42 | New Scientist | 18 July 2020
become more abundant – ironically largely as
a result of plants photosynthesising – it has
become a roadblock to better photosynthesis.
Over the past 100 million years, some
plants have found a workaround, evolving a
process known as C4 photosynthesis. This
splits the metabolic pathway involved in
normal photosynthesis between two parts of
their anatomy. First, they capture CO 2
molecules in spongy cells called mesophylls
beneath a leaf ’s waxy protective layer, where
they produce a four-carbon molecule. This
molecule is then transported through special
channels to cells clustered around leaf veins,MICHA
EL
DEFR
EITAS
CENT
RAL^ AMERICA/ALAMYTe q u i l a
sunset?
The blue agave may be fancied as
a plant to save the world because it
uses the more efficient CAM form of
photosynthesis (see main story). For
most of us today, however, it is the
raw ingredient for making the spirit
tequila. In the US, sales of this drink
have rocketed over the past few
years, spurred by the introduction of
smooth brands you sip rather than
knock back. In 2018 came a shot
of bad news: a tequila shortage.
The slow pace at which CAM
photosynthesising plants such as
blue agave generally grow means
farmers guess at future levels of
demand years in advance. If a lot of
agave matures at once and floods
the market, prices drop and farmers
then lack the incentive to grow
more. This can lead to cycles of
over and undersupply.
Even more tricky, farmers try to
harvest agave before they flower
because flowering drains the plant’s
sugar content. Getting this right also
involves guesswork because the
timing of flowering is unpredictable.
“Other species respond to
changes in temperature and light
intensity, whereas agave ignores
these stimuli for many years,”
says June Simpson at the National
Polytechnic Institute in Irapuato,
Mexico. Simpson and her team
are working to pin down the
triggers that initiate flowering by
sequencing the agave genome.
The hope is that this will help to
take some of the unpredictability
out of agave and tequila supplies.
Here’s to her success.where it is broken down to release CO 2 again.
Only here does rubisco come in, and with
higher concentrations of CO 2 present, it has
fewer chances to grab oxygen. C4 plants also
have enlarged chloroplasts, the parts of the
cell where photosynthesis is conducted,
which gives them an extra boost.
The benefits of these adaptations are
stark. Although only about 4 per cent of
plant species use C4 photosynthesis, they
are responsible for about 23 per cent of the
biomass produced on land. C4 crops include
major sources of food such as maize and
sugar cane, and pasture grasses that feed
many of the animals we consume.
The warming planet is adding fuel to the
idea that we could make more of these potent
photosynthesis machines , for example by
using genetic engineering to prod C3 plantsThe drought-hardy agave
is finding favour beyond its
traditional Mexican heartland