Dynamics of pp highlighted the typical inversion phenomenon of the diel curve in olive leaves from trees under deficit irrigation . In NB, DI-66 and DI-33 leaves exhibited the half-inverted state , whereas DI-0 leaves showed a total inversion of the curve . In MN trees, a clear shift from state I to II was not observed, despite the slight tendency to enter state II at 219 and 221 DOY, with no apparent differences among irrigation levels . This suggests that MN leaves can maintain high cell turgor, probably by reduced cell wall elasticity or osmotic adjustments, as found in other olive genotypes . The highest RRfruit always occurred early in the night as fruit quickly rehydrated their tissues . As expected, the most negative RRfruit rate always occurred in the warmest hours of the day. RRfruit dynamics were also affected by deficit irrigation in NB, as the diel RANGE was greater in DI-0 and DI-33 than in DI-66 and FI fruit . A completely different behavior was observed in MN fruit, which instead had the widest diel RANGE in FI trees . In addition, the overall diel RANGE of RRfruit in MN was almost double than in NB, implying larger water in- and out-flows per unit of fruit volume in the former, determined by high fruit sink power for water. A general positive peak of RRleaf was exhibited early in the morning , representing a quick leaf turgor loss after pre-dawn highest turgor in the 24-hour timeframe. Even in this case, the two cultivars responded differently to water deficit, growing blueberries in pots with NB DI-0 trees exhibiting minimal diel fluctuations while MN DI-0 trees showing the largest RANGE. This suggested that the oscillations of RRleaf might be linked to those of RRfruit. Another 5-day interval was considered at stage III of fruit development .
Differently from stage II, FD responses were characterized by an evident diameter increase across the 5 days and within the 24-hour period in both cultivars, as in stage III fruit are in full cell enlargement phase . Daily curves of pp did not show pronounced inversion phenomena, as this week was characterized by high rainfall and general higher midday 9 stem . Only NB DI-0 trees showed a partially inverted pp curve. Diel RANGE of RRfruit was found to be greatly reduced at stage III compared to stage II . In the former, low VPD and good soil water availability determined by abundant precipitations led to an increase of water content in fruit and lower fruit water exchanges. For similar reasons, the diel RANGE of RRleaf was reduced in stage III , although NB and MN maintained the same differences in response to deficit irrigation levels observed at stage II . Considering the interesting findings from RRfruit and RRleaf dynamics, these two indices were further related to each other regressing their diel data at 15-minute intervals in a day at stage II and stage III . Scatter plots in Figure 10 show anti-clockwise hysteretic relationships between RRfruit and RRleaf, both for NB and MN . Hysteresis are common when relating outputs from different sensors of plant water status mounted on different organs , as there is generally a lag in tissue water de- and re-hydration, and in our case, also a likely different pattern of the RRleaf to RRfruit relationship between day and night. An overall decrease of the hysteretic loop area occurred from stage II to stage III in both cultivars . This is probably driven by the different fruit growth pattern at stages II and III which induced a reduction of the RRfruit diel range . In both DOY 223 and 287, the hysteretic loops in NB progressively flattened along the RRfruit axis with increasing water deficit due to the change in the ratio between RRfruit and RRleaf.
In other words, on one hand, at increasing water deficit and in a diel interval, it seems that NB leaves significantly reduce water exchanges, as the values of RRleaf stay around 0 Pa kPa−1 min−1 . On the other hand, increasing water deficit caused MN loops to flatten along the RRleaf axis , with MN leaves keeping high water exchanges at low 9 stem, while fruit water exchanges were significantly reduced, as RRfruit did not change much from 0 µm mm−1 min−1 .This opposite trend suggests a completely different mechanism of leaf and fruit water exchanges in response to increasing water deficit in the two cultivars, which might be driven by different osmotic adjustments, cell-wall elasticity and tissue water content. The statistical diel, nocturnal and diurnal parameters of RRfruit were associated to the corresponding RRleaf parameters to assess fruit and leaf responses to water deficit. Subsequently, data were analyzed by MANOVA to determine whether the combined response of parameters was affected by cultivars, irrigation levels, and cultivar × irrigation interaction. The cultivar did not influence significantly diel, diurnal and nocturnal RRfruit/RRleaf when statistical parameters were considered together . Diel and diurnal RRfruit/RRleaf parameters changed significantly in response to irrigation levels, but the cultivar × irrigation interaction had the strongest effect , indicating that RRfruit/RRleaf responses to deficit irrigation differ between the two genotypes under study. Specifically, the highest F was found in the MANOVA that tested diurnal RRfruit/RRleaf responses to cultivar × irrigation. These results suggest that, under increasing water deficit, the differences in genotype-specific fruit and leaf sink power to water are predominant in day hours.Human persistence depends on many natural processes, termed ecosystem services, which are usually not accounted for in market valuations. Global degradation of such services can undermine the ability of agriculture to meet the demands of the growing, increasingly affluent, human population . Pollination of crop flowers provided by wild insects is one such vulnerable ecosystem service , as their abundance and diversity are declining in many agricultural landscapes . Globally, yields of insect-pollinated crops are often managed for greater pollination through the addition of honey bees as an agricultural input . Therefore, the potential impact of declines in wild pollinators on crop yields is largely unknown, as is whether increasing application of honey bees compensates for losses of wild pollinators, or even promotes these losses. Wild insects may increase the proportion of flowers that develop into mature fruits or seeds , and therefore crop yield , by contributing to pollinator abundance, species number , and equity in relative species abundance . Increased pollinator abundance, and therefore visitation rate to crop flowers, should augment fruit set at a decelerating rate until additional individuals do not further increase , or even decrease fruit set . Richness of pollinator species should increase the mean, and reduce the variance, of fruit set , because of complementary pollination among species , facilitation , or “sampling effects” , among other mechanisms . Pollinator evenness may enhance fruit set via complementarity, or diminish it if a dominant species is the most effective pollinator . To date, drainage gutter the few studies on the importance of pollinator richness for crop pollination have revealed mixed results , the effects of evenness on pollination services remain largely unknown, and the impact of wild insect loss on fruit set has not been evaluated globally for animal pollinated crops.
