It has been reported that mild water stress applied during the intermediate developmental period of slow fruit growth has no effect on crop yields but significantly reduces vegetative growth in peach . Fruit developmental stages may differ in time of initiation and duration among peach varieties, an example of this would be an early vs. late harvested cultivar as demonstrated by . Fruit growth occurs in stages from fruit set to harvest, in all cultivars, and during the final growth phase of peach fruit is when 65% of a fruit’s dry weight and 80% of a fruit’s fresh weight are accumulated . Available water varies throughout the growing season, including diurnal fluctuations brought on by daily temperature fluctuations , day-to-day changes brought on by a shift in evapotranspiration , and possible seasonal changes brought on by the formation of new xylem . Water conduction in the tree is largely dependent on newly formed xylem each spring and the new xylem cells are smaller in size-controlling root stocks. It is thought that the spring flush of vegetative growth is limited in trees on size controlling root stocks compared to growth on vigorous root stocks because of temporary reductions in root hydraulic conductance caused by smaller xylem vessels. A question that arises from these findings, does the reduction of water conductance in dwarfed peach trees also limit fruit growth? In peach production, fruit size is often manipulated with the use of a management practice known as fruit thinning. With fruit thinning, shortly after fruit set, a portion of immature fruit is removed from the tree to reduce carbohydrate competition among those remaining. It is widely recognized that fruit size is largely influenced by crop load, with larger fruit size obtained as the crop load is reduced . Quality of fruit may also be affected by crop load, nft growing system low-cropped trees have been shown to produce larger and firmer fruit than those from heavily cropped trees .

Although minor in comparison to carbohydrate demand, fruit size may also be diminished by inducing higher water stress with larger crop loads. An experiment by found that larger crop loads were responsible for reducing midday stem water potential in nectarines. MacFayden et al., concluded that an increased crop load also increased the fruit water deficit which may reduce fruit growth in peach. According to another study by , root stocks also influenced the crop load’s effect on fruit size, and more vigorous root stocks had larger fruits at specific crop loads. The for ementioned findings relay the importance of better understanding the relationship between fruit size and crop load among vigorous and reduced-vigor root stocks. While crop load per tree is controlled by thinning, crop load per area is most influenced by planting density. The reduced vigor and overall size of trees on size-controlling root stocks facilitates the establishment of high-density plantings . The primary principle in establishing an appropriate planting density for an orchard using trees on size controlling root stocks is that total tree dry matter production and crop yield are related to total light interception . This principle holds for essentially all crops . However, although higher light interception often leads to higher yields, yield may also vary significantly with other environmental stressors such as available water, nutrients, temperature, and amount of time the fruit has for growth . Orchard systems with increased planting densities have also been shown to reach maximum yield capacity earlier than conventional plantings since the trees are able to fill out their allotted space more quickly . In a small trial using the ‘Summer Bright” nectarine cultivar, trees that were pruned to a standard height of 12 to 13 feet or limited to heights of 8 or 9 feet produced similar sized fruit and crop yields. The reasoning for this was that, despite the height difference, both tree shapes had equal planar volume and therefore intercepted similar amounts of photosynthetically active radiation . The goal of this study was to address three production characteristics and their relationship with four different orchard systems. 1) Fruit size: can peach orchard systems using trees on size controlling root stocks produce fruit of equal size compared to orchard systems with trees on vigorous root stocks? 2) Fruit count: if crop load per area is similar among size-controlling and vigorous systems is fruit size also similar? 3) PAR interception and yield: is there a difference in the relationship of fruit production vs light interception among orchard systems with vigorous root stocks and those with size-controlling root stocks?

A better understanding of production capabilities will allow researchers and growers to better estimate the potential of an orchard system on size-controlling root stocks as a commercially viable option.In April 2015, an orchard system trial was established at the University of California Kearney Agricultural Center, Parlier, CA. The research block consisted of two peach [Prunus persica Batsch] scion cultivars, June Flame and August Flame grafted onto three different root stock genotypes: HBOK 27 , P-30-135 , and Nemaguard . Controller 6 was used in two of the four training systems . The C-6 V was a high-density planting system with an in-row spacing of 1.2m and trained to the KAC-V perpendicular V pruning system . The C-6 Quad system was pruned to a Quad V where four main scaffolds are selected in each tree and pruned to resemble an open vase, the system also had a larger in-row spacing of 2.4 m . The Controller 9 Quad system was identical to the C-6 Quad system with the only difference being the root stock. Between-row spacing was 4.6m in all systems using size controlling root stocks. Nemaguard was used as the commercial standard root stock with a planting density of 2.4m in-row spacing and 5.5m between-row spacing . Shortly after harvest, orchard systems using size-controlling root stocks were topped to a height of 2.5m while systems using the Nemaguard root stock were topped at 3.5m. The four systems were divided into three replications for each of the two scion cultivars making a total of eight unique orchard systems. Each replication consisted of four rows of trees with the northern and southern most rows used as guard rows, the first and last two trees in each data row were also considered guard trees making nine trees in each of the two inner rows the sample size per replication . In total, each cultivar was represented by approximately 54 data trees . All systems with size-controlling root stocks were irrigated and fertigated using sub-surface drip to maintain a soil moisture between -20 and -60 cbar throughout the growing season. Microsprinklers were used for irrigation and fertigation in the Nema Quad system. The soil at the site is a well-drained Hanford, fine sandy loam.

