Physiological and hydraulic mechanisms of drought tolerance in plants : implications of CO2 and irradiance Mecanismos fisiológicos e hidráulicos da tolerância a seca: implicações de CO2 e irradiância

Abstract

Herein, it is present a series of experiments divided into four chapters with the purpose to immerse deep into some physiological and hydraulic responses to drought and also how some of them interact with environmental variability such as elevated [CO2] and irradiance. On the first two chapters it is presented drought responses of coffee plants, one of the most important commodities worldwide, under elevated (700 ppm) and ambient (400 ppm) [CO2] On the first chapter we found that drought-stressed 700-plants were able to keep hydraulic conductance for longer, transpiring more than 400-plants. Correlative evidence is shown that aquaporins may play major roles in these processes. In addition, Well-watered 700-plants displayed lower whole-plant transpiration rates than their 400- counterparts. This was not associated with maximum gs per se, but rather with an increased stomatal closure rate upon vapor pressure deficit transitions, which occur innumerous times over the course of the day. On the second chapter we found that elevated [CO2] improved carbon assimilation, water use-efficiency and biomass accumulation regardless watering, in addition to decreasing the oxidative pressure under drought conditions. Elevated [CO2] also promoted key allometric adjustments linked to drought tolerance, e.g. more biomass partitioning towards roots with a deeper root system. Improved growth under enhanced air [CO2] was unlikely to have been associated with global changes on hormonal pools but rather with shifts on carbon fluxes. Altogether, results from the chapters 1 and 2 suggest that [CO2] is perceived by the plant as a key environmental factor having profound implications on how plants respond to drought, thus permitting 700-plants to have an improved fitness under drought when compared to 400- plants. In the third and fourth chapters, efforts were focused on analyzing hydraulic aspects of several different species. O the third chapter, we focused on finding anatomical drivers related to inter- and intraspecific xylem embolism resistance. Vessel lumen fraction was the only anatomical trait measured that correlated with xylem embolism resistance across scales and species. Light was found to drive only minor differences in stem and not leaf embolism resistance. Our data suggest that conduits highly dispersed in a matrix of imperforate elements may be better protected against the spread of embolism than conduits that are packed in close proximity, which may contribute to our understanding of the mechanisms behind air-seeding. Finally at the fourth chapter, it is presented deep insights into the possible existence of a well-established water potential threshold beyond which vessels and tracheids will embolize We found that, in vessel-based xylem species, individual xylem conduits had a more well-defined water potential at which embolism occur, with considerable pre-existing embolism being able to influence the vulnerability of the xylem. In contrast, conduits in tracheid-based xylem did not display a well-defined individual water potential threshold at which embolism occurs and thus pre-existing embolism did not alter the vulnerability of xylem.