Harnessing nighttime transpiration dynamics for drought tolerance in grasses

Abstract

Non-negligible nighttime transpiration rates (TR_N) have been identified in grasses such as wheat and barley. Evidence from the last 30 years indicate that in drought-prone environments with high evaporative demand, TR_N could amount to 8-55% of daytime TR, leading several investigators to hypothesize that reducing TR_N might represent a viable water-saving strategy that minimizes seemingly 'wasteful' water loss that is not traded for CO₂ fixation. More recently however, evidence suggests that actual increases in TR_N during pre-dawn hours, which are presumably controlled by the circadian clock, mediate drought tolerance - not through water conservation - but by enabling maximized gas exchange early in the morning before midday depression sets in. Finally, new findings point to a previously undocumented role for leaf sheaths as substantial contributors (up to 45%) of canopy TR_N, although the extent of their involvement in these two strategies remains unknown. In this paper, we synthesize and reconcile key results from experimental and simulation-based modeling efforts conducted at scales ranging from the leaf tissue to the field plot on wheat and barley to show that both strategies could in fact concomitantly enable yield gains under limited water supply. We propose a simple framework highlighting the role played by TR_N dynamics in drought tolerance and provide a synthesis of potential research directions, with an emphasis on the need for further examining the role played by the circadian clock and leaf sheath gas exchange.

Keywords: Circadian clock; climate change; food security; nocturnal transpiration; sheath gas exchange; stem photosynthesis; stomatal conductance; vapor pressure deficit; water conservation.

Figure 1.

Examples of time courses liking nighttime transpiration rate (TR_N) dynamics to differential drought tolerance for three hypothetical genotypes. The orange time course represents a standard line with a non-negligible TR_N and no pre-dawn increase in stomatal conductance (genotype 1). The dotted orange line represents a variation of this behavior (genotype 2) which exhibits an identical baseline TR_N relative to genotype 1, while expressing a pre-dawn increase in TR_N. The blue line represents a genotype with a negligible or null baseline TR_N, with a pre-dawn increase in TR_N. Genotypes 2 and 3 are considered to be more drought tolerant compared to genotype 1 since they express a pre-dawn increase in TR_N which enables maximizing early morning gas exchange. Genotype 3 is the most drought tolerant since it also exhibits a negligible TR_N during most of the night, leading to water conservation. Time courses assume a non-zero VPD throughout the night with early morning dynamics amplified for illustration purposes

Figure 2.

Examples of time courses of vapor pressure deficit (VPD, a) and sheath transpiration in barley (b), expressed as a percentage relative to whole-plant transpiration rate during a 24-h greenhouse experiment (unpublished data). The relative contribution of sheath-based transpiration was estimated gravimetrically, as a ratio of transpiration rates of plants with excised blades to those measured on intact plants, under the same conditions. Each datapoint on panel (b) represents a 10-minute average of three replicate plants