Stress and Crossing

[Karl_K]

Seed parents that are suffering from environmental stress may be more willing to accept foreign pollen than they would when thriving. Also, pollen from non-stressed plants tends to be more successful in siring offspring than pollen from the same plant (or type of plant) when stressed.

Science 248(4963): 1631-1633 (29 June 1990)

Influence of Environmental Quality on Pollen Competitive Ability in Wild Radish

Helen J. Young and Maureen L. Stanton

Pollen of Raphanus raphanistrum produced under low nutrient conditions sired fewer seeds than pollen produced under better conditions when the two types were applied on a stigma together. No difference was seen in single-donor crosses. Male mating success can be strongly influenced by the environmental conditions of pollen-bearing plants, a factor overlooked in studies of plant reproductive biology and in standard quantitative genetic crossing designs, where effects of male parent are equated with heritable genetic variation.

American Journal of Botany. 2001; 88: 242-257.

Mechanisms of differential pollen donor performance in wild radish, Raphanus sativus (Brassicaceae)

Diane L. Marshall and Pamela K. Diggle

The process of pollen tube growth and fertilization differed substantially among maternal watering treatments, with many early events occurring more quickly in stressed plants. Seed paternity, however, was somewhat more even among pollen donors used on stressed maternal plants, suggesting that when maternal tissue is more competent, mating is slowed and is more selective.

American Journal of Botany. 2007 Mar; 94(3): 409-418.

Do differences in plant and flower age change mating patterns and alter offspring fitness in Raphanus sativus (Brassicaceae)?

Marshall, et al.

Abstract: When more pollen is present on stigmas than needed to fertilize all ovules, selection among pollen grains may occur due to effects of both pollen donors and maternal plants. We asked whether increasing plant age and flower age, two changes in maternal condition, altered the pattern of seed paternity after mixed pollination. We also asked whether changes in seed paternity affected offspring success in an experimental garden. While flower age did not affect seed paternity, there was a dramatic shift in pollen donor performance as plants aged. These differences were seen in the offspring as well, where the offspring of one pollen donor, which sired more seeds on young plants, flowered earlier in the season, and the offspring of another pollen donor, which sired more seeds on old plants, flowered later in the season. Thus, change in maternal condition resulted in altered seed paternity, perhaps because the environment for pollen tube growth was different. The pattern of seed paternity and offspring performance suggests that pollen donors may show temporal specialization.

The first two items, taken together, indicate that a stressed pollen parent will be more successful on a stressed seed parent than on one that is not stressed. A non-stressed pollen parent has a definite advantage on a non-stressed seed parent (which is more selective), but has a smaller advantage on a stressed seed parent (which is less selective).

The third indicates that the same seed- and pollen-parents may produce different types of offspring when crossed early or late in their lives. I don’t know whether this is strictly age-related or seasonal, since age and season would be correlated in short-lived plants.

It is odd to think that a bad year for a plant, due to unusual environmental stress, may be a good year for crossing it. This might explain Michurin’s report that his attempts to cross Sorbus aucuparia x melanocarpa for 7-8 years gave only one or two seedlings from around 1000 seeds. Finally, with no change in method, he got mass germination. Michurin didn’t mention any difference in the weather, but I’m guessing that unfavorable years (for the trees, but good for crossing) were less common than favorable (for the trees, but bad for crossing.

Deliberate stress has also proven effective. Christy Hensler successfully crossed a Japanese iris (Iris ensata) with a Siberian (I. sibirica), and Iris cristata with a modern dwarf bearded. She wrote, “All of the irises I’d used as pod parents in the first two types of crosses were under enormous stress and two of the three plants had died shortly after ripening their seeds.”

Some of her later generation plants from the first cross have the larger flowers of the Japanese irises on drought resistant plants.

Karl

[philip_la]

Interesting…

I’ve never considered such from a crossing perspective.

I have, not infrequently, induced blooming in, i.e. bougainvillea by stressing them (They certainly follow the “Life is tenuous: Procreate” philosophy) but stressing a plant when fertilizing, while it logically follows that philosophy, seems counter-intuitive on a gut level. I’ve seen stressed plants abort their fruit to save themselves, so I guess I projected the wrong end of the equation onto the fertilization stage.

