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Effect of regulated deficit irrigation (RDI) on the vegetative growth of pear variety Triunfo de Viena (Pyrus communis L.)

One of the consequences of a water deficit in the vegetative growth is the modification of the plant vigor, which determines the extent and the rate of growth and lengthening in shoots, which, in this phase, is more important than fruit growth, which is very slow in terms of dry mass accumulation. The objective of this research was to determine the effect on the vegetative growth of the pear variety Triunfo de Viena (Pyrus communis L) of regulated deficit irrigation (RDI), which was compared with a control irrigated at 100% crop evapotranspiration (ETc). The irrigation treatments consisted of the application of watering regimes of 74 and 48% ETc in treatment T2 and 60 and 27% ETc in treatment T3 in 2014 and 2015, respectively, during the period of rapid fruit growth with the same watering regime used in T1 (control) applied during the rest of the season. The irrigation reduction used in T2 and T3 represented water savings of 26% and 40% in 2014 and 52% and 73% in 2015, respectively. In the deficit treatments, there were no significant differences with respect to the control for the length, or absolute and relative growth rates (AGR and RGR) of the shoots. The weight of the wood from the fructification pruning had a significant difference between T3 and the control.


INTRODUCTION
An alternative that improves water use efficiency in fruit trees is RDI, which consists of making small water applications during periods of the crop cycle that have low sensitivity to water deficits and applying the necessary amount during the rest of the phenological cycle (Vélez Sánchez et al. 2012). Monitoring the evolution of the daily growth of buds allows for controlling the intensity of deficits to avoid losses in fruit production and quality (Moreno Hernández et al. 2017).
The vigor conditions of plants determine the extent, rate of growth, and elongation of shoots; a high-water deficit can affect trees or their parts (Bolat et al. 2014). The effect depends on the intensity, duration, and timing of the deficit. Imposing moderate levels of water stress during crop development improves fruit quality and saves water (Silveira et al. 2020).
The initiation and differentiation of vegetative and reproductive structures and cell lengthening once the cells are differentiated are very sensitive to water stress and, physiological processes, are not necessarily affected to the same degree (Marsal et al. 2002). Wu et al. (2013) and Molina Ochoa et al. (2015b), in the pear, found that the effect of water stress manifests itself clearly in vegetative growth, so one of the objectives of RDI is to avoid the excess vigor that affects fructification (Vélez Sánchez et al. 2007). Vegetative development is limited proportionally to reductions in the water supply (Moreno Hernández et al. 2017).
Growth curves show the behavior of a plant in a particular ecosystem over time, irreversible increases in dry matter or volume, and changes in size, mass, shape, or number of structures as a function of genotype and environment, resulting in a quantitative increase in the size and weight of the plant or a specific organ (Álvarez et al. 2021). The growth rates commonly used for analyses are absolute growth rate (AGR), which indicates changes in size per unit of time, and relative growth rate (RGR), which indicates variation in size per unit of initial size (Ardila et al. 2011).
The objective of the research was to determine the effect on the vegetative growth of the pear variety Triunfo de Viena of regulated deficit irrigation RDI during the rapid fruit growth stage in 2014 and 2015, taking into account the water status of the soil and of the plants.

