Planta 135, 93-94 (1977)
P l a n t a 9 by Springer-Verlag 1977
Evidence for a Graft-transmissible Substance which Delays Apical Senescence in Pisum safivum L. W.M. Proebsting and P.J. Davies Section of Genetics, Development and Physiology, Division of Biological Sciences, Cornell University, Ithaca, NY 14853, USA G.A. Marx Department of Seed and Vegetable Sciences, New York State Agricultural Station, Geneva, NY 14456, USA
Abstract. Apical senescence in an early flowering line o f pea, G2, is greatly delayed by short days. This behavior is controlled by two d o m i n a n t genes. Apical senescence o f ungrafted, insensitive (I) lines is unaffected by photoperiod. W h e n I - t y p e scions with one o f the two required genes were grafted onto G2, apical senescence o f the I-type was delayed in short days, but not in long days. Flowering o f the I-type was unaffected. The apex o f the G2 stock was unaffected as well. Apical senescence of an I-type line lacking b o t h p h o t o p e r i o d genes was not delayed when grafted on G 2 in short days. It is concluded that G2 plants grown in short days p r o d u c e a graft-transmissible factor which delays apical senescence o f photoperiodically insensitive lines.
Key words: Apical senescence - Pisum - P h o t o m o r phogenesis Senescence.
G r o w t h regulators
Apical senescence is delayed in an early-flowering genetic line o f peas ' G 2 ' when the plants are grown under short days (SD), apical g r o w t h continuing for a prolonged period o f time despite the presence o f n u m e r o u s flowers a n d fruits (Marx, 1968). This response is light-dependent but not strictly photoperiodic because the controlling influence is the total a m o u n t o f light received per 24 h. It seems to be determined by the d o m i n a n t alleles o f two different genes designated Sn and Hr (Murfet and Marx, 1976). Other early-flowering lines senesce and die after producing a limited n u m b e r o f reproductive nodes, regardless o f photoperiod. These insensitive, or I-type, lines comprise three different genotypes with respect to Sn and Hr: I~=sn,Hr; I z = S n , h r ; I3=sn,hr. L e o p o l d et al. (1959) and L o c k h a r t and Gottschall (1961) showed that reproductive structures are a prerequisite for the induction o f apical senescence, a find-
ing that also applies to G 2 peas g r o w n under tong days (LD) (Proebsting et al., 1976). I-type lines of peas senesce regardless o f p h o t o p e r i o d if fruits are allowed to develop. Since fruit p r o d u c t i o n thus clearly plays a role in senescence, the behavior o f G2 plants in SD m a y be explained (1) by the absence o f the senescence-inducing activity o f the fruits in SD, or (2) by the f o r m a t i o n in SD o f a substance which counteracts the senescence-inducing activity o f the fruits that m a y be present in both SD and LD. The fruits m a y influence senescence either by exporting an inhibitor o f apical growth, or alternatively, by acting as sinks for growth regulators required for apical growth. Grafting has been used by others to obtain evidence for a flowering h o r m o n e (Zeevaart, 1976). We used grafting procedure to study senescence, specifically to distinguish between the alternative control mechanisms cited above. Seedlings of the G2 line of pea (Pisum sativum L.) were grown individually in 15-cm clay pots filled with a mixture of sand, soil and peat (1:1:1, by volume). The plants were supplied with a complete nutrient solution every 2 weeks, and the lateral branches were routinely trimmed. The stocks were grown in SD (9 h) and were used for grafts just prior to flowering, at the age of 4 weeks. The I-type seeds were sown in vermiculite and 10-day-old seedlings were used as scions. The scions were trimmed into a wedge at the base, inserted into an incision made in the axil of the uppermost expanded leaf of the stock, and then secured with masking tape. The