Amer. J. Bot. 74(9): 1458-1459. 1987.
INDUCED POLLEN TUBE DIRECTIONALITY
GABRIELLA BERGAMINI MULCAHY AND DAVID L. MULCAHY
Department of Botany, University of Massachusetts, Amherst, Massachusetts 01003

ABSTRACT

Pollen grains placed within longitudinal cuts in styles germinate and produce pollen tubes which grow equally well towards the stigma or the ovary. If there are simultaneous stigmatic pollinations, the growth of the intrastylar pollen tubes toward the stigma is significantly impeded. This observation indicates the presence of an influence which may or may not be related to pollen tube tropism, one which is inducible rather than constitutive.

Received for publication 24 December 1986; revision accepted 10 April 1987.
We thank Prof. Richard Durbin, University of Wisconsin, for supplying seeds of Nicotiana alata. This research was supported by NSF Grant BSR-8407472.

POLLEN TUBE TROPISM is presently experiencing renewed attention because, in wide crosses, potentially useful as sources of valuable genetic variation, pollen tubes often fail to complete the passage from stigma to ovules (Heslop-Harrison, 1986). An understanding of pollen tropism might allow such crossing barriers to be overcome.

Despite a long history of study, the phenomenon is only erratically demonstrable in vitro. Furthermore, its physiological basis, in vivo, is poorly understood. Chemical (Mascarenhas and Machlis, 1964; Rosen, 1968), electrical (Sinyukhin and Britikov, 1967), and mechanical tropisms (Tilton and Homer, 1980) have each been considered. Of these, the second and third have been demonstrated in vivo. Furthermore, pistils, ovules, ovaries, mixtures of amino acids, amines, and sugars have all been reported to attract, and sometimes to stimulate, growth of pollen tubes on agar surfaces (Mascarenhas, 1978). An important question about tropisms is whether or not long-distance signals operate throughout the style. In a recent review of this topic, Heslop-Harrison (1986) concludes that the evidence for a long distance chemotropism "leads to nothing but disquiet." In fact, strong evidence against a preexisting pollen tube tropism is provided by observations of pollen tube growth when pollen grains are placed in longitudinal slits within styles. Pollen tubes originating from such intrastylar pollinations are as likely to grow towards the stigma as towards the ovary. (See Buchholz, Doak, and Blakeslee [1932] and Iwanami [1959], who is quoted in Shivanna and Johri [1985]).

MATERIALS AND METHODS—We used two self-incompatible but intercompatible genotypes of Nicotiana alata, one, N. alata sensu strictu and N. alata cv. Sensation (W. A. Burpee Seed Co., No. B-41715). Intrastylar pollinations were made by cutting a longitudinal slit, 1.2cm long, in the central portion of styles. Compatible pollen was then inserted into this slit. Within 3-5 min before each intrastylar pollination, the stigmatic surface of each style was either left unpollinated or pollinated with compatible or incompatible (self) pollen. Styles were fixed, 17-20 hr after pollination, a time in which pollen tubes growing from the stigma had not yet intercepted those originating within the style.

Fixation was in 70% ethanol, and styles were cleared with 20 min in 8 N NaOH at 60 C, rinsed several times with tap water, and then with 0.025 M Tris-glycine buffer (pH 8.4). Cleared styles were stained with decolorized aniline blue (0.1% aniline blue in 0.1 M K3P04). For each style, the distances between the front of growing pollen tubes and the nearest pollen grain within the stylar slit were measured.

Data from three separate experiments gave similar results and the pooled data are presented in Table 1.

RESULTS AND DISCUSSION—The results indicate that, when the stigma has not been pollinated, there is no significant difference in length between the intrastylar pollen tubes growing towards the ovary and those growing towards the stigma. But when the stigma has been pollinated, with either compatible or incompatible pollen, the length of intrastylar tubes growing toward the ovary exhibit a significantly greater length than do those growing towards the stigma. More specifically, growth of intrastylar pollen tubes towards the stigma appears to be impeded by stigmatic pollination whereas growth of intrastylar pollen tubes toward the ovary is not affected.

TABLE 1. Direction and lengths of pollen tubes originating from pollen grains inserted within midregions of the style. Values are in mm, mean ± SE

Type of pollen placed
on the stigma
Average distance
toward the stigma
Average distance
toward the ovary
None 4.8 ± 0.4
N=14
4.8 ± 0.5
N=14
Compatible 2.6 ± 0.6
N=15
5.6 ± 0.7
N=15
Incompatible 2.4 ± 0.7
N=8
4.0 ± 1.1
N=8

Previous studies have, of course, documented that stigmatic pollinations induce changes in the pistil. The best studied of these is self-incompatibility (de Nettancourt, 1977). Furthermore, synergid degeneration in Gossypium, occurring well before pollen tubes enter the ovary, is known to be induced by pollination (Jensen and Fisher, 1968). Finally, it has recently been demonstrated that, in Nicotiana alata, pollination induces the ovary to produce a growth stimulus for pollen tubes (Mulcahy and Mulcahy, 1986). In the present study, we report that, with intrastylar pollinations, the growth of pollen tubes towards the stigma is somehow impeded by stigmatic pollinations. Does this restraint indicate the functioning of a long-distance chemotropism? The fact that intrastylar pollen tubes growing toward the ovary have not exhibited a significant response to stigmatic pollinations suggests that it may not. Nevertheless, this is the first report of any phenomenon which has a long distance influence on the direction of pollen tube growth in vivo. Its relationship to chemotropism or other phenomena within the pistil has yet to be determined.

LITERATURE CITED