MATERIALS AND METHODS
This experiment was carried out during 2014 and 2015 in the municipality of Sesquilé, Cundinamarca, Colombia, on a plot of 0.32ha, with 172 pear trees of the variety Triunfo de Viena, planted in 1998 at 4 x 4m. The soil, typical of the area has a clay-loam texture (IGAC, 2010). The weather information was obtained from a WS-GP1 portable weather station (AT delta-T Devices, USA) located next to the plot. The climate is cold and dry, the average temperature was 13.0°C and 13.14°C, relative humidity was 80.17 and 79.78%, and accumulated precipitation was 509. The experiment design consisted of randomized complete blocks, taking into account the slope of the land and the distribution of the trees in the plot, with three treatments and four repetitions (12 plots); the experimental plot was formed by 4 or 5 contiguous rows of three, four and five trees each (with a total of 12, 15, 16 and 20 interior trees per plot).
The irrigation regime was determined by taking into account crop evapotranspiration (ETc). During the crop cycle, all treatments were irrigated at 100% ETc; in the rapid fruit growth phase, the following were applied: Control Treatment (T1), irrigated for two years at 100% ETc; and Deficit Treatments (T2), irrigated at 74% and 48% ETc, and (T3), irrigated at 60% and 27% ETc, in 2014 and 2015, respectively. The volume of water applied to each treatment was controlled by varying the irrigation time, maintaining the same frequency, every two days. The water was measured with 13mm volumetric counters (Zenner®) installed in each plot. The drip irrigation system used six 8Lh -1 emitters per tree.
The water matrix potential in the soil (Ys) was measured every three days with eight granular matrix sensors (Watermark Mod. 200ss Irrometer Co., USA) per treatment, installed at depths of 15 and 30cm every 25cm on the emitter and drip line.
The leaf water potential was measured with a pressure chamber (PMS Instrument Company, Model 600, PMS Instrument Company, Oregon, USA). The determinations were made on leaves before dawn (potential at dawn, Ya) and at solar noon (potential stem, Yt) on leaves in which transpiration was prevented by covering them two hours before the measurement with hermetically sealed plastic bags covered with aluminum foil. For both the Ya and Yt, two and three adult leaves were used per tree from three and four representative trees per treatment in 2014 and 2015, respectively, which were located in the lower third on the north side of the tree. The determinations were made at the beginning of the water restrictions and every month thereafter for the Ya and every 15 days for the Yt.
On October 1 and 2 of 2013 and 2014 and September 20 of 2015, fruit pruning was carried out; after the harvest, the wood corresponding to two trees was weighed per repetition, for a total of eight per treatment, to determine whether the water regimes affected the vegetative development.
In November 2013 and 2014, 12 shoots, with an average length of 10cm, were selected and labeled by three in two trees per repetition, for a total of 96 per treatment, half on the north side and half on the south side, on which the length was measured in 2014 and 2015, every eight days with a precision flexometer (0.01mm) until harvest (May 9 of 2014 and April 10 of 2015). The growth curves were fitted to a three-parameter sigmoid logistic (Equation 1).
was carried out taking into consideration the precipitation lost through deep infiltration and runoff. The considered factor was the net storage in the soil. It permitted the planning of the deficit in each treatment with minimal differences in production. The water reserve in the soil coming from rain and irrigation before the rapid fruit growth phase mitigates the effects of reductions of water.   With the SAS/STAT program (SAS Institute Inc, 2010), the statistical analysis was carried out using analysis of variance and Tukey test, p<0.05, to compare the treatments and the control with the "glm" procedure.

RESULTS AND DISCUSSION
The volume of water applied. The irrigation treatments applied during the restriction from January 1 to (71 DAF), respectively, in 2015, without significant differences between treatments, which means that, during the two years, T3 accumulated enough dry matter for the fruits to achieve the respective compensatory growth once irrigation was resumed. , respectively, without any significative differences. Contrary to that seen for the diameter of the fruits; the AGR of the bud length in 2014 in all treatments was on average lower than in 2015.
In the two years at the beginning of the restriction, the AGR in T2 and T3 was higher than in T1, because of the water reserve in the soil coming from the rain and the irrigation before the rapid fruit growth phase. In 2014 at the end of the restriction before the harvest was lower, while in 2015, T2 was lower than T1 and T3 ( Figure 2).
Relative growth rate (RGR) of shoot length. The relative growth rate (RGR) of the shoot length during the two years showed a decreasing trend in all treatments, with a rapid decrease at the beginning. The RGR of the bud length in 2014, in T1, T2 and T3 was, before the restriction (25 DAF) 0.0174, 0.0182 and 0.0189cm day , respectively, without any significant differences.
In the two years before the start of the restriction, the RGR in T2 and T3 was higher than in T1, and, at the end of the restriction and before the harvest, they were smaller, coinciding with the conditions of the treatments. The RGR in T1, T2, and T3 reached its maximum at the beginning of growth: 0.0174, 0.0182, and 0.0189cm day