I’ve never bothered trying to work with unhappy plants.

The second summary makes it sound less like a survival tactic though, and more like the exhausted momma-plant just doesn’t have the energy to be choosy.

[Karl_K]

Philip,

You may be right about the weary momma. And Michurin found that plants flowering for the first time tend to be more tolerant of foreign pollen (Foolish virgins?). First flowers of a seedling may be somewhat defective. This defectiveness may extend to incompatibility. This seems to fit the model, and might be extended to the first flowers of the season, which may not be quite as well-developed as later ones. For once-bloomers, the last blooms of the season might be a little “off” in their perfection, and a bit less scrupulous in rejection foreign pollen.

Michurin also noted that once the virgin specimen had been “tricked” into accepting foreign pollen, it continued to accept that pollen in later years. This should be tested with suitable controls. It’s interesting, though, and is analogous to what happens in some fishes. Breeders who have successfully crossed guppies (for instance) with other species have reported better success with virgin females. I guess the inexperienced females just don’t know any better.

Then there’s Stout (1922): “In these [Brassica] species flower abortion occurs as a transitional stage between a period of vigorous vegetative vigor and a period of flower formation and seed production. The plants which exhibit abortion are not able to pass at once into complete reproductive activity in producing potent flowers. The amount of abortion is greatest in the varieties of B. pekinensis in which vegetative vigor is most marked and in which excessive vegetative growth can readily be induced by good cultural conditions and which have been selected and bred for this feature. Flower abortion occurs in numerous plants of these sorts that are grown in pots, as it does in many plants of the loose-leaved kinds, but it apparently tends to be less marked in these.”

Environmental stress would limit vegetative vigor, and hasten the transition from non-flowering to perfect flowering.

Stout went on to describe, "Sterility in Brassica pekinensis and B. chinensis

Three distinct and quite different types of sterility are in evidence during the period of bloom in both these species.

I. One type is to be classed in general with impotence, but here two very distinct types of impotence may be observed. These may be described as (1) flower abortion of the first flowers, and (2) arrested development of the last flowers that start to form.

II. In some plants of both species, axial proliferations develop from the pistils of many flowers, and the pistils of such flowers are functionless in respect to fruit production.

III. Among the flowers that open fully and are capable of functioning in certain relations, various grades of incompatibilities are in evidence, and self-compatibility whenever present is most strong during the period of mid-bloom."

It will be interesting to learn how much of this applies to roses. Are the first blooms of those excessively double varieties a bit less double? I never could find pollen or stigmas on ‘Rosette Delizy’, but I wasn’t paying attention to first blooms when I was interested in it. And as I recall, double-flowered tuberous begonias become less double, and more fertile, late in the season.

Maybe the axial proliferations that Stout observed could be developed into double flowers by selection of plants that produce them for the longest period, and show the greatest development. I’m not saying the world needs a double-flowered Chinese cabbage, but I wouldn’t mind a double flowered Sweet Rocket (Hesperis matronalis) if one could be produced in that way.

Karl

[philip_la]

It’s funny how we anthropomorphize plants when assigning cause to a correlation. Whether due to “exhaustion” or “desperation” the effect of the stressed plant’s receptiveness is the same – potential survival of the genetic material.

The extent to which generalities about plants in the mustard family apply to roses is something I would like to know more about. I would think studies on orchard fruit might exist and would have a more likely application, as mentioned in another thread.

[Karl_K]

Philip,

Some generalities are useful, others not so much. I agree that data from more closely allied species would be useful. Here’s one that is not precisely on point:

African Journal of Agricultural Research Vol. 5(25), pp. 3581-3589, December 2010 Special Review

Unfruitfulness in fruit crops: Causes and remedies

Imtiyaz Ahmad Wani, M. Y. Bhat, Abid Ali Lone and Mohd. Younis Mir

Fruit setting of flowers in different positions:

Fruit borne on terminal growth have more competition in many fruit crops and its maturity is set under normal nutritional conditions, but the percentage of the set is small. This positional competition takes place between fruits and branches as well as between different fruits influencing fruitfulness. The apical (king flower) flower in the bud of apple (Malus doestica) develops first, followed by the lateral flower which develop in sequence, beginning at the base of the spur. Some fruits like plum and cherry, bear on both shoots and spurs and it is to be expected that slightly different nutritive conditions are obtained in the different tissues and a distinctly heavier June drop occurs in shoot borne fruits (Gardner, 1952). In apple, old spurs are less likely to initiate flowers than the young ones (Jackson, 2003).

