 |
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)
.
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| 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.
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):
https://allthingsplants.com/pics/2011-10-04/eclayne/9fc853.jpg
http://zidbits.com/wp-content/uploads/2011/04/castor-seeds1.jpg
http://upload.wikimedia.org/wikipedia/commons/1/17/Ribwort_600.jpg
http://upload.wikimedia.org/wikipedia/commons/4/42/Starr_080609-7930_Malva_parviflora.jpg
http://www.florasilvestre.es/mediterranea/Gramineae/Bromus_diandrus2.jpg
http://rian.inta.gov.ar/atlasmalezas/atlasmalezascarga/sd/sop_Medicago%20polymorpha%20%20L.10-11-2011_221391352493.jpg
http://www.naturefg.com/images/a-plants/convolvulus-arvensis.jpg
http://bugwoodcloud.org/images/768x512/1459068.jpg
http://www.cooltropicalplants.com/image-files/canna-rhizome01.jpg
http://thebegavalley.org.au/uploads/tx_steverplantgallery/Canna_indica_03_canna%20or%20Indian%20shot.jpg
Black and white field bindweed images are from Frazier (1943).
We start off with our
wild and fun party stages where we stay up all night and sleep all day. Unfortunately, as we get older and have more
responsibilities, lying in bed until 2 the next day is extremely
unrealistic. Let’s be honest, those
fun-filled nights still exist but the mornings after have gotten increasingly
worse. Recently while in the midst of enjoying
a beer and writing, let’s say, an osmoregulation paper, the correlation of
hangovers and age was brought up. Now it
seems most people agree with this phenomenon except the new 21 year olds whose
response is, “HA, that’ll never be me.” So
this begs the question, do hangovers actually get worse as we age and if so,
why?
For people who
consume a moderate amount of alcohol, it is usually broken down in the liver by
alcohol dehydrogenase (ADH), which is found in the cytosol of cells. ADH converts alcohol (ethanol) to
acetaldehyde which is extremely toxic to our bodies (Lieber 2003). Acetaldehyde is then broken down further into
acetate by aldehyde dehydrogenase and an antioxidant, glutathione. If you are not following the one drink per
hour rule, you’re drinking faster than the acetaldehyde can be broken
down. Some of the acetaldehyde gets
released into the blood stream and then BAM! - you’ve got yourself a killer
hangover anxiously awaiting you tomorrow morning. 
Now remember those
responsibilities we all have? Well those
can also be to blame because people tend to stop drinking as much as they get
older. Furthermore, weight fluctuations
in either direction can have a pretty hefty impact as well. Young Chul Kim also stated that when body
weight increases, the blood alcohol level decreases because it is distributed
into a larger area, leading people to drink and build up more acetaldehyde
without realizing it. On the other hand,
people who have lost weight and still drink an amount they’re used to consuming
are subject to increased intoxication effects.
