Effect of solution pH on root architecture of four apple rootstocks grown in an aeroponics nutrient misting system.

The pH of the solution in the rhizosphere is an important factor that determines the availability and mobility of nutrients for plant uptake. Solution pH may also affect the root distribution and architecture of apple rootstocks. In this study, we evaluated the effect of solution pH on root system development of apple rootstocks using an aeroponics system designed and developed at Cornell AgriTech Geneva, USA. Four Geneva® apple rootstocks (G.210, G.214, G.41, and G.890) were grown in an aeroponic system under nutrient solution misting featuring continuously adjusted pH levels to three pH treatments (5.5, 6.5, and 8.0). Root development was monitored for 30 days and evaluated regularly for distribution and root mass. Images of the developed roots grown in the aeroponic system were collected at the end of the experiment using a high-resolution camera and analyzed using GiA Roots® software, which generates root architecture parameter values in a semi-automated fashion. The resulting root architecture analysis showed that the Geneva® rootstocks were significantly different for two architecture parameters. The length-to-width ratio analysis represented by two GiA Roots parameters (minor-to-major ellipse ratio and network width-to-depth ratio) showed that G.210 was flatter than G.890, which had a greater tendency to grow downward. Rootstocks G.214 and G.41 displayed similar growth values. The solution pH affected most root architecture parameter measurements where overall root growth was higher at pH 8 than at pH 5.5 and 6.5, which showed similar growth. In general, the average root width tended to decrease at higher pH values. While there were no significant differences in the leaf nutrient concentrations of P, K, Ca, Mg, S, B, Zn, Cu, and Fe within the four rootstocks, the pH level of the solution had a significant effect on P, Ca, and Mn. This study is the first of its kind to investigate the effect of pH on root architecture in a soil-free (aeroponic) environment and may have implications for apple root behavior under field conditions where pH levels are different.

Trunk Water Potential Measured with Microtensiometers for Managing Water Stress in "Gala" Apple Trees.

The weather variations around the world are already having a profound impact on agricultural production. This impacts apple production and the quality of the product. Through agricultural precision, growers attempt to optimize both yield and fruit size and quality. Two experiments were conducted using field-grown "Gala" apple trees in Geneva, NY, USA, in 2021 and 2022. Mature apple trees (Malus × domestica Borkh. cv. Ultima "Gala") grafted onto G.11 rootstock planted in 2015 were used for the experiment. Our goal was to establish a relationship between stem water potential (Ψtrunk), which was continuously measured using microtensiometers, and the growth rate of apple fruits, measured continuously using dendrometers throughout the growing season. The second objective was to develop thresholds for Ψtrunk to determine when to irrigate apple trees. The economic impacts of different irrigation regimes were evaluated. Three different water regimes were compared (full irrigation, rainfed and rain exclusion to induce water stress). Trees subjected the rain-exclusion treatment were not irrigated during the whole season, except in the spring (April and May; 126 mm in 2021 and 100 mm in 2022); that is, these trees did not receive water during June, July, August and half of September. Trees subjected to the rainfed treatment received only rainwater (515 mm in 2021 and 382 mm in 2022). The fully irrigated trees received rain but were also irrigated by drip irrigation (515 mm in 2021 and 565 mm in 2022). Moreover, all trees received the same amount of water out of season in autumn and winter (245 mm in 2021 and 283 mm in 2022). The microtensiometer sensors detected differences in Ψtrunk among our treatments over the entire growing season. In both years, experimental trees with the same trunk cross-section area (TCSA) were selected (23-25 cm-2 TCSA), and crop load was adjusted to 7 fruits·cm-2 TCSA in 2021 and 8.5 fruits·cm-2 TCSA in 2022. However, the irrigated trees showed the highest fruit growth rates and final fruit weight (157 g and 70 mm), followed by the rainfed only treatment (132 g and 66 mm), while the rain-exclusion treatment had the lowest fruit growth rate and final fruit size (107 g and 61 mm). The hourly fruit shrinking and swelling rate (mm·h-1) measured with dendrometers and the hourly Ψtrunk (bar) measured with microtensiometers were correlated. We developed a logistic model to correlate Ψtrunk and fruit growth rate (g·h-1), which suggested a critical value of -9.7 bars for Ψtrunk, above which there were no negative effects on fruit growth rate due to water stress in the relatively humid conditions of New York State. A support vector machine model and a multiple regression model were developed to predict daytime hourly Ψtrunk with radiation and VPD as input variables. Yield and fruit size were converted to crop value, which showed that managing water stress with irrigation during dry periods improved crop value in the humid climate of New York State.

Isolation and characterization of genes associated with shade-induced apple abscission.

Apple trees generally produce excessive number of flowers and young fruitlets, which negatively affects commercial value. Thus, thinning is an important practice by which excessive flowers and fruitlets abort. Fruit abscission is difficult to predict or control, as many factors including shading can cause fruitlets to abort. In order to understand the molecular basis of shade-induced fruit abscission, two cDNA libraries were constructed using the suppression-subtractive hybridization (SSH) method and 347 expressed sequence tags (ESTs) were obtained. 168 ESTs represent transcripts that are preferentially expressed after 24 h of shading, and the other 179 are derived from RNAs of small apple fruits that were shaded for 72 h. Sequence analyses revealed that these clones represent 68 (24 h) and 44 (72 h) unique genes; these genes belong in eight functional categories. The largest set of genes is related to carbohydrate metabolism including the sorbitol 6-phosphate dehydrogenase (S6PDH) gene that was generally believed not to be expressed in young fruits, while the second largest group contains unclassified or unknown genes. RNA gel blot analysis confirmed that at least 26 genes are up-regulated after shade treatment. Some of these known genes may serve as molecular markers for apple monitoring the induction of fruitlet abscission. Improved genetic understanding is critical to the development of targeted abscission agents to better control apple crop loads and optimize apple production.

Fire Blight Resistance of Budagovsky 9 Apple Rootstock.

Erwinia amylovora, the causal agent of fire blight, can cause a fatal infection of apple rootstocks known as rootstock blight. Budagovsky 9 (B.9) apple rootstock is reported to be highly susceptible when inoculated with E. amylovora, although results from multiple trials showed that B.9 is resistant to rootstock blight infection in field plantings. Conflicting results could stem from genetic variation in the B.9 population, appearing as phenotypic differences in rootstock material. However, genetic testing, using 23 microsatellite loci, confirmed the clonal uniformity of B.9 in commerce. Variation in growth habit between B.9 rootstocks originating from two nurseries also has been discounted as a source of disease resistance. Instead, results indicate a possible novel resistance phenotype in B.9 rootstock. B.9 rootstock was susceptible to leaf inoculation by E. amylovora, statistically similar to the susceptible rootstock Malling 9 (M.9). Conversely, inoculation assays targeting woody 4- to 5-year-old tissue revealed a high level of resistance in B.9, whereas M.9 remained susceptible. Although the mechanism by which B.9 gains resistance to E. amylovora is unknown, it is reminiscent of age-related resistance, due to an observed gain of resistance in woody rootstock tissue over succulent shoot tissue. Durable fire blight resistance correlated with tissue development could be a valuable tool for rootstock breeders.