Another may have some useful information, but only the first page is shown.

Annual Review of Ecology and Systematics 12: 253-279 (November 1981)

Flower and Fruit Abortion: Proximate Causes and Ultimate Functions

A G Stephenson

The next item is more to the point:

HORTSCIENCE 36(2):295–297. 2001.

Reassessment of Seed Influence on Return Bloom and Fruit Growth in ‘Bartlett’ Pear

Steven A. Weinbaum, Theodore M. DeJong, and John Maki

Biennial bearing has been linked to the inhibition of flower induction by seeded fruits in ‘Spencer Seedless’ apple (Chan and Cain, 1967; Dennis and Neilsen, 1999). In ‘Bartlett’ pear, however, flower number per tree was not a primary crop-limiting determinant in this experiment. Fruit yield was ?47 kg per tree irrespective of the level of flowering (Table 2). Thus, as long as some minimal number of flowers was produced, additional flower production did not increase cropping. Previously published work on oaks (Quercus alba L., Sharp and Sprague, 1967) also suggests that crop is limited ultimately by the lack of carbohydrate resources (Lloyd, 1980), which may limit flower initiation (Jackson and Sweet, 1972; Monselise and Goldschmidt, 1982), promote abortion of flowers or immature fruit (Stephenson, 1981), and restrict fruit growth (Wright, 1989).

A plant that is growing vigorously may be using up too much of its carbohydrate resources to fruit successfully. This agrees with Stout’s results with the Brassica spp. That Stout’s plants were also more self-compatible during the mid-season bloom supports evidence from other genera that a seed-parent may selectively allocate resources, providing more nourishment to favored fruit while aborting others.

For example,

Jour. Roy. Hort. Soc. London (1847)

On Hybridization amongst Vegetables

William Herbert

The very bulb [Hippeastrum organense] lately dug up in Brazil was used. It produced two two-flowered scapes; the first pair of flowers were touched with their own dust, and the germens swelled; of the second scape, which was several days later, one flower was touched with its own, and the other by the dust of a fine triple cross from H. bulbulosum, var. pulverulentum by reginae-vittatum, otherwise called Johnsoni. The ovaries of the three flowers impregnated by the natural pollen for a few days after the decay of the last flowers appeared to have the advantage, and the fourth continued smaller, and seemed likely to fail, when it unexpectedly made a rapid advance, and immediately the three others ceased to grow, and after a few days perished entirely; while the progress of the pod impregnated by the mule made vigorous and rapid progress to maturity, and bore good seed, which vegetated freely. The impregnation by the cross-bred pollen was therefore slower in taking effect, but had the same decided superiority over the pollen of the natural species as over that of any other cross-bred variety. The anthers had been taken out of the flowers before their expansion.

American Journal of Botany. (Feb., 1999) 86: 261-268.

The effects of pollen load size and donor diversity on pollen performance, selective abortion, and progeny vigor in Mirabilis jalapa (Nyctaginaceae).

Richard A. Niesenbaum

The influence of pollen competitive environment on pollen performance (pollen germination, stigmatic penetration, and pollen tube growth rate), the maturation or abortion of initiated fruit, seed size, and seedling vigor was explored by manipulating the size and diversity of stigmatic pollen loads on Mirabilis jalapa. All aspects of pollen performance significantly increased with the number of pollen grains on a stigma or pollen tubes in a style, but was not influenced by the diversity of pollen donors. Plants tended to mature single-ovulate fruits that came from flowers where pollen load size and diversity were greatest and aborted those where these were lowest. No plants from seeds resulting from pollinations with a single pollen grain survived, but other fitness measures were mostly determined by maternal plant. The data suggest that pollen performance is influenced by pollen competitive environment, and both the genetic diversity of the pollen and number of competing pollen tubes are important determinants of seed/fruit abortion.

