Comparison of Optimal and Deficient Irrigation of Avocado by Phytomonitoring

Two irrigation regimes, the one optimal (100%) and the other deficient (70% of optimal), were compared in a ‘Hass’ avocado (Persea americana Mill.) orchard at Kibbutz Ma’agan Michael during the 2001 and 2002 seasons. Changes in trunk and fruit diameter, in the volumetric moisture of the soil, and in vapor pressure deficiency in the atmosphere were phytomonitored, using electronic sensors to transmit data to a remote computer. Micronic fluctuations in growth rate and shrinkage in the trunk and in the fruit showed similar trends, and were sometimes affected by the amount of irrigation. The trunk appeared to be more sensitive to medium changes than the fruit, as growth or shrinkage or both were usually seen first in the trunk and only later in the fruit. Deficient irrigation resulted in an increase in the daily shrinkage of the trunk and slower growth of the fruit. In both seasons, fewer large fruits were obtained from trees grown under deficient than under optimal irrigation. Daily shrinkage of the trunk is the best indicator of irrigation efficacy, and monitoring of such shrinkage is currently the only reliable way to follow up water consumption in avocado. Introduction Plant parameters offer the most direct way of obtaining information about the water status of plants. Monitoring of fluctuations in water status allow us to gain a better understanding of the effects of environment on the growth and water requirements of plants. For many years the water status of plants was determined by measuring the water potential of plant tissue (usually the leaves) in pressure chambers (Scholander et al. 1965). This is a destructive measurement and is unsatisfactory because it is noncontinuous and therefore does not adequately reflect the plant water status, which varies considerably during the course of the day. A more suitable approach is one in which the water status is monitored continuously. Over the last three decades a new approach was developed for the continuous monitoring of plant water status, based on the determination of micronic changes in the trunk and fruit diameter and leaf thickness (Burquez 1987, Huck and Klepper 1977, Huguet 1985, Schroeder and Wieland 1956, Sionist and Henderson 1973, Syvertsen and Levy 1982). These changes in the diameters of plant organs were found to 1 I wish to thank the orchard team at Kibbutz Maagan-Michael for their cooperation, Isaac Adato for professional assistance, the personnel of Phytech Ltd., Yad Mordechai, for technical assistance, and the Israel Avocado Board, who financed this work. correlate with fluctuations in the plant water status. Periodic changes were observed on a daily basis in the different plant organs, i.e., expansion and growth at night and organ shrinkage during the day. The amount of expansion or shrinkage of the organ depends on the climatic conditions, the type of soil, and the availability of water in the soil. A great advance was made in the 1980s, with the use of fluctuations in the trunk diameter to indicate the water status of the tree. Li and his colleagues showed that the trunk was the first plant organ to react to water stress, and it did so by a reduction in growth (Li et al., 1989). Maximal daily shrinkage of the trunk was found to be the best indicator of the water status in peach trees (Huguet 1985; Garnier and Berger 1986). In the 1990s, Isaac Adato introduced the idea of using the maximal daily shrinkage of the trunk as a measure of the water status of avocado trees in practical farming. This approach was based on examining the effects of empirical changes in the amount of irrigation on the maximal daily trunk shrinkage. An increase in irrigation would be considered effective if it diminished the maximal daily shrinkage of the trunk. By the same reasoning, less irrigation water could be applied if the smaller amount did not lead to an increase in the maximal daily shrinkage of the trunk. In terms of this approach avocado is considered a ‘non-stress crop’, and must be irrigated with an amount causing the lowest possible trunk shrinkage for maximal yield and fruit growth. In the present study we investigated the possibility of using the maximal diurnal trunk shrinkage as a parameter for determining water status in avocado trees. Materials and Methods Avocado trees of the cultivar ‘Hass’ on seedling Degania 117 rootstock, growing in a commercial plantation of Kibbutz Ma’agan Michael on the Mediterranean coast of