Weeds were controlled by mowing the row middles and applying herbicides to maintain a 1.5m wide weed-free strip down the tree rows. All systems received a light summer pruning and heavy dormant pruning to establish desired structure and improve light interception. Approximately a week before harvest, total canopy light interception using a ACCUPAR LP- 80 meter was measured in each plot. Harvest occurred on two or three separate days, depending on root stock and cultivar,nft hydroponic system during the growing season due to variance in fruit maturity, as is common in stone fruit production. Each data tree was harvested individually, total quantity of fruit produced, and total fruit weight data were recorded which enabled calculation of mean fruit weight per tree. Harvest data were collected for growing seasons 2017-2019. A linear model was created in R markdown for each season and cultivar’s harvest. With each linear model, an ANOVA test was conducted using a 95% confidence level and Dunnett’s method adjustments to identify significant differences among the four orchard systems for both scion cultivars. A true significant difference was concluded if the comparison between two systems had a p-value less than 0.05, a t-ratio greater than 1.68 , and a confidence level range that did not include 0.During the 2017 harvest season for the June Flame cultivar, trees in all systems produced commercially acceptable mean fruit size, >200g per individual fruit . The C-6 Quad system produced significantly larger fruit when compared to the Nema Quad system . In the June Flame 2018 harvest, mean individual fruit weights in all systems were again > 200g, . Although the C-9 Quad system produced large enough fruit for fresh market sale, the mean individual fruit weight was significantly less compared to fruit in the Nema Quad system . The harvest season of 2019 for June Flame had individual fruit weight above 200g in all systems . However, it should be noted that the C-6 V and C-9 Quad systems produced significantly smaller fruit compared to the Nema Quad system . In all three seasons for the June Flame cultivar, the C-6 Quad system produced fruit of equal or larger size than the Nema Quad system. In the 2017 harvest season for the August Flame cultivars, all systems with size-controlling root stocks produced significantly larger fruit than the Nema Quad system . In a few cases, fruit size exceeded 300g per fruit in systems with size controlling root stocks, >50% larger than the minimum requirement for large sizing in the fresh market . The harvest season of 2018 for August Flame may have been the most productive of all years for both cultivars, all systems exceeded 250g in mean individual fruit weight .

The C-6 Quad system had significantly larger fruit than the Nema Quad system while C-9 Quad had significantly smaller fruit . In the 2019 harvest season all systems produced fruit sizes above 200g but were smaller than fruit from previous seasons . Although the C-6 V and C-9 Quad systems did not differ significantly from the Nema Quad system, the C-6 Quad system produced significantly smaller fruit compared to the Nema Quad system .During the 2017 harvest season of the June Flame cultivar, the C-6 V and C-9 Quad systems produced significantly fewer fruit per hectare compared to the Nema Quad system. There was no significant difference between the C-6 Quad and Nema Quad systems . For June Flame in 2018 there were no significant differences in yield per hectare among the C-6 V, C-6 Quad and Nema Quad systems. The C-9 Quad system produced significantly fewer fruit per hectare compared to the Nema Quad system . The 2019 June Flame harvest had close to identical fruit count per hectare between the C-6 Quad and Nema Quad systems .Once again, the C-9 Quad was the only system that produced significantly fewer fruit per hectare compared to the Nema Quad system . In August Flame’s harvest of 2017 there were no significant differences among systems using size-controlling root stocks and the Nema Quad system for fruit produced per hectare . In the 2018 harvest season for August Flame, the C-9 Quad system produced significantly fewer fruit per hectare compared to the Nema Quad system . Meanwhile the C-6 V and C-6 Quad systems maintained similar fruit counts per hectare as the Nema Quad system. In the 2019 harvest for August Flame the C-6 V system produced significantly fewer fruit per hectare than the Nema Quad system. The C-6 Quad and C-9 Quad systems did not differ significantly for fruit count per hectare compared to the Nema Quad system . It should be noted that during the 2019 harvest season some trees displayed signs of water stress in the field which may have hindered production and skewed results for that season.During the 2017 harvest of the June flame cultivar, there was a significant difference in the slope of the relationship between fruit size and fruit per hectare among the C-6 V and the Nema Quad systems . Data from all systems fit a linear model that had a negative correlation between fruit size and fruit per hectare. Although a negative correlation was visible between fruit size and fruit per hectare in the C-6 V system, its magnitude was not as steep as with other systems in the same season . The following season, 2018, for June Flame there were significant differences in the fruit size vs. fruit per hectare relationship among systems.