RELATIONSHIP BETWEEN FERTILIZATION, IRRIGATION, VEGETATIVE VIGOR AND CANOPY EXPOSURE, FRUIT SIZE AND QUALITY, AND POSTHARVEST BIOLOGY OF BARTLETT PEARS

INVESTIGATORS:
David Ramos
Steve Weinbaum
Patrick Brown
Gordon Mitchell
Ken Shackel
Larry Schwankl
Glenn McGourty
Ron Snyder
Tom Muraoka
Gene Mayer
Bill Biasi

ABSTRACT

The project encompasses two phases. The objective of phase one, which is exploratory and being conducted this year, is to establish a differential in tree N status (as determined by leaf N concentration) and tree moisture status by differential N fertilization and irrigation. Irrigation treatments are approximately 100%, 85% and 65% of ET in two randomized blocks with high N (500 lbs N/acre/year) imposed on 8 individual trees within each irrigation regime. The experiment consists of a total of 48 selected trees, half receiving supplemental N, from which the following data are being collected from each tree: (a) Tree nutrient status - leaves were sampled from each tree in early July and late August and analyzed for leaf concentration of N and other selected essential nutrients; (b) Tree water status - tree water stress was determined in July by measuring trunk water potential; (c) Fruit size and quality and postharvest shelf life - fruit were harvested from each tree at two dates; yield, fruit size and maturity were measured at harvest (i.e., color, firmness, soluble solids and titratable acidity) and after air and controlled atmosphere cold storage. In addition, the relationship between fruit position in the tree canopy and fruit size, maturity/quality and shelf-life is being studied. The data are being compiled and analyzed to determine whether relationships exist between tree nutrient and water status, canopy position, and fruit attributes including shelf-life.

Introduction

There has been virtually no detailed work relating tree N and water status to vegetative vigor, yield, fruit size and quality and storage life in Bartlett pear. In addition to nitrogen, there is evidence that other essential plant nutrients can have significant effects on fruit quality and storage. The supply of calcium and boron have been shown to influence fruit quality and postharvest breakdown in a range of fruit species. Magnesium and potassium supply can impact fruit quality attributes such as acidity, pH and sugar content as well as influencing fruit color. This study integrates the combined expertise of extension personnel and researchers specializing in production physiology, tree nutrition and postharvest fruit physiology. The objectives are to assess the relationship between N fertilization, leaf N concentration, tree nutrient and water status, vegetative vigor and fruit canopy position, and fruit size and quality at harvest and following storage. This was the first year of a two year exploratory project which is being conducted to establish a differential in tree N status (as determined by leaf concentration) and tree moisture status by differential N fertilization and irrigation. Besides developing background information and laying the ground work for research in the coming season, we have made some progress towards meeting the overall project goals. These will be described in relation to the specific objectives.

Objective 1. Influence of N fertilization and differential irrigation on leaf N concentration, tree water status, and fruit size.

Procedure

Initially it had been anticipated that this phase of the project, particularly pertaining to N status, might be approached through orchard survey and sampling coupled with analyses of associated historical packinghouse records. Unfortunately, it was soon learned through discussions with Broc Zoller and others in Lake County, and David Sugar and Tim Righetti in Oregon that this approach was not feasible. The problem appears to be the large variability that exists between and within tree canopies that greatly complicates tree/orchard sampling in terms of obtaining comparative fruit samples. Our conclusion was that there needed to be a concentrated effort this year within an individual orchard which would allow us to work with individual trees and even individual fruits according to canopy position. Once we are able to characterize these intra-canopy effects (e.g. inside versus outside fruit etc.) on fruit development (i.e. nutrient concentrations, soluble solids, firmness, color, storability, etc.) we might then be able to incorporate this information in implementing orchard scale experiments involving preharvest factors (e.g. nitrogen rate trials).

It also became evident during our initial planning that it was not realistic to attempt to assess the effect of tree nitrogen status without also taking into account tree water status since it is widely accepted that irrigation, particularly just prior to harvest, can greatly influence fruit size and quality. Based on these findings, the decision was made to set up a differential nitrogen fertilization and irrigation experiment within a single orchard.

The site chosen for the experiment was a block in the Thomas Brothers orchard in Ukiah consisting of 239 trees at a spacing of 12' x 20' (1.3 acres). It is farmed separately from adjoining pear blocks making it an excellent, isolated experimental unit. In addition, the nitrogen fertilization practices in the last five years have consisted of an annual application of 92 lbs of actual nitrogen per acre. This is a relatively low application rate which allows us to apply additional nitrogen to selected trees within the orchard in order to attempt to set up a differential in nitrogen status. It was felt that this would be more efficient than going into a high nitrogen status orchard and attempting to withhold nitrogen on selected trees over several years in order to lower the existing N level.