Comparison between the growth of the fruits and the buds.
In 2014 for all treatments, the percentage of fruit growth was greater than the one for the buds in the initial stage. In 2015 the opposite occurred, possibly because of rainfall occurring between 40 and 55 DAF.
The percentage growth of the buds was greater than the fruits for all the treatments as a result of the water restrictions for two years. It coincided with the period of the slow growth of the fruits, and the rapid growth of the buds, resulting from the competition between them (Yang et al. 2016).
T2 and T3 presented the highest percentage of shoot growth and the lowest percentage of fruit growth. The greatest differences in percentages of growth between the shoots and fruits were seen in T2 (17.29%), higher than T3 (14.90%) and T1 (8.47%) at 76, 69, and 90 DAF in 2014, respectively. In 2015, T2 (15.81%) was higher than T3 (13.93%) and T1 (11.06%) up to 103 DAF (Figure 4). This behavior occurs when plants have decreased water supply and accumulate carbohydrates in the cellular structures that maintain it until transferred to the fruits or stored after harvest for use in the following season. This could be verified with the lower pruned wood weights in the deficit treatments, along with the volumetric humidities (Θv), soil water potentials (Ψs), stem potentials (Ψt) and rainfall, but with the same number and average weight of the fruits, which presented no differences in 2014 and 2015.
The lower percentage of fruit growth in 2014 and 2015 in the deficit treatments was also due to other factors, such as pruning and the level of stress to which the plant was subjected. This allowed for the separation of the vegetative and fruit growth, which occurs after the end of shoot growth, similar to that found by Marsal et al. (2002), in the pear.   Pruned wood weight. One of the objectives of the RDI was to avoid excess plant vigor so as not to have negative effects on the production, which was achieved because of the clear separation that was found in this variety between the vegetative growth of the  shoots and the growth of the fruits, where part of the growth of the fruit occurred after the end of the vegetative growth ( Figure 4).
The results in 2014 showed an increase in the weight of the pruned wood in T1, which had the greatest volume of water applied, as compared to T2 and T3, 17.75% and 79.31%, respectively, with a difference, from that of T3. In 2015, there was no difference, between the treatments although the wood weight was lower in T3 and T2, 9.35% and 5.25%, than in T1. This indicates that the decrease in the vigor of the trees was achieved through the RDI.
However, in the control T1 treatment, there was no difference between the two years, while in T2 and T3 there was. The vigor increased, which could be verified in the same plot in 2013 through the effect of the deficit treatments carried out in 2012 (Molina Ochoa et al. 2015b). This variation was very marked in the pome fruit trees because the floral buds can also develop leaves and short branches, which does not happen in drupe fruit trees, where the formation of fruits and flowers has a greater separation (Fischer, 2012). The lower volumes of wood also mean important savings because of the high labor costs for pruning (Martínez et al. 2019).
When there is a decrease in the slope of the daily growth curve of the outbreaks longitudinal, the plant begins to experience slight water stress that signals the start of irrigation, which is why it is used as a highly sensitive indicator of stress.
The bud growth curve was fit to a sigmoid logistic type with three parameters, with which the AGR and RGR were determined, which allows for coordination of the irrigation in such a way that the vegetative growth is controlled to reduce water consumption through transpiration (Panigrahi & Srivastava, 2016). These results indicate the possibility of programming the automating irrigation when there is no previously established reference equation, and well-watered trees are available in the same plot. It is needed to monitor the water status of the plants continuously, so the imposed deficit does not reach too severe stress situations that could compromise productivity (Vélez Sánchez et al. 2012).