The option of directing carbohydrate resources into either vegetative or reproductive structures appears to be a useful generality that applies to a wide range of plants. The more limited the carbohydrate resources, the greater the contrast in treatment between favored and unfavored fruit. And when a plant is suffering environmental stress, restricted vegetative growth allows relatively greater allocation to reproduction. This model may even explain why the stressed radish seed-parents seemed less “picky” about the pollen it received.

Now I wonder whether it might be just as useful to provide an auxiliary source of carbohydrates. In roses and other woody plants, ringing the branch can restrict the downward flow of carbohydrates, leaving more for use in reproduction. In other plants, such as irises and daylilies, an “IV Drip” filled with a suitable sugar solution might do the trick.

Karl

[Karl_K]

After rereading Stout’s paper, a passage I had previously neglected seemed more relevant.

Botanical Gazette 73:110-132 (1922)

CYCLIC MANIFESTATION OF STERILITY IN BRASSICA PEKINENSIS AND B. CHINENSIS

A. B. Stout

“These three types of sterility develop and operate in these two species and in their hybrids in intimate correlation with the cyclic alternation of vegetative and reproductive vigor. Flower abortion occurs normally as a transitional stage between the formation of green leaves and the production of functional sporophylls. Those plants which exhibit flower abortion are not able to pass directly from producing green leaves or leaves with branches at the nodes to the production of flowers, and flower abortion occurs as a transitional stage. The abortion of flowers appears in the phase where vegetative vigor is waning, but before reproductive vigor is fully in evidence. There is also a marked agreement among the various branches of a plant as to the grade of development reached at any one date of blooming (figs. 1-3), which indicates a definite relation between the condition causing flower abortion (and also normal flower formation) and a condition of the plant as a whole. These phenomena, therefore, have many aspects characteristic of physiological correlation.”

This reminded me of another report:

The Indian Gardener, 1: 266-267 (June 9, 1885)

ROSE EDOUARD

By an Expert

“Soon after joining my appointment in Northern India, in October 1864, my predecessor (an amateur) in honorary charge pointed out this Rose, of which there were several plants in various parts of the garden, and enquired if I knew its name, stating that it was known only as the Bombay Rose in that part of India; and asked also if I could explain the cause of its flower-buds dropping off without opening. My reply was that I recognized it as the Rose “Edouard,” and the dropping its buds to be characteristic of it in England. The cause generally assigned was the climate being too cold and the soil too wet for it there; but this, I remarked, could not be the reason here, and that I was now inclined to believe that it was owing to the plant being too free in growth, that is by producing more flower-buds than it could carry out or bring to perfection, a freak of nature not altogether unusual in other instances, and amongst fruits especially. He expressed his pleasure in hearing its proper name, and requested me to take some of the plants in hand and see if I could ascertain the cause and rectify it. I did so as follows. Some of the healthiest and best plants were taken up, divided, (for there were several stems or ground shoots to each plant forming a clump), and pruned; all suckers and growing buds below the surface of the ground were cut out, so that no more suckers could form, also the buds above the ground for three or four inches up the stem were removed, to secure clean single stemmed plants, by which the plants should be handled when potting operations are being performed. It also becomes the only medium or passage for the ascending sap or life blood of the plant, from the roots to the head, and thus secures an equality of distribution to all the branches and parts alike, and that uniformity of growth and flowering so essential, so that a plant of any kind may be induced to give its flush of bloom or crop uniformly and simultaneously. If a plant is allowed to have more stems than one, or a multiplicity of suckers be allowed to predominate, they soon throw out and make roots and set up on their own account, and not being detached will rob rather than assist the parent plant, for the roots of suckers do not contribute thereto, but, on the contrary, detract therefrom, and primarily each supplies itself, hence the wretched condition the plant is soon reduced to, when unassisted by art.”

He went on to write:

“I may here state that liquid manure is applied as a rule after the flower buds appear and have been thinned, the blooming principle is benefited thereby and larger flowers are developed.”

By forcing ‘Rose Edouard’ to bloom in flushes, rather than continuously, he improved the size and quality of the blooms. No doubt the same treatment would improve the uniformity of the seeds/hips. Once the plant has been pushed into reproductive mode, fertilizer is directed to the developing flowers and fruit, rather than towards vegetative growth.

[Neil]

I have been waiting, and now would be a good time to fertilize evidentaly. Thanks, Neil