Does the film formed by the Bordeaux mixture on the leaf surface of fig trees affects photochemical ...
Nativa, Sinop, v. 11, n. 2, p. 200-206, 2023.
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Internal CO2 concentration is a limiting factor for
photosynthesis and stomatal regulation, mainly in C3
photosynthetic metabolism plants such as fig trees. Once
inside the leaf, CO2 diffuses from the intercellular airspaces
to the chloroplast and is limited by resistances in gas and
liquid phases of cytosol, as well as diffusion barriers, causing
it to accumulate in intercellular spaces, mainly when stomatal
conductance and assimilation rate are affected by external
factors, such as air temperature and humidity (CHAVES et
al., 2011; TAIZ et al., 2017).
Silva et al. (2010) evaluated gas exchange in young leaves
(recently opened and near the branch meristem) of fig trees
and found that the lowest net CO2 assimilation rates,
transpiration, and stomatal conductance resulted in high CO2
concentration in the substomatal chamber and low
photosynthetic carbon assimilation. However, the results
found in the present study showed that apical leaves had the
lowest internal CO2 concentrations and the highest means of
net CO2 assimilation, i.e., carbon dioxide was moving
towards the carboxylation stage of the photosynthetic
process, followed by middle and basal leaves.
Transpiration rate (E) refers to water loss through
stomatal pores, combined with the guard cells, at the time of
opening. González-Rodríguez and Peters (2010) evaluated
leaf sprouting in pruned and unpruned fig trees in Spain and
found transpiration rates ranging from 3 to 7.6 mmol m-2 s-1
over 200 days; the highest rates were found during the
summer and were similar to the range of 4.6 to 6.1 mmol m-
2 s-1 found in the present study (Table 2). Additionally, Silva
et al. (2010), assessed gas exchange in leaves of fig trees of
the cultivar the Roxo de Valinhos in Botucatu, SP, Brazil, and
found transpiration rates ranging from 2.19 to 4.78 mmol m-
2 s-1, indicating that the transpiration rates found in the
present study are consistent with those previously reported
for the species.
The means of stomatal conductance (gs) varied from
0.280 to 0.584 mol m-2 s-1, which are similar to results
reported in the literature. Ammar et al. (2020) evaluated
physiological dynamics in 5-year-old fig trees over 259 days
in Tunisia and found maximum means of 0.370 and 0.435
mol m-2 s-1 during late spring when temperatures ranged from
28 to 31 °C; however, the lowest means (below 0.100 mol m-
2 s-1) were found during summer when temperatures were
higher than 35 °C.
Can et al. (2008) and Ammar et al. (2020) reported that
the maximum gs was found when daytime temperatures were
around 30 to 32 °C, as at these temperatures, water viscosity
decreases and mesophyll conductance increases, increasing
guard cell turgor pressure and stomatal opening. Conversely,
gas exchange rates were significantly lower during the hottest
periods of the year compared to those found in early summer.
The net assimilation rate showed localized variations in
leaf positions on the branch regarding the evaluation days,
specifically in apical and middle leaves in the treatment
without Bordeaux mixture application at 0 and 3 DAA.
Regarding the variation among leaf positions within each
treatment, a significant difference was found, with the highest
rates in apical leaves and the lowest rates in basal leaves were
inversely proportional to the results found for internal
carbon concentration (Ci).
The results obtained differed from those reported by
Silva et al. (2010), who found the lowest rates of net CO2,
transpiration, and stomatal conductance in younger leaves,
resulting in higher CO2 accumulation in the substomatal
chamber. Only stomatal conductance and transpiration rate
remained within the expected range, with the lowest rates
found in apical leaves in the present study.
The lower CO2 concentration found in apical leaves,
combined with the higher assimilation rate, may be related to
environmental conditions, mainly intense light and radiation,
as leaf temperatures did not vary significantly and the lowest
transpiration rate and stomatal conductance were found in
apical leaves, denoting that the leaves maintained the CO2
assimilation even with partially open stomata.
The mean assimilation rates found (6.345 to 13.018 μmol
m-2 s-1) are consistent with results reported in the literature
for fig tree crops. Costa et al. (2020) found rates ranging from
6.06 to 10.49 μmol m-2 s-1 when studying photosynthetic
dynamics in different numbers of branches of fig trees
(cultivar Roxo de Valinhos) in Erechim, RS, Brazil.
According to Ferraz et al. (2020), the net assimilation rate
represents the photosynthesis performed by the plant.
Therefore, superior performance in CO2 assimilation results
in increased quantum efficiency and better utilization and
conversion into light energy, leading to higher allocation of
biomass and the formation of better plant architecture. The
authors compared commercial accessions of fig trees and
found means ranging from 5.74 to 12.94 μmol m-2 s-1 and a
mean of approximately 12.59 μmol m-2 s-1 for the cultivar
Roxo de Valinhos, which is similar to that found in the
present study.
Regarding the evaluations of chlorophyll a fluorescence,
the maximum photochemical efficiency of PSII (Fv/Fm)
varied over time in the treatments, mainly with decreases in
Fv/Fm on isolate evaluation days in the apical and middle
leaves, without significant variations. These results were
expected since characteristics such as cultivar, time, and leaf
position on the branch do not affect Fv/Fm, but significant
variations are due to environmental factors such as high
radiation incidence (PALLIOTTI, et al. 2009).
The correlation between maximum fluorescence (Fm)
and variable fluorescence (Fv) enables a better understanding
of qualitative and quantitative analyses of light energy
absorption and use by the photosystem II (PS II); it is an
indicator of use efficiency of photochemical radiation and,
consequently, carbon assimilation by plants, contributing to
the diagnosis of integrity of the photosynthetic apparatus
after exposure to environmental adversities, especially when
combined with gas exchange analyses (TATAGIBA et al.,
2014; FREIRE et al., 2014).
The means found for Fv/Fm (0.689 to 0.788) were
consistent with other Fv/Fm results reported for arboreal
species. Ammar et al. (2020) found means ranging from 0.493
to 0.741 in an experiment in Tunisia, with the lowest means
found in summer and early autumn when temperatures and
light incidence were higher. Similarly, Gomes et al. (2008)
found Fv/Fm means close to 0.741 in a study characterizing
the photosynthetic performance of fig trees in the state of
Espirito Santo, Brazil; the authors noted that other fruit tree
species grown in the same area, such as coconut and mango
trees, had Fv/Fm means of 0.74 and 0.76, respectively, which
are similar to the results found in the present study.
The triple interaction was significant for the electron
transport rate (ETR) and effective quantum yield of PSII
(ΦPSII) at 1% and 5% probability levels, respectively. It was
significant for ΦPSII due to apical leaves, as the treatment
