Tuesday, March 3, 2015

Sex in the Weeds (or a Lack Thereof)

R. communis, one of the world's most
toxic plants.
Imagine you are walking down the street, minding your own business, only to have your eyes land on a particular plant you have seen before. You turn your head towards this chlorophyll-producing beauty and notice its glossy palm-shaped leaves, upright growth habit, red flowers, and….whoa….whoa….whoa! Don’t touch that thing! That’s the castor bean plant (Ricinus communis), a common invasive species in the San Luis Obispo area, and the source of ricin. Haven’t you seen Breaking Bad? Just four castor bean seeds can kill a full-grown Homo sapien (1)
R. communis seed.

Phew, that was a close one. Ok, so you continue on your walk, smelling a flower here, picking a blade of grass there—really enjoying the fresh air—and you start to notice more than just the occasional castor bean plant. You see ripgut brome (Bromus diandrus), little mallow (Malva parviflora), California burclover (Medicago polymorpha), and narrow-leaf plantain (Plantago lanceolata), over and over and over again. “What the bajeezers,” you think, “these same plants are everywhere!” Well, that is because they are…….WEEDS.

B. diandrus
M. parviflora
M. polymorpha
P. lanceolata
Weed scientists (yep, that’s what they are called, chuckle chuckle) distinguish between weeds and invasive plants. Weeds are found in agricultural settings, while invasive plants exist in wild land areas. These often despised leafy organisms have been variously defined as: “a plant out of place”, “a plant causing economic or environmental harm”, or “a plant that grows so luxuriantly or plentifully that it chokes out all other plants that possess more valuable properties” (2). Noxious weeds are found in both agricultural and wild land settings. While weeds can be a nuisance, noxious weeds are so readily able to spread that they pose a high likelihood of causing economic harm and displacing natives (3).

Noxious weeds exhibit one or more reproductive strategies that make them so successful (3):

1.)  Immense seed production: not just a hundred or even a thousand seed, we are talking hundreds of thousands to over a million seed produced by a single plant.

2.)  Unpredictable germination rates due to dormancy (the inability to germinate under favorable conditions).

3.)  Asexual reproduction.

In California there are 251 state listed noxious weeds (4). One of the worst noxious weeds in temperate zones of the world is field bindweed (Convolvulus arvensis). It is a beautiful specimen in the morning glory (Convolvulaceae) family (see pic below) with a dark secret.

Field bindweed makes full use of an asexual reproduction strategy to consistently land on lists of the world’s worst weeds (2). It frequently invades bean, cereal, and potato fields (1), and can reduce crop yields by 60 percent (5). In 2003, crop losses due to field bindweed were estimated to be around 377 million dollars per year in the United States alone (5). Field bindweed also serves as an alternate host for several viruses, and a bacterium that affects grapevines known as Pierce’s disease (Xylella fastidiosa) (6). To top all of that, efforts to control field bindweed chemically with roundup (glyphosate) are becoming less effective due to multiple cellular mechanisms that lend resistance to the plant (7).

A grain crop infested with C. arvensis.
Of the many noxious weeds in California, what is it about field bindweed that makes it such an agricultural threat? There is a one-word answer to that: rhizomes. These are modified underground stems that store starches and proteins and allow the plant to reproduce asexually (8). These plants are able to reproduce without pollen from the plant’s male sexual organ combining with the ovule of a plant’s female sexual organ to form a zygote (immature seed). No plant sex needed!  

Rhizomes are full of buds that can sprout to grow new shoots and roots. Researchers found that field bindweed can have upwards of 60 buds per 2.5 grams of root tissue (9). A bud contains meristematic cells, undifferentiated embryonic cells, that divide rapidly so that a plant can grow (10). The rhizome’s buds and associated meristematic cells, as well as carbohydrate energy reserves, allow the same plant to grow back year after year without producing a single seed.

But wait a second, lots of plants have rhizomes. Ginger is a rhizome. Irises have rhizomes. Even the beautiful tropical plants known as cannas (e.g. Canna indica, see pic) have rhizomes. If all rhizomatous plants were as successful as field bindweed, we would be swimming in ginger and tropical flowers (actually, that sounds kind of pleasant).
C. indica, an ornamental that spreads via rhizomes.