The irrigation treatments in the Thomas orchard were established by utilizing smaller nozzles to produce the desired reduction in amount of applied water and indirectly set up a differential in the amount of evapotransporation (ET) demand which is met. This allows the orchard to be irrigated at one setting (e.g. 36 hours per irrigation) and still provide differing amounts of applied water per irrigation. Three irrigation treatments were established with each treatment randomized in two blocks. The 100% treatment (current grower practice) involved simply retaining the nozzles already in place (3/32"). Two smaller nozzles were installed to establish the other treatments. A "50 drill" nozzle was used to obtain approximately 80% of the current applied water. A 1/16" nozzle was used to establish the third treatment of approximately 65% of current irrigation. A catch can test was performed on July 10, 1992 to determine the uniformity of sprinkler application in the three treatments, and the results are shown as three D plots in figure 1, figure 2 and figure 3. Analysis of the data is shown in Table 1. The results indicate that we hit our treatments pretty much as planned. It was hoped that the "50 drill" treatment would give approximately 80% of full water and it turns out to be slightly above that (85%) which is probably more than acceptable. The 1/16" nozzles gave approximately 65% of the 3/32" treatment as desired. The uniformities are a little on the low side but it is important to note that the application rates are also quite low which matches the soil intake rate. It was felt that the large root zone of the tree can integrate the non-uniformity so that it should not adversely impact the experimental results. The orchard received three irrigations (May 27, June 12, June 27) prior to renozzling. Starting on July 10, four differential irrigations were applied (July 10, August 11, September 8, September 24). Each irrigation was about 36 hours.

To establish a nitrogen differential, four experimental trees in each of the six irrigation plots (3 treatments x 2 blocks) received five nitrogen fertilizer broadcast applications equivalent to 100 lbs N per application or 500 lbs of actual N for the year. Four of these applications were preharvest (May 13, June 8, June 17, July 9) using calcium nitrate. The fifth application was postharvest (September 4) using urea. The switch to urea was made on the basis of some experimental work conducted several years ago which showed that pears seem to take up ammonium nitrogen more readily than nitrate and it is our intention to establish as high a level of nitrogen status as possible in the fertilized trees. Four comparable trees in each of the six irrigation plots were identified and left unfertilized as controls for the nitrogen treatments. Thus, there are a total of 24 high nitrogen fertilized trees and 24 control trees in the experiment. Leaf samples were taken from each of these trees on July 2 and again on August 31 to determine tree nitrogen status. Analyses were also run on other essential plant nutrients (e.g. P, K, Ca, Mg, B, Zn, Mn, Fe and Cu). In addition, soil samples were taken on August 18 in the upper three feet of the root zone to monitor the status of nitrate nitrogen in the soil profile as a result of the fertilizer applications.

Results

Tree water potential was measured mid-season on two clear, warm days (7/9/92 and 7/24/92) using a pressure chamber (AKA "The Bomb") at midday on all 48 treatment trees. Results were very consistent between the two sample dates: some trees were under stress, but the level of stress was apparently not related to the three irrigation levels (100%, 85%, 65%) that had been imposed (figure 4). In general, trees located in the East block (block 2, closest to the Russian River), especially the SE corner, had very high water potentials (were not stressed) relative to the trees of the West block, especially trees in the NW corner. This was particularly interesting because the SE and NW corners both corresponded to the lowest (65%) irrigation treatment. Statistical analysis showed no significant irrigation treatment effect on average fruit size (gram per fruit) or any measure of yield per tree (total weight, number of fruit) or per trunk XS.

Even though irrigation treatments did not influence fruit yield or size (figure 5), there was a clear correlation between fruit size and tree water potential size (figure 6). Size ranged from 120 g/fruit in trees under some stress (-1.5 MPa) to 160 g/fruit in trees under little stress (-O.7 MPa). Statistical analysis (stepwise regression) showed that of all the parameters which could influence fruit size (water, N, K, crop load, etc.) the most significant single factor was tree water potential. These results indicate that pear fruit sizing appears to be very sensitive to tree water stress, and that irrigation quantity may only be one of a number of factors (root distribution, soil characteristics, etc.,) that determine what that level of stress will be.

