In MGCNL 56 we reported that at 27 C coleoptiles of sweet corn variety Silver Queen were longer following seven days of continuous light treatment, compared with those grown for the same period in continuous darkness. At this temperature we have found most of the 15 different lines examined show quite the opposite: continuous light inhibits both coleoptile and epicotyl growth. This unusual behavior prompted a closer examination of the light dependence of coleoptile and epicotyl development.
Using the same methods reported in MGCNL 56, we have investigated the relation of short periods of white light on coleoptile and epicotyl elongation. Germinating seedlings were exposed to a single period of growth chamber light lasting for 1/2, 1, 2, 4, 8, 16, or 24 hours; seedlings remained in darkness for the remainder of the seven-day incubation period. Duplicate pans of germinating seeds of each of four different lines were subjected to each of the light treatments. Every twelve hours a new set of seedlings was given the above light treatments; the new seedlings were thus twelve hours older than the previous set. The last set of treatments was begun at 72 hours and completed at the 96th hour. Differences in coleoptile and epicotyl measurements represented the effect of a single light exposure in the seven-day period. Light treatments, therefore, represent a photo probe for the developmental potential of coleoptiles and epicotyls during these first 96 hours of seedling development.
Figures 1, 2, 3, and 4 show length of seven-day-old coleoptiles and epicotyls following exposure to single light periods of 1/2, 1, 2, 4, 8, 16, or 24 hours. Light periods are represented by points on lines at the termination of the light period. For each line, representing seven different light treatments, it is possible to associate the length of the light period with the age of the seedling and the resulting length of coleoptile or epicotyl. In each figure, the six solid lines and six dotted lines represent new sets of experiments begun at twelve hour intervals. The straight lines indicate the length of tissues grown under continuous light or darkness. Epicotyls fail to grow in continuous light.
In Fig. 1, seedlings of sweet corn variety Gold Cup, grown under conditions of continuous light, produce coleoptiles approximately 2 cm. In the dark these same coleoptiles reach 3.2 cm in length. One must conclude that continuous light inhibits coleoptile development. However, short light treatments can cause Gold Cup to reach 5.6 cm, nearly double the lengths found after continuous darkness. Figs. 1 and 2 show in four different lines that as the light treatments are lengthened from 1/2 hour to 24 hours, an increase in coleoptile length takes place. Depending on the line, age of seedling, and length of treatment, light can stimulate as well as inhibit coleoptile elongation. Coleoptiles of Silver Queen show a gradual increase in length in response to longer light periods, reaching a maximum at about 70 hours before increased inhibition becomes evident. In Sprite, a yellow sweet corn, coleoptiles reach a maximum after the first 24 hours, and after this period of development increases in light reduce coleoptile lengths as the seedling ages. It is remarkable, too, how soon seedlings are light receptive: Figures 1 and 2 show that 1/2 hour of light after 12 hours of imbibition produces a measurable increase in coleoptile length. Gold Cup, another yellow sweet corn variety, shows maximum coleoptile length following 1/2 hour light when seedlings are 60-72 hours old, resulting in coleoptiles twice the length of dark-grown and three times light-grown coleoptiles. Silver Sensation, a white-seeded sweet corn, shows erratic behavior, especially for light treatments of 1/2 to 8 hours for all 12 hour periods. To a limited extent this behavior is reflected in the first few light treatments of all lines. One may speculate that this same behavior is found and reported in phototropic behavior of coleoptiles as first positive and first negative curvature as a result of increasing light dosages.
Compared to coleoptiles, epicotyls show less pronounced increases in length in response to light over dark controls. Silver Queen epicotyls in Fig. 3 exceed dark controls for a few light treatments administered within the first 36 hours. Epicotyls longer than dark controls were recorded in Fig. 4 for Silver Sensation for treatments up to 60 hours. As seedlings age, Figs. 3 and 4 show that epicotyls tend to shorten with increased exposure to light. Epicotyls of the yellow sweet corns, Sprite and Gold Cup, remain shorter than those grown under continuous dark conditions.
From a practical standpoint, the behavior of coleoptiles and epicotyls of Silver Sensation and Silver Queen could represent a more desirable seedling quality for areas where seed emergence is a problem. Continued growth of both tissues under weak light conditions near the soil surface could result in higher percentages of emergent seedlings. Seedlings with greater light sensitivity could result in premature cessation of growth in both tissues before complete emergence. The methods used above provide a simple system for selecting the seedling behavior needed. Another interesting point from the four figures is that the greatest differences among the four lines can be seen during the early germination period. As one approaches the 70th hour a convergence of behavior is evident, especially among the coleoptiles. This might be expected, since under field conditions selection for seedling behavior is likely to be practiced at emergence, and most lines would be exposed to convergent selection at this time.
Bernard C. Mikula and Amy Smith
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