A C. indica rhizome.

What differentiates field bindweed from the rhizome-sprouting plants previously mentioned is its extensive root system. Doctor John C. Frazier (1943) methodically excavated the root systems of field bindweed plants at various stages of growth. What he found was nothing short of remarkable. Field bindweed roots grow in a predictable manner (11):

1.)  A vertical taproot first penetrates deep into the soil.

2.)  As it grows downwards it produces multiple lateral roots. Some of these lateral roots (known as “permanent lateral roots”, see pic), usually in the top 4-6 inches of soil, grow more extensively than others.

3.)  Permanent laterals continue their horizontal growth for 10 to 30 inches away from the initial vertical root before dramatically bending downward themselves. While bindweed is able to produce shoots from any part of its permanent root system, at this bend is the point of heaviest shoot development.
An initial vertical root (P) produces multiple lateral roots. More developed laterals turn downward to form new vertical roots (S). This bend (Z) is the point of heaviest shoot development.  

An analogy might clarify what is going on here: it is as though the original shoot growth is an insect at the center of a spider web of roots. At each location where a horizontal to vertical root bend occurs another insect appears in this incessantly growing web (i.e. a new shoot emerges).

Frazier found that the majority of permanent lateral roots grow away from the original vertical root (the one at the center of the analogized web). In this way, the plant is able to essentially move itself via spreading radial growth. In fact, an established plant can grow 30 feet in radius in a single growing season!

Just half of a 29 week-old excavated C. arvensis root system with shoot growth.
How do the rhizomes mentioned earlier fit into this subterranean story? Buds form all along the main vertical and horizontal roots. Each below ground bud gives rise to a rhizome (11). Typically just a two-inch segment of root can produce an entire new plant (1). Taking into account the conceivable 30-foot per year growth radius, and that field bindweed has an astonishing potential rooting depth of over 20 feet, it is clear that a single plant has a whole lot of two-inch sections. Oh yeah, and another thing, field bindweed roots are extremely brittle (1). If a farmer cultivates an infested field in an attempt to destroy the plant, all the farmer is doing is spreading vegetative propagules everywhere.
5: Permanent lateral root sections forming buds. E: A bud becoming a rhizome. 6: Rhizomatous growth at the bend between a vertical and lateral root. 7: A rhizome producing shoots.
Let’s say a farmer lucks out and is able to control field bindweed with cultivation or an herbicide. Field bindweed has a back-up plan: water impermeable seeds that can remain viable for 50 years in the seed bank.

Given the ability of just this single noxious weed to resist multiple management strategies and spread asexually, humans are fighting a battle that they can not possibly win. We might as well get used to seeing field bindweed and other non-native weeds and invasive plants. They are simply masters of reproduction, even without sex.

Written by Eli Weissman

References (in order of appearance):

1.) DiTomaso, J.M. and H.A. Evelyn. Weeds of California and Other Western States. Oakland: University of California, 2007. Print.

2.) Radosevich, S.R., J.S. Holt, and C.M. Ghersa. Ecology of Weeds and Invasive Plants: Relationship to Agriculture and Natural Resource Management, Third Edition. New Hoboken: John Wiley & Sons, Inc., 2007. Print.

3.) Steinmaus, Scott. “Advanced Weed Science”. California Polytechnic State University, Clyde P. Fisher Science Building, January-March, 2014.

6.) Wistrom, C., and A.G. Purcell. 2005. The fate of Xylella fastidiosa in vineyard weeds and other alternate hosts in California. Plant Disease 89(9):994-999.

7.) Westwood, J.H., and S.C. Weller. 1997. Cellular mechanisms influence differential glyphosate sensitivity in field bindweed (Convolvulus arvensis) biotypes. Weed Science 45:2-11.

9.) Degennaro, F.P., and S.C. Weller. 1984. Growth and Reproductive Characteristics of Field Bindweed (Convolvulus arvensis) Biotypes. Weed Science 32(4):525-528.

11.) Frazier, J.C. 1943. Nature and Rate of Development of Root System of Convolvulus Arvensis. Botanical Gazette 104(3):417-425.

Image Sources (in order of appearance):


Black and white field bindweed images are from Frazier (1943).