Seasonal changes of maximum daily shrinkage reference equations for irrigation scheduling in olive trees: Influence of fruit load

Maximum daily shrinkage (MDS) is the parameter of daily cycle of trunk diameter most widely suggested in irrigation scheduling for several fruit trees. However, as in other plant-measured approaches, the irrigation decision may be difficult due to the influence of the environment on the values obtained. Reference equations of MDS have been established in order to avoid the effects of environmental conditions. Such equations are usually related to simple meteorological data, in order to easily estimate MDS values in full-irrigated conditions. This paper studies the influence of fruit load and the inter-annual variations on the reference equation of MDS in olive trees. These reference equations were calculated during 4 seasons in a full-irrigated orchard and the equations were validated with the data from a different season. The MDS values were related to vapour pressure deficit (VPD) and temperature taken near the experimental orchard. In addition, meteorological data were considered as mean daily or as midday values; only for temperature was the maximum daily value also used. The validation of the equations was made using the fits with all the meteorological data considered (midday and mean daily of VPD and temperature). In addition, two different fits were used in each meteorological data—one according to fruit load and other with the complete pool of data. The equations fit were significantly different each season in all the meteorological data considered. However, seasons with similar fruit load were more similar to each other. In both meteorological data considered (VPD and temperature) the midday values improved the fit in respect to mean daily values. The equations obtained with maximum daily temperature were similar in accuracy to the one of midday. The reference equations in which temperature was used obtained a better fit that the ones calculated with VPD. No significant differences were found in the validation when equations according to fruit load or the complete pool data were compared. The limitations and usefulness of these reference equations are also discussed.     

Usefulness of trunk diameter variations as continuous water stress indicators of pomegranate (Punica granatum) trees

Pomegranate trees (Punica granatum L.) is a deciduous fruit tree included in the so-called group of minor fruit tree species, not widely grown but of some importance in the south east of Spain. Pomegranate trees are considered as a culture tolerant to soil water deficit. However, very little is known about pomegranate orchard water management. The objective of this research was to asses the feasibility of using trunk diameter variation (TDV) indexes, obtained by means of LVDT sensors, as a plant water stress indicators for pomegranate trees. The experiment was carried out with mature trees grown in the field under three irrigation regimes: control well watered trees; trees continuously deficit irrigated at 50% of the control regime (SDI); and trees that had a summer water stress cycle being irrigated at 25% of the control rates only in July and August (RDI). The seasonal variations of maximum diurnal trunk shrinkage (MDS) and trunk growth rates (TGR) were compared with midday stem water potential (Ψstem) measurements. During the course of the entire season, control trees maintained lower MDS values than the SDI ones. In the RDI treatment, as water restrictions began, there was a slow increase in MDS, in correspondence with a decrease in Ψstem. When water was returned at full dosage, the RDI quickly recovered to MDS and Ψstem values similar to the control. However, lower MDS for a given Ψstem values were observed as the season advanced. The magnitude of differences between well watered and deficit irrigated trees was much larger in the case of MDS than for Ψstem. However, the tree-to-tree variability of the MDS readings was more than four times higher than for Ψstem; average coefficient of variation of 7.5 and 36% for Ψstem and MDS, respectively. On the other hand, TGR did not clearly reflect differences in tree water status. Overall, results reported indicated that MDS is a good indicator of pomegranate tree water status and it can be further used for managing irrigation. However, the seasonal changes in the MDS-Ψstem relationship should be taken into account when attempting to use threshold MDS values for scheduling irrigation.     

Scheduling deficit irrigation of citrus trees with maximum daily trunk shrinkage

The feasibility of scheduling deficit irrigation using maximum daily trunk shrinkage (MDS) was evaluated during two consecutive seasons in a citrus orchard planted with mature ‘Clementina de Nules’ trees, in Valencia, Spain. Results showed that MDS in well irrigated trees varied largely according to the environmental conditions (higher correlation was obtained with global radiation), and therefore, the absolute values of MDS cannot be employed as the only variable to schedule irrigation. To avoid the effects of the climatic conditions we scheduled deficit irrigation using the MDS ratio, which is the MDS of any treatment related to the MDS of a control, well irrigated, treatment located in the same plot. We explored the feasibility of scheduling irrigation based on the MDS ratio in a deficit irrigated treatment, where water was applied as necessary, from July until mid October, in order to maintain the MDS values at 125% of that of the control treatment. Despite the large variability observed in the MDS measurements in both years no significant reduction in yield and fruit weight was observed in the deficit irrigated treatment compared with the control, allowing seasonal water saving between 18 and 12%.     

Maximum diurnal trunk shrinkage is a sensitive indicator of plant water, stress in Diospyros kaki (Persimmon) trees

Persimmon tree (Diospyros kaki L.f.) is a deciduous fruit tree included in the so-called group of minor fruit tree species. Worldwide, it is not widely grown but, nowadays, Kaki culture is of some importance in the south-east of Spain because of the high fruit commercial value. Currently, neither it is known about Kaki trees water needs, nor crop responses to the irrigation regime. The objective of the present research was to assess the feasibility of using maximum diurnal trunk shrinkage (MDS) as a plant water stress indicator for Kaki trees. During two drought cycles, in trees under either full or deficit irrigation, the MDS obtained by means of LVDT sensors was compared with a reference indicator of fruit trees water status, the midday stem water potential (Ψ_stem). In addition, stomatal conductance and fruit diameter variations were also followed. As water restrictions began, there was an immediate increase in MDS, in correspondence with a decrease in Ψ_stem. Pooling data from both drought cycles and irrigation regimes, MDS and Ψ_stem were linearly correlated (r² = 0.77***). The magnitude of differences between well watered and deficit irrigated trees was much larger in the case of MDS than for Ψ_stem. However, the tree-to-tree variability of the MDS readings was three times higher than for Ψ_stem; average coefficient of variation of 14% and 38% for Ψ_stem and MDS, respectively. Overall, results reported indicated that MDS is a sensitive indicator of Kaki water status and it can be further used as an irrigation scheduling indicator for optimum irrigation management of this crop. However, the large MDS tree-to-tree variability should be taken into account when selecting the number of trees to monitor within an orchard.