Nighttime transpiration: does it contribute to water stress in grape?

Climate change is driving the search for grapevine cultivars and/or rootstocks that use water more efficiently. Recently, there has been increasing attention on nighttime transpiration. The reasoning is simple. While daytime transpiration results from the necessity to have stomata open so the plant can take up carbon dioxide for photosynthesis, nighttime transpiration could be considered a “waste” of water since no photosynthesis occurs at night. So how significant is nighttime water use in grape, and does it serve some other purpose that benefits the vine?

How to differentiate nighttime transpiration and water recharge in nocturnal sap flow?

<p>Nocturnal sap flow (<em>Q<sub>n</sub></em>) affect not only forest carbon and water budgets but also their responses to climate change as it consists of two ecohydrological and ecophysiological significant components: nighttime transpiration and water recharge. A vapor pressure deficit (<em>VPD</em>) based sap flow partitioning method has been developed to estimate nighttime transpiration, which is normally quantified through the discretely measured nighttime stomatal conductance, from the widely and continuously measured sap flow. However, given the increasing knowledge of <em>Q<sub>n</sub></em> mechanisms, whether <em>Q<sub>n</sub></em> could be partitioning simply by <em>VPD</em> and whether this method is valid in semi-arid regions remain unclear. We measured sap flow of <em>Pinus tabuliformis</em> and <em>Acer truncatum</em> in a middle-aged and a young monoculture forest stand, respectively, in a semi-arid mountainous area of northern China. We found the influence of <em>VPD</em> on <em>Q<sub>n</sub></em> conditioned by soil moisture. Meanwhile, a considerable impact of wind speed on <em>Q<sub>n</sub></em> was observed. In the stands with relatively dry soils, both increased and decreased soil moisture promoted <em>Q<sub>n</sub></em>, which might be due to enhanced nighttime water recharge for two distinct purposes, i.e., capacitance refilling and avoiding hydraulic failures. For these three environmental factors (i.e., <em>VPD</em>, wind speed, and soil moisture) that have been considered most in previous studies, their total effect explained less than 55% of the <em>Q<sub>n</sub></em> variations. This study highlights that physiological influences of <em>VPD</em> on nighttime stomatal water loss were uncertain. Furthermore, it suggests that there could exist considerable nighttime water loss induced by wind, possible region-specific patterns of nighttime water recharge, and limited concurrent environmental controls on <em>Q</em><sub>n</sub>. Our findings are helpful to improve the <em>VPD</em>-based sap flow partitioning method to differentiate nighttime transpiration and water recharge.</p>

Reduced nighttime transpiration is a relevant breeding target for high water-use efficiency in grapevine

Increasing water scarcity challenges crop sustainability in many regions. As a consequence, the enhancement of transpiration efficiency (TE)—that is, the biomass produced per unit of water transpired—has become crucial in breeding programs. This could be achieved by reducing plant transpiration through a better closure of the stomatal pores at the leaf surface. However, this strategy generally also lowers growth, as stomatal opening is necessary for the capture of atmospheric CO2 that feeds daytime photosynthesis. Here, we considered the reduction in transpiration rate at night (En) as a possible strategy to limit water use without altering growth. For this purpose, we carried out a genetic analysis for En and TE in grapevine, a major crop in drought-prone areas. Using recently developed phenotyping facilities, potted plants of a cross between Syrah and Grenache cultivars were screened for 2 y under well-watered and moderate soil water deficit scenarios. High genetic variability was found for En under both scenarios and was primarily associated with residual diffusion through the stomata. Five quantitative trait loci (QTLs) were detected that underlay genetic variability in En. Interestingly, four of them colocalized with QTLs for TE. Moreover, genotypes with favorable alleles on these common QTLs exhibited reduced En without altered growth. These results demonstrate the interest of breeding grapevine for lower water loss at night and pave the way to breeding other crops with this underexploited trait for higher TE.