Water relations and hydraulic architecture in Cerrado trees: adjustments to seasonal changes in water availability and evaporative demand

Abstract

We determined adjustments in physiology and morphology that allow Neotropical savanna trees from central Brazil (Cerrado) to avoid water deficits and to maintain a nearly constant internal water balance despite seasonal changes in precipitation and air saturation deficit (D). Precipitation in the study area is highly seasonal with about five nearly rainless months during which D is two fold higher compared to wet season values. As a consequence of the seasonal fluctuations in rainfall and D, soil water potential changes substantially in the upper 100 cm of soil, but remains nearly constant below 2 m depth. Hydraulic architecture and water relations traits of Cerrado trees adjusted during the dry season to prevent increasing water deficits and insure homeostasis in minimum leaf water potential Ψ L and in total daily water loss per plant (isohydry). The isohydric behavior of Cerrado trees was the result of a decrease in total leaf surface area per tree, a strong stomatal control of evaporative losses, an increase in leaf-specific hydraulic conductivity and leaf hydraulic conductance and an increase in the amount of water withdrawn from internal stem storage, during the dry season. Water transport efficiency increased in the same proportion in leaves and terminal stems during the dry season. All of these seasonal adjustments were important for maintaining Ψ L above critical thresholds, which reduces the rate of embolism formation in stems and help to avoid turgor loss in leaf tissues still during the dry season. These adjustments allow the stems of most Cerrado woody species to operate far from the point of catastrophic dysfunction for cavitation, while leaves operate close to it and experience embolism on a daily basis, especially during the dry season.. Relações hídricas e arquitetura hidráulica em árvores do cerrado: adequação às variações sazonais de disponibilidade hídrica e de demanda evaporativa: O objetivo deste estudo foi determinar os ajustamentos na morfologia e fisiologia que permitem árvores das savanas neotropicais do Brasil Central (Cerrado) de evitar déficits hídricos e de manter um balanço hídrico interno praticamente constante apesar das variações sazonais da precipitação e no déficit de saturação do ar (D). A precipitação na área de estudo é fortemente sazonal, com cerca de cinco meses praticamente sem chuva durante os quais D é duas vezes maior aos valores medidos na época chuvosa. Como conseqüência da flutuação sazonal das chuvas e de D, o potencial hídrico do solo muda substancialmente, nos primeiros 100 cm do solo, mas permanece quase constante abaixo de 2 m de profundidade. A arquitetura hidráulica e os parâmetros relacionados a relações hídricas das árvores do Cerrado se ajustaram durante a estação seca para evitar o déficit hídrico crescente e assegurar a homeostase nos valores mínimos de potencial hídrico foliar Ψ L e na perda total diária de água pela planta (iso-hidria). O comportamento iso-hídrico das árvores do Cerrado foi o resultado de uma diminuição da superfície foliar total por á...

Root to branch wood anatomical variation and its influence on hydraulic conductivity in five Brazilian Cerrado species

“…In contrast, bottlenecks could also increase flow resistance and improve conductance efficiency above the stem base. According to Bucci et al (2008), Cerrado woody plants have deep roots that can get water even in the driest periods, which may account for the lack of significant seasonal variations in the transpiration of Cerrado trees. Moreover, these plants respond to a rigid stomatal control.…”

The length of the dry season may be associated with leaf scleromorphism in cerrado plants

“…Forest species are less sclerophyllous (have high SLA) than savanna species (Hoffmann et al 2005) since the main limitation in forests is the capture of sunlight (Givnish 1988, Habermann andBressan 2011), while in savannas the main challenge is surviving the dry season (Prado et al 2004, Bucci et al 2008. Less sclerophyllous species invest more in photosynthetic tissues, have higher ratios of mesophyll to epidermis, and mesophyll to leaf thicknesses (Delgado et al 2013, Somavilla et al 2014; moreover, these species accumulate more N, P, Ca and Mg in their leaves on a leaf mass basis (Hoffmann et al 2005).…”

Water relations and organic solutes production in four umbu tree (Spondias tuberosa) genotypes under intermittent drought

“…Some species can adapt to water deficiency by modifying the solute level inside the cells; thus, turgor, stomatal aperture, and physiological activities can be maintained under low leaf water potential (Chartzoulakis et al, 2002;Zhu et al, 2005;Bucci et al, 2008). This mechanism, known as osmotic adjustment, contributes to drought tolerance with desiccationavoidance and is defined as the ability to accumulate osmotically active solutes in response to drought (Quezada et al, 1999;Pagter et al, 2005, Farooq et al, 2009).…”