straight-neck summer squash of about 800 cc. Aside from these lines of Cucurbita Pepo, two from Lagenaria vulgaris were studied: line TR, a pearshaped type with a fruit volume of about 650 cc., and MB, a small bottle gourd, of about 60 cc. A few fruits from plants of watermelon grown from commercial seed were also measured.