High GA-like concentrations in midsummer reduce generative bud development, and the highest GA-like substance in fruits occurred in June and July . These collective results suggest that gibberellins in developing seeds of fruits on one year old growth suppress development of fruit buds on the apical current year’s shoot growth, resulting in alternate bearing. Thus, alternate bearing is due to inhibition of floral induction , and floral induction in the current year is inhibited by fruit load from the previous year. There is a negative relationship between the current year’s flowering intensity and the previous year’s production . In ‘Manzanilla de Sevilla’ cultivar, 58% of the variance in the number of flowers was explained by production in the previous year. The current year’s yield depends on flowering intensity, which depends on the previous year’s production. This relationship was also found in pistachio. The yield of the previous-year harvest is most strongly and negatively correlated with the yield of current year . However, the specific mechanism of suppressing floral bud induction and development is unknown. The role of ethylene in precipitating plant organ abscission, including floral organs, is clearly defined .It increased to a maximum seven days after FB and this peak coincided with massive flower shedding . Inhibition of ethylene results in longer-lived carnation flowers . In the ethylene biosynthesis pathway, methionine is catalyzed to SAM by SAM synthetase, SAM is then converted to 1-aminocyclopropane-1-carboxylic acid by ACC synthase,macetas cuadradas and ACC is further converted to ethylene by ACC oxidase . The ACC synthase is the targeted compound in limiting ethylene synthesis in tomatoes .
The compound aminoethoxyvinylglycine competitively inhibits ACC synthase activity by binding to the substrate’s active site, preventing ethylene synthesis . AVG is now used on multiple fruit crops. AVG applications to apple trees during harvest slowed ripening, effectively extending the harvest period . AVG inhibited ethylene biosynthesis, increasing peach fruit quality . Similarly, AVG decreased ethylene generation and increased fruit firmness in plums post harvest . AVG treatment delayed cocoa flower abscission and decrease ethylene generation in pear flowers, increasing fruit set and yield . Similar results were found in apples . Whole-tree applications of AVG on ‘Regina’ and ‘Kordia’ cherry trees significantly improved fruit set and yield . AVG application in walnuts increased yield because the high ethylene concentrations in female flowers caused of pistillate flower abortion . The effect of AVG on ethylene suppression has been confirmed multiple times. For example, AVG prevented ethylene generation in rapeseed and sunflower plants . AVG applications inhibited fruit ethylene production in ‘Golden Supreme’ apples . The primary role of AVG is inhibition of ethylene biosynthesis . Extensive research of AVG applications in walnut demonstrated that ReTain™ applied to pistillate flowers successfully decreased ethylene production and resulted in reduced flower abortion . Subsequent orchard trials confirmed these findings. The AVG treated walnut trees produced higher yields than control trees because the AVG reduced pistillate flower abortion, improving fruit set . We hypothesized the short longevity of olive pistils is the primary reason for the low fruit set and yield in olives. We further hypothesized that AVG applications, as ReTain™, during bloom can potentially decrease ethylene generation by olive inflorescences, extend the pistil and ovule longevity for a longer effective pollination period, and increase yield.
Therefore, we tested the ability of AVG, as ReTain™, applied at 25 to 50% bloom to decrease ethylene production by olive flowers, extend the pistil viability, EPP, and potential successful fertilization, and increase fruit set and oil yield in Arbequina olives. The experimental site was a 12 year-old Oleae europeae cv. ‘Arbequina’ orchard located at 38.07°N, -121.21°W, farmed commercially by Lodi Farming Inc. The orchard comprises 59 rows oriented on a north-south axis with ~ 220 trees in each row. Trees are spaced at 5 feet inrow x 13 feet between rows, for 670 trees per acre. We designated this orchard as Orchard A. A randomized complete block design was used in the experiment, with six, eight-row blocks containing two, three-row sets within a block, separated by two buffer rows. The ReTain™ treatments were applied once in every block to three contiguous rows . The center row of the three contiguous rows was used to collect samples and yield data.In 2019, after spraying on May 13th, 2019 , sampling of ethylene evolution started and was repeated daily for 13 days, until there was no difference in ethylene generation between control and treatment trees. It rained on 10 of the 13 sampling dates. From each treatment in each block, two trees, each from the north and south ends of the row, were selected for ethylene measurements. From each tree, two uniform shoots, similar in floral load, facing two directions , and at equal position in the canopy were collected daily for 13 days. They were placed on ice and transported to the lab immediately. In the lab, ~ 0.6 g, or three to five inflorescences were cut from each shoot, weighed on a Mettler balance and placed in a 15 cm airtight test tube. The tubes were placed in a controlled temperature room at 20°C to equilibrate for 1 hour. After equilibration, 10 mL of air was withdrawn from each sealed tube with a 10 mL syringe and injected into Series 400, AGCCarle Gas chromatograph for ethylene measurement. Ethylene production was expressed as µL/.In 2019, olive inflorescence samples were collected and preserved in FAA solutions .
