How may plants respond to the Global Climate Changes in the Neotropics?
Marcos S. Buckeridge
Department of Botany, Laboratory of Plant Physiological Ecology (LAFIECO),
Institute of Biosciences, University of São Paulo, msbuck@usp.br
Abstract
The responses of plants to the effects of the Global Climate Changes (GCC) have
been studied for many years. Such studies have been focused primarily on
temperate species and on crops of high economical importance, soybean
(Glycine max) being one of the most studied species to date. Although a
significant amount of the biomass is held in the tropical regions of the world,
native and crop species to these regions have been poorly studied. In 1999, my
laboratory initiated a research program to study some of these species. We
started by growing native tree species to the Atlantic Forest, all from the family
Leguminosae: Hymenaea courbaril (jatobá), Dalbergia nigra (jacarandá-dabahia), Schyzolobium parahyba (guapuruvú), Piptadenia gonoacantha (paujacaré) and Sesbania virgata (feijão-do-mato). This monophyletic approach was
adopted on the basis of the fact that these species belong to different stages of
ecological succession. The idea was that, being from the same family, their
adaptation strategies could be compared without having much interference of
the differences among families. At the same time, we initiated a program of
studies of some important crops for Brazil: sugarcane, common bean and
soybean. Later on, our studies were extended to comparisons among Hymenaea
species from savannah and forest and also to understand the physiological and
biochemical responses of two Amazonian species: Senna reticulata (mata-pasto)
and Euterpe oleracea (assai palm). Our experiments have been performed with
plants growing in elevated CO2 (twice the ambient concentration) and more
recently combinations of elevated CO2 and temperature and also CO2 and
flooding.
Our results led to the following general findings:
1) all species present responses to elevated CO2 in the form of increased
photosynthesis rate, growth and source-sink relationship;
2) the effect is stronger in fast-growing species, what implies that ecological
succession in the Atlantic Forest may be differentially affected by GCC;
3) the proportion o carbon in relation to nitrogen increases in all species
analysed for this parameter;
4) elevated temperature in combination with elevated CO2 (750ppm) had a
positive effect in Hymenaea courbaril (+3oC) and soybean (+5oC). In the
former species, the combination of elevation of the two factors led to a
modified temperature optima for photosynthesis, increasing its range in
about 5oC;
5) Sugarcane presented a strong response to elevated CO2, with modified
gene expression and proteomic patterns. Common bean and soybean
produced more fruits and seeds in elevated CO2 and the combination of it
with elevated temperature respectively;
6) Elevation of CO2 and temperature affected carbohydrate metabolism,
especially the non-structural sugars. In all species, a significant increase
in starch has been observed and in some of them cell walls were also
altered.
7) In all cases studied, stomatal indexes decreased with the elevation of
CO2, corroborating the observations made for leaves obtained from
herbarium exsiccates collected in the beginning of the 20th Century
Although our results refer to relatively small number species, the
comparison with the data available in the literature for photosynthesis and
growth, indicates that all species will respond positively to elevation of CO2 and
temperature up to the levels of 800ppm and 3-5oC respectively. Although little,
but significant changes in development have been observed (stomata is one
example) in all cases in elevated CO2, we observed that non-structural
carbohydrates changed their metabolism and plants accumulated more starch.
Carbon flux was altered and the plants increased their proportion of carbon in
relation to nitrogen, indicating that carbon allocation and partitioning were
affected in most species. However, in at least one species (H.courbaril), elevation
of temperature reverted this effect.
The partial conclusion is that tropical species will continue to respond to the
GCC, through responses to the elevation of CO2 and temperature, with increased
growth and biomass accumulation. The next step will be to test the effects of
another important factor related to GCC: water availability. This is expected to
complement the three most important factors related to the GCC for plants. Our
hypothesis is that water stress may revert the positive effects observed for the
two other factors. Preliminary results with flooding of plants of the Amazonian
species Senna reticulata, corroborates this hypothesis.
References
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