Transcript Heat load/water stress and leaf size in three mesquite woods in

Heat load/water stress and leaf
size in three mesquite woods
in Sonora, Mexico
Paul R. Earl
Facultad de Ciencias Biológicas
Universidad Autónoma de Nuevo León
San Nicolás de los Garza, NL, Mexico.
[email protected]
Is there enough rain ? No
Replacement of tall old trees by stunted
ones of more desertic foliage is ongoing at
Guaymas & Hermosillo, but not at Navajoa,
Sonora. The reason is that Guaymas &
Hermosillo have had their watertables
lowered during the past half century by civic
demand for water. In contrast, trees over 10
m in height at Navajoa live on Río Mayo.
Introduction
The enormous change in 60 years in all
Mexico is severe drops in the watertables
such as over 20 m. Country water, some from
deep wells, is canalized to the cities so that
some rivers are dry beds, except for hurricane
flooding. Many mesquite trees (Prosopis,
Mimosaceae) more than 10 m tall are being
replaced by ones of 2-4 m, all of unknown
ages. Mesquite replacement trees in arroyos
(streams) may not find enough groundwater to
show rapid growth. The tall trees seen in a
community were there long before the
watertable dropped.
Mexico with almost 2 million km2 surface
has vast northern areas of desertic
matorral (short open thorn woodlands)
with perhaps 250-600 mm rainfall/year
distributed irregularly with most in the 3
months of July-September. The purpose
of this work is to phenotypically describe
a presumed major mutant with tiny leaves
that arose in Guaymas and to its north in
response to heat load/water stress.
Gates (1968) noted that, “The smallness of the
leaves reduces the danger of leaf temperatures
rising unduely high. Furthermore, if water
becomes limiting, the stomata will close, the
internal diffusion resistance will rise, the
transpiration rate will drop and the leaf
temperature will rise.” See also Sinha et al. (1997a,
b, c, 1998) who wrote on the midday depression of
photosynthesis, including stomatal closure due to
the aridity of the air. Add that these mesquites fold
in their leaflets at night. At sunrise, leaves open
by 10-20 minutes. Mesquites close their leaves
whenever light intensity drops sufficiently as in a
daytime thunderstorm.
Drought occurred in 1940-60 killing many
mesquites across the north of Mexico. This
same period is when hydraulic works began
diverting rural water to civic use in national
antieconomic agriculture made oncompetitive
by lack of rain. Now we have trees older than
50 years still standing, and the younger ones
that have grown till today in much drier
conditions range 5-30 years old. This ongoing
replacement has strong genetic overtones
(Earl, 1998, 2000, 2003) and fairly presents
evolution in action.
The flora of much of the state of Sonora
was given in a classic superpaper “Rio
Mayo plants” by Gentry (1942). A beforeand-after study is now strongly suggested.
With Gentry’s predrought record, the
record over 60 years later should provide
acute insights into the Sonoran water
stress story as well as important facts
about the warming trend such as learning
about the plants that have been extincted.
Mesquites combat heat load by genetically
shortening the rachis to produce a smaller
leaf so that 6 mm length of leaflets (6 LL) is
found in the south and 36 LL in the north of
Mexico. See also Hilu et al. (1982) on
California mesquites. Velutine mesquites (not
seen in this study area) defined by dark green
overlapping leaflets about 9 LL have been
often misidentified, because some have
become dilute by repeatedly backcrossing P.
glandulosa var. torreyana Benson.
Water stress leads to dry air then stomatal
closure and less biomass that is seen as
shorter trees less than 5 m. Again, see Sinha
et al. (1997). When photosynthesis is shut
down for long midday periods, the end result
is a short tree of a few meters. See also
Bohnert et al. (1995). If a permanent water
table is available, mesquites can reach over
20 m, and height could be gained at even 1 m
per year. Read also Felker et al. (1981).
However in poor conditions, a tree of 80 years
or more might be 8 m tall. Also, lowering a tall
tree’s watertable will eventually kill it
categorically.
Over centuries or even decades by water
deficit, the more heat resistant mesquite races
introgress the more northern mesquites, and
plantation planners should know this. All
Mexican races are, originally, mixtures of
microphyllans and glandulosans, although
some have different taxonomic species
designations. See Table 1. Although leaf
designs (Earl, 2003) vary radically by locality,
all North, Central and South American
mesquites could be considered as the same
biological species as they cross. A taxonomic
species results from an arbitrary decision. All
Mexican & US mesquite trees cross freely.
In Argentina, Saidman et al. (1998a, b) using
molecular markers reported the accumulation
(fixation) of mutants in mesquite hybrids not
present in either parent population,
sometimes called private genes. This case
seems to be parallel to the origin of the race
Tiny in Guaymas, but no genetic markers have
been tried there. Race Tiny can have a private
gene, a gene absent from the genomes of
both parent population. One crucial point is
that a large group of races are being
examined that are simple hybrids. These
taxonomic species are all interfertile.
Interspecific hybridization is nonexistent and
common backcrossing is interracial.
The emergence of a race with especially tiny leaves
in Guaymas depended upon endemic dessication
without outside influences, conforming to the model
of private genes by Saidman et al. (1998a, b).
