Relações hídricas e fotossíntese em espécies lenhosas de um cerrado stricto sensu em São Carlos, SP.
Abstract
Values of pre dawn, initial, and morning (between 10:00-11:30 h) leaf water potential
(Ψaa, Ψi, and Ψ, respectively), and morning leaf gas exchange were obtained during wet
and dry seasons in woody species of cerrado stricto sensu vegetation located at
Southeast of Brazil, in São Carlos municipality (21°58'S-47°52'W, 850 m a.s.l.). At
first eight species were studied measuring the leaf water potential during daily courses
in dry and wet seasons. Two daily courses were carried out per season. In wet season
the meteorological conditions were similar between the daily courses, when the
maximum air temperature, vapor pressure deficit (VPD) and the total hours of direct
solar radiation values were equivalent. Notwithstanding, the studied species showed
different mean values of minimum (Ψmin) and integrated (IΨ) leaf water potential
between the daily courses. In dry season the values of maximum temperature, VPD and
the total hours of direct solar radiation values were much more different between the
daily courses but there were not significant differences between Ψmin and IΨ mean
values. The conditions at dry season (specially a moderate water stress at the rizosphere)
could induce one distinct reaction since the early morning, promoting stomatal closing
and avoiding great alterations in leaf water status. In chapter 1 were compared the
average values of Ψmin among cerrado stricto sensu vegetation, Northern Australian
savanna and Mediterranean Climate Vegetation. Because the rainfall is greater and more
constant during the year in cerrado, and by reason of their woody species are able to
cope the leaf water status during dry season, the values of Ψmin in cerrado were lower
than Australian savanna (Mediterranean vegetation showed the lowest Ψmin). The
results in chapter 2 confirmed the initial explanations in chapter 1 about the differences
in Ψmin and IΨ in wet and dry seasons. In chapter 2 there are presented results of leaf
gas exchange in 23 woody species: net photosynthesis (A), stomatal conductance (gs),
transpiration (E), and sub stomatal CO2 concentration (Ci). All species studied
(including those in chapter 1) showed reduced values of gs in dry season (78% on
average basis) confirming that during the winter drought occurs sharp reduction of gs.
This reduction was able to decrease the E values (-33%) due to the stomatal narrowing
(lower values of gs under high values of VPD). It was probably responsible to increase
the resistance for CO2 incoming to sub stomatal chamber, lowering the CO2
concentration available to photosynthesis. It could explain, at least in part, the lower
mean A values under drought condition. Because A and E values decreased
proportionally (around 32%), the water use efficiency mean values were kept equivalent
during dry and wet seasons. In chapter 2 is proposed a diagram where are pointed out
the principal causal events and the probable physiological responses for cerrado woody
species under dry season.