While the effect of climate change on diurnal temperature, predominantly during summer months, has been thoroughly discussed, little attention has been paid to the proportional increase in nocturnal temperatures. Indeed, previous research revealed that the nocturnal temperature increase rate is approximately 1.4 times greater than the corresponding diurnal temperature increase [
1], resulting in the concomitant increase in nighttime vapor pressure deficit (VPD). These changes in VPD have been correlated with increase in nocturnal plant water losses, which in turn could significantly reduce water use efficiency (WUE) in cultivated species [
2,
3]. In this frame, several scientific results suggest incomplete stomatal closure and increased sap flow velocity values during the night in olive trees [
4], grapevine [
5,
6], tomatoes [
2], sunflower [
7], and bean [
8]. However, it is not yet well understood why stomata remain open during the night even though CO
2 fixation via photosynthesis is suspended, and additionally, there is no need to reduce leaf temperature through transpiration fluxes [
9]. Specific experiments support that stomata remain open to facilitate the absorption and translocation of water and dissolved inorganic nutrients from the soil to various plant parts [
10]. Other studies suggest that maintaining stomatal conductance throughout the night serves the purpose of supplying dissolved oxygen to woody tissues [
11] while preventing cell turgor loss [
12]. On the other hand, the observed increased values of nocturnal sap flow do not necessarily imply water losses through transpiration, as a significant part of the up-streamed amounts of water can be used to refill the water reserves of the plant tissues predominately those in the sapwood where the most considerable amounts of water are known to be stored [
13]; a process commonly referred as ‘stem refilling’ [
11,
14]. The water reserves are utilized to fulfill the transpirational requirements of plants during the day and are replenished during the night period [
15]. The stem refilling process is controlled by complex ecophysiological mechanisms that alter hydrodynamic parameters [
15], reducing the risk of embolism within the conducting xylem [
16] which is an essential mechanism that allows plants to adapt to various environmental conditions. The study of this mechanism has been well-documented in forest species [
17,
18,
19,
20]. However, limited research has been conducted on the dynamics of stem refilling in cultivated species, such as olive trees. In addition to its well-known high resistance and capacity to produce fruit under arid conditions [
21], this species is characterized by differentiated fruit production between two consecutive years owing to the phenomenon of alternate bearing, i.e., the production of a substantial fruit load in one year (commonly termed in the literature as "ON" year), followed by a subsequent year characterized by a diminished fruit load ("OFF" year) [
22]. This characteristic essentially affects the olive species' water relations and consequently influences water resource utilization [
23,
24]. Beyond the fruit load, the phenological growth stage also exerts a considerable impact on the water relations of olive trees, with a more pronounced effect during the "ON" year. Specifically, during the phenological growth stage of 'pit hardening,' olive trees exhibit resistance to water deficit without detrimental effects on yield [
25,
26]. Conversely, in the subsequent phenological growth stage ("second phase of rapid fruit growth"), there is an elevated demand for water [
27].
Hence, the objective of this study is (a) to systematically observe and document the alteration of water status during the night in olive trees subjected to varying fruit loads, and (b) to comparatively assess the impact of alternate fruit bearing on the dynamics of stem refilling process.