Centerfire dispersal, a term from Simpson (1944), will
extend the superior hybrid’s domain. Furthermore, in
the case of Tiny, the niche is expanding, drying out.
That is, desertification is extending to the north and
northeast. Having gained the effecive heterozygosity,
phenotypically expressed as a jump to a much shorter
rachis, Tiny is swamping its torreyana parents. Still,
we know nothing about the implied genes. In the past
and less than a century ago, the Navajoan type
(P. glandulosa var. torreyana Benson) dominated
Guaymas. Navajoa has length of rachis of 83 mm
(83 LR), whereas Tiny has 21-24 LR.
Outstanding papers in evolution that apply
here are Stebbins (1950), Fisher (1958),
Clausen & Heisey (1958), Clarke & Sheppard
(1959), Maynard Smith (1975), Gottlieb
(1984), Gould (1980), Gould & Lewontin
(1979 ), Hill (1982), Kimura (1983), Lande
(1983), Lande & Arnold (1983), Arnold &
Wade (1984), Endler (1986), Mackay (1989),
Abbott (1992), Orr & Coyne (1992),
Rieseberg (1997), Orr (1998) and Kim &
Rieseberg (1999).
There has been a sudden phenotypic
jump in race Tiny, yet no genetic
information in this study. Although the
logic seems convincing, the genetic
evidence awaits future studies. Not only
is Lande’s statement confirmed, but also
smaller leaves are gained gradually over
great distances like Guerrero to Texas as
well as by jumps. Two more papers that
require consideration are Shrimpton &
Robertson (1988) and Zeng et al. (1990).
Foliar morphometrics were studied. Fifty-six
trees were studied with 1501 leaves, averaging
about 27 leaves per sample. Hermosillo had 18
trees with 4 more trees at Mazatán and 3 at El
Novillo. Navajoa has 16 and Guaymas 15 trees.
The foliar variables, all in mm, were 1) leaflet
length, LL, 2) length of the longer rachis, LR, 3)
number of pairs of leaflets per rachis, PL, 4)
number of pairs of raches, PR, and 5) length of
the petiole, LP. PL & PR are counts, and only 1
PR occurred in Navajoan samples. For contrast,
tree T113 of Mina, NL is added to Table 1. A
guide to velutine mesquites as a single tree from
Altar, Sonora that is # 10 in Table 1. Discriminant
analysis was run in SPSS, Chicago, IL.
Fig. 1. The area
of Sonora that
was investigated.
The bar
measures 100
km. The distance
from Hermosillo
to Guaymas is
132 km, thus the
bar is 76 % of
this.
Fig. 2. Trees of Hermosillo, Navajoa and Guaymas, Sonora
ordered by PL. Navajoa is undisturbed. Race Tiny is group
6. Group 7 is parents backcrossed by Tiny, and Group 8 is
old torreyana parental trees.
GRP
1
2
3
4
5
6
7
8
9
10
LL
14
9
6
9
14
5
13
21
5
10
SD
2.56
1.63
1.78
1.14
2.77
0.92
3.33
3.19
1.18
1.54
LR
94
70
47
67
83
24
70
125
35
85
SD
16.87
14.71
12.42
11.80
18.38
4.89
12.00
30.78
6.60
22.58
PL
16
21
19
24
15
13
16
16
21
31
SD
2.42
3.03
3.70
4.14
1.96
3.25
1.85
2.68
2.83
7.00
PR
1.000
1.088
1.203
1.053
1.000
1.000
1.087
1.035
2.070
1.670
SD
0.000
0.284
0.403
0.226
0.000
0.000
0.283
0.185
0.300
0.810
LP
45
20
15
21
33
9
26
52
22
14
SD
17.4
9.02
6.80
8.43
16.5
3.05
11.1
19.1
7.40
3.76
N
130
228
231
75
440
150
161
86
248
100
Table 1. The foliar morphometrics of all 56 trees, grouped.
Group 1 is CIAD torreyana trees at Hermosillo, Group 2 is
Hermosillo intermediates, 3 is Mazatán & 4 Novillo. Navajoa is
Group 5. Tiny is 6, 7 intermediates & 8 torreyanas of Guaymas.
Single tree T113 of Mina, NL (9) is added for constrast as it
converges by having a short rachis. # 10 is a standard for the
velutine race from Altar, Sonora. SD is standard deviation. N is
number of leaves.
Table 2. Cases classified to their own group by
discriminant analysis. Cases has the number of leaves.
Actual Group Cases
Group
1
130
Group
2
228
Group
3
231
Group
4
75
Group
5
440
Group
6
150
Group
7
161
Group
8
86
1
58.5
5.7
0.0
0.0
26.4
0.0
0.6
2.3
2
3.1
53.9
13.9
25.3
0.9
0.0
3.1
0.0
3
0.8
13.6
62.3
12.0
0.5
4.0
9.3
0.0
4
0.0
19.7
11.3
61.3
0.0
0.0
0.0
0.0
5
17.7
0.9
0.0
0.0
42.3
0.0
34.8
4.7
6
0.0
0.9
12.1
0.0
0.0
96.0
5.6
0.0
7
4.6
5.3
0.4
1.3
23.4
0.0
46.6
3.5
8
5.4
0.0
0.0
0.0
6.6
0.0
0.0
89.5
Table 3. The 5 types of trees at Guaymas, including
Tiny as # 1. In mm. N refers to number of leaves. SD
is standard deviation. N is number of leaves.