Israel, were studied during the 2000−01 and 2001−02 seasons, when the plantation was 5 and 6 years old. Spacing was 6 m between rows and 4 m between trees in a row. Water was applied through two lines of drip irrigation along the row placed on either side of the tree, 1 meter apart, with drippers spaced at 0.5-m intervals in each line. Irrigation frequency varied during the season, and ranged from one pulse of irrigation every 2 days in the spring and autumn to three pulses of irrigation a day during the hot summer. The “optimal” irrigation was based on the standard pan evaporation coefficients which varied during the season from 0.35 to 0.85, with corrections for the daily maximal trunk shrinkage. Fertilization through the irrigation water was adjusted where necessary to maintain a constant concentration. Two irrigation regimes were applied: the “optimal” irrigation (determined as described above), and “deficient” irrigation with about 30% less water than the optimal. For the optimal irrigation regime five replicates, each about 0.15 to 0.2 hectares, were established. For the deficient irrigation there were only three replicate of 0.2 to 0.25 hectares each. Data were collected by a Local Phytomonitoring Station (LPS; Phytech Technologies, Yad Mordechai, Israel), comprising data loggers, a computer program for data display, and a cellular forecasting system to transfer the data to a remote computer. Meteorological sensors were used to measure temperature and relative humidity. The volumetric humidity of the soil was measured by a Theta probe (Delta-T) from Phytech. Changes in trunk diameter were measured by electronic dendrometers (Phytalk; Phytech), using a Linear Variable Differential Transformer (LVDT). These dendrometers are sensitive enough to measure tissue fluctuations in micrometers. The dendrometer was set up with a screw inserted into the xylem in the center of the trunk, while its cylinder was in contact with the circumference of the trunk and moved with it (Fig. 1). Each dendrometer was connected to an electronic data logger and a forecaster, which transmitted the data through a cellular network to the remote computer. A computer program supplied by Phytech calculated the daily shrinkage and expansion of the trunk. Fluctuations in fruit size were measured by sensors FI-3EA from Phytech based on LVDT. For each treatment we used two electronic trunk dendrometers and two fruit sensors. The maximal diurnal trunk shrinkage and the rate of fruit growth were calculated according to the continuously monitored changes in the trunk diameter and fruit size. Each season the yield of every replicate was harvested separately, and the distribution of fruit size was determined according to commercial standards. Results and Discussion Background data. Rainfall during the winter of 2000/01, prior to the start of the study, was only 397 mm, compared to 823 mm in the winter of 2001/02. Similar amounts of irrigation were applied between April and November in the two seasons of the study, i.e., 8400 m per hectare under the optimal irrigation regime and 5900 m per hectare under the deficient irrigation regime. Daily growth and shrinkage of the trunk in the spring. The second half of Apr. 2001 was taken to be representative of the spring period. The last significant spring rains (85 mm) were in February, an additional 9 mm fell in March, and at the beginning of April the trees were receiving irrigation water according to the optimal (100%) or deficient (70%) irrigation regimes established for this study. Spring in Israel is characterized by periods of hamsin, when hot dry east winds cause a significant drop in the atmospheric vapor pressure (VPD). Between 15 and 30 Apr. 2001 there were three periods of hamsin. The first occurred on 18 Apr. and lasted for 1 day only. The others each lasted for 3 days (21−23 Apr. and 28−30 Apr.; Fig. 2). The maximal VPD (MaxVPD) during each hamsin was higher than 5 kPa (Fig. 2). In trees irrigated according to the optimal regime, during each hamsin there was a sharp decrease in the daily trunk growth until growth stopped altogether. In trees with deficient irrigation the decrease in the daily trunk growth was much more pronounced than the Fig. 1. Electronic trunk dendrometer device on an avocado tree. increase, and instead of trunk growth there was negative growth (shrinkage) of 0.02−0.07 mm per day (Fig. 2a). Trunk growth (mean ± SE) under optimal and under deficient irrigation during that period is recorded in Table 1. A. -5 0 5 10 April 2001