A Cultivar-Sensitive Approach for the Continuous Monitoring of Olive (Olea europaea L.) Tree Water Status by Fruit and Leaf Sensing

Abstract

Sustainable irrigation is crucial to reduce water use and management costs in modern orchard systems. Continuous plant-based sensing is an innovative approach for the continuous monitoring of plant water status. Olive (Olea europaea L.) genotypes can respond to drought using different leaf and fruit physiological and morphological mechanisms. This study aimed to identify whether fruit and leaf water dynamics of two different olive cultivars were differently affected by water deficit and their response to changes of midday stem water potential (stem), the most common indicator of plant water status. Plant water status indicators such as leaf stomatal conductance (g s) and stem were measured in the Sicilian olive cultivars Nocellara del Belice (NB) and Olivo di Mandanici (MN), in stage II and III of fruit development. Fruit gauges and leaf patch clamp pressure probes were mounted on trees and their raw data were converted in relative rates of fruit diameter change (RR fruit) and leaf pressure change (RR leaf), sensitive indicators of tissue water exchanges. The analysis of diel, diurnal and nocturnal fluctuations of RR fruit and RR leaf highlighted differences, often opposite, between the two cultivars under water deficit. A combination of statistical parameters extrapolated from RR fruit and RR leaf diurnal and nocturnal curves were successfully used to obtain significant multiple linear models for the estimation of midday stem. Fruit and leaf water exchanges suggest that olive cultivar can either privilege fruit or leaf water status, with MN likely preserving leaf water status and NB increasing fruit tissue elasticity under severe water deficit. The results highlight the advantages of the integration of fruit and leaf water dynamics to estimate plant water status and the need for genotype-specific models in olive.

Detecting Mild Water Stress in Olive with Multiple Plant-Based Continuous Sensors

“…The increase in P p daily minimum values has been previously described as an indicator of mild water stress in olive [ 13 ], as also supported by the significant relationship we observed between this indicator and stem water potential ( Figure 7 a). Similarly, the RGR daily range increased strongly with irrigation withholding, and it was also highly correlated with tree water status ( Figure 7 b), highlighting a pronounced fruit shrinkage in response to water stress as previously reported in several crops [ 26 , 27 , 28 , 29 , 30 ] and, most recently, also in olive [ 7 , 31 ]. Changes in sap flow density fluxes between irrigation events were less noticeable than those in leaf P p and fruit RGR, most probably because of the high impact that the environment (namely VPD) has on this parameter [ 32 ], further affected by tree water status [ 8 , 33 ].…”

“…One sensor of each type was installed in each selected tree, for a total of 18 sensors (three sensor-types per three trees per two cultivars). The sampling procedure and number of measurements corresponded to the standards for these settings and type of investigation [ 7 , 19 ] and fully support the strength of the presented outputs.…”

“…Most of these sensors do not measure plant water status directly but they monitor specific physiological processes that correlate in different ways and degrees with plant water status. Since tree physiological responses are mediated by several factors, including tree phenological stage, environmental conditions, or genotype-specific traits, the development of simplified and uniform water management protocols using these sensors is complex [ 7 , 8 ]. As an example, the use of the leaf patch clamp pressure (LPCP) probe [ 9 ] could be very promising for irrigation scheduling in ‘Arbequina’ olive orchards [ 10 , 11 ].…”

“…Some recent studies suggested combining the use of different plant-based sensors to improve water stress detection in olive. Scalisi et al [ 7 ] reported that combining leaf and fruit sensing improves water stress detection in two olive cultivar. Girón et al [ 15 ] investigated physiological responses of olive fruits under different levels of water deficit (with midday Ψ stem < −2.5 MPa) and found that, at moderate water stress, fruits are weaker water sinks than leaves, as opposed to what is observed in other species [ 16 , 17 ].…”

“…In addition, two Sicilian olive cultivars (Olivo di Mandanici and Nocellara del Belice) were investigated to assess if the combined use of these physiological indicators could provide useful information, regardless of the cultivar-specific response to water deficit. In particular, ‘Olivo di Mandanici’ (OM) showed a slower decrease in Ψ stem and a lower tendency to show semi-inversion or inversion of the P p curves in response to water withholding with respect to ‘Nocellara del Belice’ (NB) [ 7 , 13 , 19 ], resulting in lower irrigation needs.…”