graft was covered with a clear plastic bag for 1 week. The experiments were conducted in growth cabinets illuminated by fluorescent tubes supplemented with incandescent bulbs. The average full light intensity was 3400 gW cm -2 at pot level. SD and LD were 9 h and 18 h, respectively, and day and night temperatures were 20~ C and 15~ C, respectively. Plants receiving LD treatment were transferred from SD as soon as there was evidence of scion growth (3-4 weeks). The number of reproductive nodes developed on the scion was taken as a measure of response. The results o f the I-type/G2 grafting experiments are given in T a b l e x l . Iz (Sn,hr) and Ii(sn,Hr) p r o d u c e d twice the n u m b e r o f reproductive nodes in
W.M. Proebsting et al. : Senescence-delaying Substance in Pisum
Table 1. Number of reproductive nodes and fruits developed by three different I-type scions grafted on G2 stocks and grown in short- or long-day conditions The number of plants in each treatment is in parenthesis Scion
I3 (sn hr)
It (sn Fir)
I2 (Sn Hr)
Short days (9 h) Reprod. nodes Fruits
70 413 (6)
11.3 5.3 (7)
23.3 18.4 (11)
Long days (18 h) Reprod. nodes Fruits
54 216 (7)
5.7 3.1 (13)
7.6 6.1 (7)
Non-grafted control (9 h) Reprod, nodes Fruits
6.6 (19) 4.3
5.3 (10) 2.3
10.7 (28) 10.0
LSD0.01 forf SD vs. LD 1.76 reprod. ~ SD vs. nongrafted 1.48 nodes t LD vs. non-grafted 1.40
2.45 2.58 1.54
5.83 3.64 5.1
The G2 line has been produced by crossing the I1 and I2 types, and its behavior is based on the complementary action between the Sn and Hr genes. Although genetic complementarity need not be reflected by complementarity in grafts. 11/I 2 grafts were grown in short days to test the possibility that I1 and I2 could produce complementary factors which delay apical senescence when combined in this way. However, no complementarity was observed in the 11/ 12 grafts, since the Ix scion produced only 4.1+0.4 reproductive nodes. These data indicate that the G2 response is under hormonal control. Additional support is offered by our earlier observation that G2 senesces under nonphotosynthetic light intensities (Proebsting etal., 1976). It is not yet known why the 13 line did not respond like Ix and I2 when grafted to G2. This study was supported by National Science Foundation grant PCM76-05616.
SD as their counterparts in LD and the ungrafted controls. I3(sn, hr) in contrast, produced about the same number of reproductive nodes under all three conditions. Moreover, senescence of I3 scions was not delayed when the scions were defoliated to ensure the movement of organic compounds from the stocks. This indicates that the presence of leaves on the scions was not a limiting factor in the I2 and Ix grafts. All scions, despite having fruited normally, did not inhibit apical growth of the G2 stock. The results indicate that a graft-transmissible substance(s) is produced by G2 plants in short days and that this substance(s) delays senescence by counteracting the influence of I1 or I2 fruits. If the delay were caused by the absence of the fruit influence, then the scions should have senesced normally.
Leopold, A.C., Niedergang-Kamien, E., Janick, J. : Experimental modification of plant senescence. Plant Physiol. 34, 570-573 (1959) Lockhart, J.A., Gottschall, V. : Fruit-induced and apical senescence in Pisum sativum L. Plant Physiol. 36, 389-398 (1961) Marx, G.A. : Influence of genotype and environment on senescence in peas, Pisum sativum L. BioSci. 18, 505-506 (1968) Murfet, I.C., Marx, G.A.: Flowering in Pisum: comparison of the Geneva and Hobart systems of phenotypic classification. Pisum Newslett. 8, 46-47 (1976) Proebsting, W.M., Davies, P.J., Marx, G.A. : Photoperiodic control of apical senescence in a genetic line of peas. Plant Physiol. 58, 800-802 (1976) Zeevaart, J.A.D. : Physiology of flower formation. Ann. Rev. Plant. Physiol. 27, 321-348 (1976)
Received 20 December 1976; accepted 17 February 1977