Applying the Dickson quality index (DQI) is an efficient but destructive and time-consuming method for predicting sapling quality in nurseries. Fast and low-cost prediction of the quality, establishment, and growing strength of fruit tree saplings in the orchard through the identification of morphological characteristics, which are so highly correlated with this index, is necessary for different stages of sapling production.
Materials and Methods
To evaluate the relationship between the morphological characteristics in peach and nectarine saplings, bare-root saplings of 16 commercial cultivars with three replicates from 10 nurseries, located in four provinces, were studied. The samples provided from two types of non-certified sapling, grafted on the seedling rootstocks, and certified sapling on the vegetative rootstocks during the transplanting seasons of 2020-2021. Sapling height, diameter above grafting line (DAGL), root length, number of roots and branches, shoot and root fresh and dry weights, height to diameter ratio, and DQI were evaluated, and simple correlations between them were estimated. The correlation coefficients were broken down into direct and indirect effects through path analysis, with DQI as the dependent variable.
Results and Discussion
The results showed that the average DQI for non-certified saplings produced in different climates was 14. The higher values occurred in areas with more than 3100 accumulated growth degree-days (AGDD). In certified saplings, DQI reached to 47, which is likely due to more biomass and higher sturdiness (less height to diameter ratio) of certified seedlings. These seedlings were grafted on GF 677 vigorous rootstocks. In addition, appropriate geographical location of certified nursery in case of AGDD might have been a factor in increasing the quality, and also the DQI of these seedlings. Seedling provided from certified nursery also have a better chance of being free from harmful pathogens including viruses, thereby may leading to seedlings with higher DQI. The linear relationship between seedling height and the number of branches is remarkable in case of fruit precocity. As the height of the sapling increases, the number of branches increases more than the number of roots, which results in photosynthetic capacity improvement. However, in stressful conditions, as the photosynthesis rate and plant growth decreases, the absorption of water by the newly developed roots does not compensate the seedling's transpiration, thereby the taller seedlings may die faster. Therefore, taller stone fruit seedlings will not necessarily be a better seedling after planting in the garden. Based on the results of the path analysis for DQI evaluation, the highest direct effect was related to the DAGL, which indicates the significance of this index in evaluating the quality of seedlings.
Standard seedlings have a healthy, developed root system without abnormal symptoms. In addition, the selection of peach and nectarine seedlings, which are thicker than 13.6 mm above the grafting line means direct selection of seedlings having more root and shoot dry weight, higher sturdiness quotient and DQI higher than the average. Such seedlings are likely to be more vigorous and of higher quality and will have a better establishment in the orchard. Among the evaluated morphological indices, DAGL is the most efficient index in the quality assessment of the saplings due to having the highest correlation with the DQI and most morphological traits.