The leaf analyses from the early sampling date (July 2) showed that nitrogen levels were nearly the same size (figure 7 and figure 8), with an average of 2.31% for fertilized trees and 2.26% for unfertilized ones. This difference was significant at the 7% level but not statistically significant at the 5% level. This suggested that a trend might be developing but that a clear difference in tree nitrogen status had not yet been established in July. However, by the late leaf sampling date (August 31) the difference between fertilized and unfertilized trees was highly significant, P = .0001 (figure 9 and figure 10). On that date, leaf nitrogen concentration was 2.04% in the fertilized trees compared with 1.89% for the unfertilized trees. This suggested our summer and fall nitrogen fertilizer applications were being utilized by the trees. Soil analyses confirmed that there was substantially more nitrate nitrogen in the profile near fertilized trees than in the soil associated with the unfertilized trees.

Objective 2. Effect of N fertilization and differential irrigation/tree water status on fruit maturity, quality and storability.

Procedure

The 48 selected test trees in the experiment were divided into three vertical sectors for harvest. The plan was to harvest all trees three times with one third of each tree harvested during early, mid and late season pickings. Unfortunately, the shortness of season limited harvest to two pickings. The first pick was on July 22 and the second, one week later, on July 30. The fruit designated for normal (air) cold storage was brought to Davis and weighed, counted, sorted and a sample used for initial workup before storage. The initial workup on a 20 fruit sample from each tree (10 large and 10 small) included color, firmness, percent soluble solids, and percent titratable acidity. This procedure was followed for both first and second pick fruit from 36 of the trees (18 high N and 18 low N) which were then stored for up to four months under normal cold storage (30 F) at Davis. The fruit from the other 12 trees (6 high N and 6 low N) were placed under CA storage (first pick only) at Ukiah. Samples of fruit from both first and second picks held under air storage at UCD were pulled after two and three months and worked up for color and firmness. At four months, all of the fruit (air and CA storage) were evaluated both before (color and firmness) and after ripening (scald, internal breakdown, etc.).

Results

Data on maturity/quality and postharvest condition are being summarized for computerization and analysis. We will attempt to see what relationships may exist between yield, size, maturity, quality and storability as influenced by tree water and nitrogen status.

Objective 3. Effects of fruit location within a tree canopy.

Procedure

In order to determine the influence of canopy position on fruit size, quality and storability, 75 fruitful spurs were tagged in each of three locations in the canopy of five trees on July 2. The three exposure conditions were inside shaded, top exposed, and outside semi-exposed. Two leaves were collected from each tagged spur and brought to Davis for determination of specific leaf weight (SLW). SLW or leaf mass per unit leaf area is determined by measuring leaf area, then drying and determining the dry weight of the leaf. It has been shown that SLW and N increase with available light within tree canopies. Thus, SLW can be indicative of natural light exposure in a pear canopy and perhaps allow us to use this measurement to assess the effects of light exposure on fruit development associated with these tagged spurs.

Just prior to harvest, fruits were identified with the corresponding spur numbers to allow them to retain their identity with the spur on which they were borne. All tagged fruits were harvested at the time of the first pick of the experimental block (July 22) and brought to Davis for evaluation and storage. The fruits were categorized according to 3 light exposure levels based on SLW (i.e. SLW 6-8 [highly shaded], 9-10 [intermediate], 11-13 [highly exposed]) and subdivided into three groups. One sample representing each SLW treatment was used for the initial workup (size, color, firmness, percent soluble solids, and percent titratable acidity). The data were recorded on the basis of individual spur/fruit number. A second set of samples were stored for three months in air storage and worked up in the same manner. The third set of samples were also removed from cold storage at three months and ripened and evaluated (size, color, firmness, internal condition). In addition, representative samples of the stored/ripened fruit were freeze dried and analyzed for N, Ca, Mg, K, and B concentrations. Cuticular examination was conducted on some of the stored fruit to determine if there were differences in thickness or other features which might affect susceptibility to fungal pathogens.

Results

All of the data from the fruit location study have been computerized and are being analyzed. We will attempt to determine what correlations may exist between various fruit parameters and the specific leaf weight associated with the spur positions.

Conclusion

Data on fruit maturity/quality and postharvest condition are still being analyzed and results are not available for the annual report. These data will form the basis upon which research will proceed in the second year of this two year study. In the coming season, water application rates, distribution patterns and infiltration/penetration will be more closely monitored with the aid of neutron probe measurements commencing early in the season in an attempt to establish a stronger relationship between tree water stress and the three differential irrigation treatments. In addition, it is anticipated that there will be a nitrogen differential established since results of the August leaf sampling show a statistically significant difference in leaf N concentration between trees receiving supplemental N and those unfertilized. A more intensive study of tree positional/exposure effects on fruit development will be undertaken with particular emphasis on attempting to develop this information for determining the best way in which to sample trees/orchards to assess the influence of various preharvest factors on fruit maturity/quality and postharvest condition.

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