COMPOSITION OF FRUITS AND PHLOEM EXUDATE OF CUCURBITS

Abstract

Phloem ex?date has been used in many studies on solute transport in plants. As far as can be determined by microscopic observation, it comes from the sieve tubes and represents, therefore, an unmixed sap that moves rapidly within the plant. If the ex?date from cut peduncles of cucurbit fruits represents a true sample of a solution of food materials moving through the phloem by mass flow to nourish those fruits, there should exist a fairly constant proportionality between the various compounds present in the ex?date and in the fruits. That the process of phloem exudation has been variously interpreted in the past is apparent from the following review of literature. Since the researches of H artig and N?geli (6) it has been commonly assumed that the sieve tubes of the phloem tissue of plants are continuous elements interconnected by open pores through which substances in solution may pass more or less freely. Phloem exudation from cut cucurbit stems, described in detail by N?geli, was cited as confirming evidence for an open system; and many considered it a manifestation of the normal process of food transport. Zacharias and Kraus (6), analyzing such ex?date, showed that it contained sugar, organic nitrogen compounds, potassium, and phosphorus, all of which were presumably translocated in the phloem. The works of Fischer, Strasburger, and Hill (6) tended to confirm this early interpretation. The cucurbit sieve tube became the accepted type of food-conducting element, illustrated in elementary texts, and often cited as an example of adaptation of structure to function. To many botanists the structural aspects of the process of food transport in plants seemed clear. In 1855, however, von Mohl questioned the actual perforation of the sieve plate, contending that the middle lamella remained intact. Lecomte described the protoplasm as penetrating into the striations of the callose plates in the form of filaments terminating at the swellings in knobs. Such structures are commonly found in the position of the middle lamella of the sieve plate. The researches of Hill, Strasburger, and other early workers (?) emphasized the extreme fineness of the sieve-tube connections in many plants, the gymnosperms in particular. Kuhla (16) denied the perforation of the protoplasmic connection of the sieve plate, and since then others have shared his view (6, 9, 26, 27). If the sieve tubes are not perforate elements through which ready mass flow of solutions can take place, how are foods transported in plants? Birch-Hirschfeld (1), Dixon and Ball (12), and Mason and Lewin (18) concluded that they move through the xylem. Later, Dixon (11) and