Type 1
1
2
3
4
5
LL
4.7
16.6
15.6
21.2
11.0
SD
0.92
1.07
6.55
3.02
1.90
LR
24.3
83.8
95.0
126.
0
65.0
SD
4.89
2.19
4.12
28.3
4
9.22
PL
12.5
16.0
16.4
16.3
15.6
SD
3.25
0.60
2.67
2.72
2.03
PR
1.00
1.00
1.13
1.02
1.11
SD
0.00
0.00
0.34
0.13
0.31
LP
8.6
37.2
37.3
58.6
20.7
SD
3.05
9.03
24.0
3
27.5
2
7.67
N
150
53
55
55
84
Discussion
The creation of a tiny leaf is taken as a major
mutation that has been fixed. A uniquely
small leaf has been selected in harmony with
the newly changing drought environment at
Guaymas. Algarroban mesquites with very
small leaves, e. g., 6 LL, pop up in farflung
parts of Mexico, and race Tiny has the
smallest ones. Moderate-sized (17 LL) leaves
occur on the Oaxacan Pacific coast and 6 LL
leaves occur 100 km north after crossing the
chain of Sierra Madre del Sur, whereas 36 LL
occur in Coahuila, Mexico through New
Mexico, USA (unpublished).
The Sierra Madre del Sur stops the coastal rain
from entering the closeby Balsas river valley
which is extremely desertic. Despite a 6-fold
difference in leaflet length, all of these
mesquites cross, meaning that no fertility
reduction occurs.
Tall old trees living in an environment now dry
can do so since their roots have reached the
watertable. Their offspring cannot establish in
this site.
What is a hybrid? What is a species? What is a
macromutation? People can be talking at
crosspurposes, because they are depending on
different definitions of the same thing.
Mesquites seem to be a single species, although at
least 83 taxonomic species are well-recognized, and
some of these artificial species are claimed to be
supported as even higher taxa by random amplified
polymorphism DNA (RAPD) without sequencing
(Ramírez et al., 1999), which is impossible.
The change in leaf size from moderate to very small
that is isolated at Guaymas is considered as caused
by a major mutation that has been fixed by the
especially dry niche. The same gene or one much
like it is expected to be fixed in any near-identical
niche, and sometimes called a private gene since it
is absent phenotypically in the parent trees.
The scenario for the hot Balsas valley evolution
of microphyllan mesquites from glandulosans
follows the model of Tiny and its torreyana
parents. A new type arises from an old one by
the selection of mutants. Leaves are partially
adapted to the humidity of the niche in all cases.
As the Holocene warmed up starting 10,000
years ago, P. glandulosa all through Mexico
including the entire Pacific coast seems to have
mutated to P. microphylla when heat load
became the survival factor. P. glandulosa
occupied all Mexico and much of southwest US
following the retreat of Wisconsin glatiation.
What appears to occur in the mesquites is
heterozygosity by mutations, some of which
are favored by the niche and fixed. This
heterozygosity is induced in the original
occupants by new selection pressures.
Of course, crossing continues, but now new
private genes (mutations) are being backcrossed.
Tiny of Guaymas is swamping its parents,
because it is better adapted to the new much drier
niche. The rate of racial evolution seems great,
assumedly because heat load/water stress
selection has been strong over some 50
generations and is intensifying as the human
population increases.
Selection for <6 LL occurs at El Infernillo,
Michoacán, Victoria, Tamaulipas and Mina,
NL (unpublished) as well as at Guaymas.
These reoccurrences are the necessary and
sufficient proof of convergence. The jump to
about 24 LR from 83 LR might involve major
genes. Read Orr & Coyne (1992), Hoffmann
et al. (1995), Orr (1998) and Kim & Rieseberg
(1999).
Since convergent mesquites have been
found most distant from each other by
different people, many taxonomic species
have been described.
The anterior scenario is a plausible story. It then
can become the matter of sifting fact from
fiction. The story qualifies as best information to
date, even though mainly an impression. Also
see Gould & Lewontin (1979). Still without doubt,
swamping is the main mesquite activity, and this
differs from hybridization with some sterility.
Similar leaf types like small leaflet-packed, dark
green 4 PR populations in Sonora, Chihuahua,
New Mexico, Tamaulipas, northern Michoacán
demonstrate convergence. The single migratory
source of any character may be illusionary, and
a convergent character may have emerged by
mutation more than once.
Sonora has internal environmental
deterioration due to increased civic use of
water that is far more evident than the
effects of competitive interracial mesquite
migrations and racial introgression, even
though introgression is the main feature in
most other areas. The effects on all the flora
and fauna remain unstudied, although
Gentry’s (1942) superwork encourages such
an effort.
References