| dc.description | Aluminum (Al) is one of the most abundant metals in the earth's crust
(comprising about 7%). Fortunately, in agicultural terms mostly of Al is primarily in
the form of insoluble aluminosilicates or oxides. Only under conditions of low pH, Al
is soluble as monomeric form (Al3+) the most toxic for plants, which can inhibit their
growth and development causing toxicity. This toxic form predominates in soils with
acid pH (≤5.5), as those of the Andisol Order. The first symptom of Al3+ injury in
plants has been identified in roots affecting the water and nutrient uptake efficiency.
However, in upper parts of plants such as stem, leaves and/or fruits, Al3+ harmful
constraint are still controversial. In this way, some reports indicate that Al3+ affects
physiological, biochemical and metabolic pathways, because triggers an excessive
generation of reactive oxygen species (ROS). These ROS induces oxidative stress in
cellular organelles and organic molecules, resulting even in cell death. It has been
reported that Al3+ stress affects photosynthesis as result of a partial inhibition of
photosynthetic electron transport rate (ETR) and closure of reaction centers in
photosystem II (PSII). By other hand, plants have evolved defensive mechanisms
against oxidative damage, by antioxidant systems both enzymatic and non enzymatic,
which act by scavenging of ROS. This performance is regulated by a network of
signal transduction molecules including receptors and messengers, among others. One
of the key components is calcium (Ca), which is essential for several physiological
and biochemical processes, playing an important structural and functional role,
related to the cell wall, biological membranes, cytoplasm, vacuoles, and other cell
organelles. Although Ca is less abundant than other nutrients in the plant cell is, is
essential for development and plant production.
Chapter two focuses on reviewing the interaction between Al3+ and Ca2+ and
its effects on the physiological and biochemical processes in crops growing on acid
soils, and then to determine the alternatives of calcareous amendments used in acid
soils providing the background support for followed chapters. This report indicates
that interaction between Ca2+ and Al3+ is probably the most important factor that
affects Ca2+ and other nutrients uptake and transport in plants grown in acids soil.
This relation can be described as Ca/Al molar ratio (Ca/Al) and it has been discussed how this interaction takes place, because both are inhibitors of another one depending
on the conditions under which it developed. Literature reports that the degree of Al
stress in plants is correlated with the Ca/Al rather than the Al concentration in the soil
solution. Thereby, an inadequated Ca/Al disrupts the role of Ca in cell function.
On the other hand, it has been recognized that Al toxicity is ameliorated by
basic cations in particularly Ca. The role of Ca adding (e.g. calcareous amendments)
on reduction of Al3+ concentration in acid soils has been studied, but differences
regarding the application manure, effectiveness, and the plant species or genotype
have been matter of controversy. Currently, the use of calcareous amendments have
been studied by several authors, who reported results demostrating the effectiveness
of amendments to reduce Al toxicity to a greater or lesser extent depending on the
source of Ca used, soil condition and the treated crop.
In South Central Chile highbush blueberry (Vaccinium corymbosum L.) is an
important cultivated plant. It is well known that this high priced small fruit crop is
increasingly cultivated due to its flavor and nutritional properties. This crop is well
adapted to acid soils as Andisol but is sensible to high level of Al toxicity which
decreases its productivity. Nevertheless, the effect of Ca addition to reduce Al3+ has
been little studied for this fruit crop in our Andisol conditions. Therefore, this thesis
aimed to study the relation between Ca and Al that allow a better nutrient balance and
Al detoxification reflected by photosynthetic and antioxidant performances in
highbush blueberry cultivars amended with calcium sulfate (CaSO4) or gypsum
grown in an Andisol of Southern Chile.
Therefore, before to study highbush blueberry plants subjected to high Al
saturation in an Andisol, it was necessary to evaluate CaSO4 treatments under
controlled conditions. Chapter three to ascertain the CaSO4 addition on Ca/Al and its
effect on chemical, physiological and biochemical features in cultivars of highbush
blueberry with contrasting tolerance to Al Legacy (Al-tolerant) and Bluegold (Alsensitive)
grown in Al-toxified Hoagland`s nutrient solution containing increased
CaSO4 concentrations (2.5, 5 and 10mM) and Al3+ (100 and 200µM), in an
experiment of 15 days. According with chemical determinations both cultivars
exhibited increased Ca content and Ca/Al (up to 180%) by adding CaSO4 in parallel to a reduction of Al (leaves and roots). The increase in Ca/Al molar ratio had a
significant effect on physiological parameters such as photochemical efficiency of
PSII [effective quantum yield (ΦPSII) and ETR]. For biochemical determinations, an
adequated foliar Ca/Al exhibited a reduction of oxidative stress, and an increase of
antioxidant activity of phenols. In summary, CaSO4 treatments at 5 and 10mM
showed positive effect on Ca/Al and evaluated parameters.
Afterwards, in chapter four were evaluated above studied cultivars under a
soil conditions. A second experiment in an Al-saturated (~70%) Andisol from
Southern Chile was carried out in plants of Legacy and Bluegold grown for 60 days.
Soils received amendment with CaSO4 at doses of 0 (control), 700, 1400 and 2800 mg
kg-1 soil. Determinations on chemical (nutrient content), physiological (growth,
hydric relations, and photochemical performance), and biochemical (lipid
peroxidation, antioxidant capacity, and antioxidant compounds) features were
performed. At the end of experiment improved nutrients content, Ca/Al and growth
were found, as well as photochemical parameters, carotenoid contents, and relative
water content (RWC) in leaves, especially in cultivar Legacy. However, CaSO4 did
not show clear effects on chlorophyll contents (Chl) and leaf water potential (Ψleaf).
On the other hand, CaSO4 addition decreased lipid peroxidation (LP) and total
flavonoids content (TFA) in both cultivars, whereas radical scavenging activity
(RSA), total phenols content (TPC), total anthocyanins content (TAN), and
antioxidant enzymes superoxide dismutase (SOD) and catalase (CAT) were
significantly increased. It is concluded that CaSO4 can be an effective amendment to
ameliorate Al3+ toxicity and improve physiological and biochemical performance in
highbush blueberry. Nonetheless, we suggest, that higher doses of this amendment
could be required to prevent harmful effect of Al3+ in Al-sensitive cultivars grown in
Andisols.
In summary, we propose that an adequated Ca/Al in tissues of highbush
blueberry according to genotypic features will allow a better aclimation to this
species against environmental stresses as Al toxicity. | |