They were further dyed with Aniline Blue and observed using a fluorescence microscope. Pictures of olive ovaries were taken, and pollen tube widths were measured using ImageJ. The pollen tube width was used as a measure of pollen tube growth, which reflected the effect of ReTain™. The treated and control rows were individually harvested on November 15th, 2019, by a Vinestar canopy contact parallel bow rod harvester with a single detached fruit bin traveling at a ground speed of 1.5 mph. The weight of the detached fruit bin was determined before harvest. The fruit weight per row was then determined using a digital in-ground scale with the bin tare entered. After weighing, a 5 kg sample was collected and put on ice for transport to the UC Olive Center Laboratory of Dr. Selina Wang at UC Davis for oil quality analyses.The same experiment was not repeated in 2020 due to strong alternate bearing and a lack of flowers. However, a grower trial was performed; selected rows in two orchards were sprayed on with the same ReTain™ treatment and control, and ethylene emission was measured from inflorescences collected from the sprayed and unsprayed rows. The sprayed and control rows in Orchard B and C were harvested and yields compared on November 15th, 2020, in both orchards. The yield from Orchard A of the 2019 experiment was measured for comparison with the yield of 2019. In 2019, the six treated rows in Orchard A produced an average yield of 3320 lb, with a standard deviation of 408, while the 6 control rows produced an average of 2970 lb, with a standard deviation of 98. With a p-value of 0.067, the difference in yield between ReTain™ treatment and control rows was not significant when the alpha level equaled 0.05. However, it was significant when the alpha level equaled to 0.1 . In 2020, the six treated rows in Orchard A produced an average yield of 1160 lb, with a standard deviation of 290,maceta cuadrada plastico while the six control rows had an average of 1185 lb, with a standard deviation of 369. The p-value was 0.9, showing no significant difference in yield between ReTain™-sprayed and unsprayed rows . The yields across the orchards in 2019 and 2020 were also determined . In 2019, the significant difference in ethylene generation between treated and control trees suggested that ReTain™ worked as assumed: it decreased ethylene generation. Day 0 was denoted as the ReTain™ application date. The confidence bands for ethylene generation were very narrow before day 3 and after day 12 . During those days, some ethylene measurements appeared to be zero and were omitted, because it is impossible to have zero ethylene generation . Fewer ethylene measurements before day 3 and after day 12 resulted in narrow confidence bands. The primary finding was not affected: from day 3 to day 12, the ethylene generation from ReTain™-treated trees was significantly lower than that from control trees. We hypothesized that delayed floral senescence was a consequence of decreased ethylene generation; however, the visual inflorescence rating data suggested the opposite. The senescence ratings of treated trees were significantly higher than the control trees, indicating ReTain™ increased the rate of flower senescence. Throughout the experiment, the first-opened flowers senesced first.
The blocks on the edge of the orchard senesced earlier than blocks in the center of the orchard, east sides of the rows than the west, and the south end of the rows earlier than the north. The flowers in the row on the edge of the orchard and the ones on the south end of the rows also bloomed first. This could be potentially explained by that those flowers were at locations to receive the most heat. The visual evaluation of inflorescence senescence was based on petal color change and petal drop. It might not indicate the ovule viability throughout the experiment. Therefore, it is possible that the petal drop was not correlated with the effective pollination period and ovule viability. Aniline blue fluorescence was argued to be an accurate method to measure ovule senescence in olives The findings on pollen tube growth supports this possibility. There was no significant difference between treated and control flowers in pollen tube width. This suggested both treated and control flowers were pollinated equally, even though the treated flowers senesced more rapidly than the control flowers. The non-significant difference in pollen tube width suggested that pollination an ovule viability were not affected by ReTain™. It is possible that pollen tube width is not a good indicator of pollination. The pollen tube width is primarily a measurement of the amount of pollen deposited on the stigma that germinated and produced a pollen tube. We selected this measurement reasoning that a longer effective pollination period provides more time for pollen grains to land on the stigmas. Determining whether the pollen tube has reached the ovary might be a better indicator of successful pollination . However, after reaching the ovary, the pollen tube was unrecognizable . The tissue in the ovary was too thick to observe under a fluorescence microscope. In Arabidopsis, both ethylene-dependent and ethylene-independent pathways are required to initiate and progress through floral senescence . It is possible that the floral senescence in olives is not regulated primarily by ethylene. Pollination induces a series of post-pollination developmental events, including petal senescence . Pollination-triggered senescence has multiple advantages. Once sufficient pollen has been set on the stigma, additional pollen deposition is wasteful, and excess pollen tubes may compete for nutrients. In addition, maintenance of floral structures is costly . In 2020, the ReTain™ treatment was applied to two different orchards at the same concentration. However, no ethylene was detected from either treated or control flowers, which may be due to reduced number of flowers in an “off” year. In 2019, heavy rain during bloom could have prolonged the flowering time. The heavy rain and low temperature combined with the spray of ReTain™, while the control rows were not sprayed, could potentially explain the earlier senescence of the treated rows. The lowest temperature during the bloom reached 8°C, while the optimal temperature for olive pollen germination and pollen tube growth is 20 to 25°C . The low temperature during the bloom may have negatively impacted pollination in both ReTain™-treated and control trees. In 2019, the difference in yield between ReTain™-treated and control rows was not significant at 0.05 but was significant at 0.1. However, yield fluctuated greatly in treated rows, while the yield among control rows was stable. ReTain™ strongly improved yield in five out of six rows, but the increase was not consistent and the effect of ReTain™ was not uniform among rows .