Fruit-induced & apical senescence in Pisum sativum L.

Abstract

Annual plants normally grow vegetatively for a limited time, flower, fruit, and then die. Rapid stem elongation of the Alaska pea plant continues until several fruit have begun development. During fruit development growth slows, and has stopped by the time fruit development is complete. Death of the vegetative parts soon follows (cf. 20). It was Reichart (17) apparently, who first showed that flowering and fruiting are antagonistic to vege-tative growth. He found that when flower buds were removed from the Vienna wall flower, normally an annual plant, this plant would continue vegetative growth for two or more years. When flowers were allowed to develop, the plant would rapidly set fruit and die. Numerous workers have substantiated and extended these results with other species (e.g. 6, 8, 9, 11, 12, 13, 19, etc.). Various lines of evidence suggest that fruit may not act simply by diverting organic reserves from vegetative growth. Rather, the reproductive structures appear to cause death of the plants by some other mechanism. Preliminary experiments indicated that periodic gibberellin treatments would substantially delay senescence in the Alaska pea. Further experiments were undertaken to study in greater detail the effect of gibberellin and fruiting on senescence of this plant. The evidence presented here indicates that the effect of gibberellin on senescence is indirect. Further experiments revealed a separate apical senescence in Pisum. The nature of this senescence is described. Senescence will be considered here to mean those progressive, deleterious changes common to all organisms of a given species which ultimately lead to the death of the organism (18). METHODS & MATERIALS Plants of Pisum sativum L., var. Alaska, and in one experiment var. Alderman Improved (Ferry-Morse Seed Co., Los Angeles) were grown in the Earhart and Campbell Plant Research Laboratories as described previously (7). Seeds were soaked in deionized water for 4 to 5 hours, then several seeds were sown in each of a number of 0.9 liter plastic containers in a 50-50 volume mixture of Vermiculite (expanded mica) and crushed ' rock. After germination for 4 days at 26° C in darkness , all except two vigorous seedlings were discarded from each pot. The two remaining seedlings were immediately moved to the appropriate greenhouse room and experimental treatments were begun. The plants were watered three times weekly with Hoag-land's nutrient solution and with deionized water as required. The plants were grown throughout the year under solar radiation with the photoperiod extended to 16 hours by incandescent illumination. Substantial differences in growth rates were observed in the different experiments conducted during different seasons of the year. These effects can be ascribed principally to differences in solar radiation. Growing temperatures are expressed as: day temperature (0800-1600 hr)/night temperature (1600-0800 hr). High growing temperatures (30°/24° C) were used when practical as a matter of convenience. With this temperature regimen, plants mature rapidly , are short, and easy to handle. The results indicate the same factor limits growth at high and at more moderate temperatures. Since plants grown at lower temperatures can be grafted much more successfully, plants for grafting experiments were always grown at 230/170 C. Co., Indianapolis, Ind.) was applied where indicated as a single 4 ,Al ethanolic drop to the stem apex or to the youngest leaf. Three micrograms of gib-berellin A3 were applied weekly to each plant, beginning the day the plants were transferred to the greenhouse and terminating when growth had ceased. This gibberellin treatment is saturating so far as stem growth is concerned. Indole-3-acetic acid (IAA) and kinetin treatments were applied in a similar manner. For sprays, solutions of 2 % sucrose, 1 % casein hydrolysate (Nutritional Biochemical Co., enzymatic-ally hydrolyzed) and 1 X 10-3 AI cobaltous chloride were made up in distilled water with 0.1 % Tween-20 (polyoxyethylene sorbitan monolaurate, a detergent produced by Atlas Powder Co.). Spraying was repeated twice weekly from germination until growth had ceased. Controls were sprayed with Tween-20 solution on the same schedule. When root temperatures were to be different from top temperatures, seeds were planted directly in 1-gallon glazed crocks in which aluminum water coils had been placed previously. The seeds were germinated in the dark at 260 C and moved to the greenhouse rooms as usual. Thus, germination occurred under uniform conditions and temperature treatments were started only after the epicotyls had emerged. From the time the plants were moved to the greenhouses , water at the desired temperature was passed 389