We tested four predictions arising from the assumption that wild insects effectively pollinate a broad range of crops, and that their role can be replaced by increasing the abundance of honey bees in agricultural fields: for most crops, wild-insect and honey bee visitation enhances pollen deposition on stigmas of flowers; consequently, for most crops, wild insect and honey bee visitation improves fruit set; visitation by wild insects promotes fruit set only when honey bees visit infrequently ; and pollinator assemblages with more species benefit fruit set only when honey bees visit infrequently . To test these predictions we collected data at 600 fields on all continents, except Antarctica, for 41 crop systems . Crops included a wide array of animal-pollinated, annual and perennial fruit, seed, nut, and stimulant crops; predominately wind-pollinated crops were not considered . Sampled fields were subject to a diversity of agricultural practices, ranging from extensive monocultures to small and diversified systems , fields stocked with low to high densities of honey bees , and fields with low to high abundance and diversity of wild insects . For each field, we measured flower visitation per unit of time for each insect species, from which we estimated species richness and evenness . We quantified pollen deposition for 14 systems as the number of pollen grains per stigma, and fruit set for 32 systems as the percentage of flowers setting mature fruits or seeds. Spatial or temporal variation of pollen deposition and fruit set were measured as the coefficient of variation over sample points or days within each field . The multilevel data provided by fields within systems were analyzed with general linear mixed-effects models that included crop system as a random effect, and wild-insect visitation, honey bee visitation, evenness, richness, and all their interactions as fixed effects. Best-fitting models were selected based on Akaike’s Information Criterion . In agreement with the first prediction, crops in fields with more flower visits received more pollen on stigmas, with an overall 74% stronger influence of visitation by honey bees than by wild insects . Honey bee visitation significantly increased pollen deposition in seven of ten crop systems, and wild insects in ten of 13 systems . Correspondingly, increased wild insect and honey bee visitation reduced variation in pollen deposition among samples . Contrary to the second prediction, fruit set increased significantly with wild-insect visitation in all crop systems, but with honey bee visitation in only 14% of systems . In addition, fruit set increased twice as strongly with visitation by wild insects than by honey bees . These partial regression coefficients did not differ simply because of unequal abundance, or disparate variation in visitation between wild insects and honey bees. In crop systems visited by both honey bees and wild insects, honey bees accounted for half of the visits to crop flowers , and among-field CVs for visitation by honey bees and by wild insects were equivalent. Furthermore, wild-insect visitation had stronger effects than honey bee visitation, regardless of whether honey bees were managed or feral and, comparing across systems, even where only wild insects or honey bees occurred . Moreover, wild-insect visitation alone predicted fruit set better than honey bee visitation alone . Correspondingly, the CV of fruit set decreased with wild-insect visitation, but varied independently of honey bee visitation . This contrast likely arose from pollen excess, filtering of pollen tubes by post pollination processes, and seed abortion , and so reflects pollination quality, in part. Intriguingly, the difference in coefficients between pollen deposition and fruit set for honey bees greatly exceeds that for wild insects , indicating that wild insects provide better quality pollination, such as greater cross-pollination . These results occurred regardless of which crop systems were selected , sample size , the relative frequency of honey bees in the pollinator assemblage among systems, the pollinator dependence of crops, or whether the crop species were herbaceous or woody, or native or exotic . Poor-quality pollination could arise if insect foraging behavior, based on focal resources typical of honey bees , causes pollen transfer between flowers of the same plant individual or the same cultivar within a field, thereby limiting cross pollination and increasing the incidence of self-pollen interference and inbreeding depression . The smaller difference in coefficients between pollen deposition and fruit set for wild insects, and the stronger effect on fruit set of wild-insect visitation, suggest that management to promote diverse wild insects has great potential to improve global yield of animal-pollinated crops. The third prediction was also not supported, as fruit set increased consistently with visitation by wild insects, even where honey bees visited frequently . In particular, the best-fitting model for fruit set included additive effects of both visitation by wild insects and honey bees , suggesting that managed honey bees supplement the pollination service of wild insects, but cannot replace it. Overall, visitation by wild insects and honey bees were not correlated among fields , providing no evidence for either competition for the resources obtained from crop flowers , or density compensation between wild insects and honey bees at the field scale.