Coping with low light under high atmospheric dryness: shade acclimation in a Mediterranean conifer (Abies pinsapo Boiss.)

Abstract

Plant species living in the understory increase carbon (C) allocation toward leaf production for maximizing light capture at the expense of roots and stems, with negative consequences for the whole-plant hydraulic conductance. Moreover, under some conditions, the high atmospheric evaporative demand occurring in Mediterranean areas may be not well buffered by the canopy, which might be the case for relict conifer Abies pinsapo Boiss. growing in the forest understory. We hypothesized that acclimation to combined understory shade and high atmospheric dryness can be achieved through the adjustment of water losses to cope with the restriction in water transport. The results reveal high structural plasticity in A. pinsapo that allows light harvesting of this species to maximize light capture in the forest understory, and maintain a positive C balance under low light conditions. However, growth in the understory resulted in reduced leaf-specific conductivity, up to approximately four to five times, implying decreased plant capacity to supply water to the leaves. In order to cope with the high atmospheric evaporative demand in the understory, there is an adjustment of the stomatal conductance to the hydraulic conductivity by means of a reduction in the stomatal density in understory individuals, which is due to the almost complete lack of stomata in the adaxial side of the needles. To the extent of our knowledge, such a drastic phenotypic response found in a conifer when growing under shaded conditions had not been previously reported.

Carbon Limitation and Drought Sensitivity at Contrasting Elevation and Competition of Abies pinsapo Forests. Does Experimental Thinning Enhance Water Supply and Carbohydrates?

“…Indeed, the timing of this sequential reduction, in functions, such as gas exchange and growth, has been largely related to recent events of forest decline and mortality worldwide [11,[16][17][18][19]. Nonetheless, the functional and structural dynamics of these critical processes in response to competition are still not well understood.We focused on the hydraulic characteristics and seasonal patterns of sap flow density (SFD), gas exchange, secondary growth, and non-structural carbohydrate (NSC) of the drought-sensitive fir Abies pinsapo Boiss [20][21][22][23]. We assumed that xylem vulnerability to embolism places a physical limit for A. pinsapo tolerance to water shortage while the dynamics of NSC would be related to drought sensitivity, for the extent of isohydric regulation of the water potential, as well as the persistence of the drought [13,14,19].…”

Regulation of Water Use in the Southernmost European Fir (Abies pinsapo Boiss.): Drought Avoidance Matters

“…A higher resistance to embolism might explain this enhanced drought tolerance, through a plastic response of the xylem physiology to different levels of water stress [39]. However, our field data suggest that the risk of embolism was small because xylem water potentials were quite different from the A. pinsapo published values of xylem water potential at which embolism begins [35]. Therefore, both the high and low elevation populations showed a similar and high safety margin, suggesting that A. pinsapo responds to low soil water availability and high evaporative atmospheric pressure by maintaining xylem water potential within a safety range, rather than being able to resist a higher tension in the xylem [29,35].…”

“…These results indicate that on a daily scale, the stomatal dynamics of A. pinsapo are different to those typically found in Mediterranean trees. Thus, stomatal conductance of A. pinsapo generally indicates midday maximums, except in late summer, where values are permanently low [35], while the daily dynamics of Mediterranean trees are characterized by early morning maximums, a sharp decline at midday, and a relative recovery of stomatal conductance values during the afternoon [17].…”

“…Such a reduction in LSC could be an adaptive response to achieve a balance between the water flow to the leaves and the water flow from the leaves to the atmosphere, reducing the risk of drought-induced cavitation due to excessive water tension in the xylem [40]. This hypothetical difference in LSC, based on the contrasting gs in the two populations may be explained by the existence of anatomical differences, for instance, by means of a reduction in the stomatal density [35]. However, a most logical explanation for the difference in behavior between the two populations in this study likely relies on the synthesis of the plant hormone abscisic acid (ABA) to maintain stomatal closure during sustained drought [44].…”

“…Drought causes a decrease in the xylem water potential, which, among other physiological mechanisms, reduces tree photosynthetic carbon uptake as a result of stomatal closure to prevent hydraulic failure. Consequently, under the expected climate scenario of increasing temperature and declining water availability [15], the growth of drought-sensitive species such as A. pinsapo will likely be reduced [11,35], while carbohydrate reserves are consumed to maintain metabolism, which can ultimately cause the death of the tree by carbon starvation or hydraulic failure [10,50,51]. Tree species showing drought-avoidance mechanisms prevent xylem damage by early stomatal closure, before exhibiting significant changes in xylem or needle water status.…”