To test the influence of avocado rootstocks on the scion leaf ionome, the nutritional status of avocado cultivars Hass and Ettinger grafted onto 15 rootstocks was compared over 3 years. The rootstocks were of different genetic origins (West Indian or Mexican) and were clonally or reproductively propagated. The trees were grown in a high-density orchard, and were continually fertigated as is common in modern avocado orchards. Leaf mineral composition was analysed and found to be correlated with crop load. ‘Hass’ leaves had significantly higher levels of B, Ca, Mg, Na, P and K than ‘Ettinger’ leaves. Rootstocks of Mexican origin produced higher foliar Cl levels, but lower levels of Mg and Mn. Rootstocks grown from seedlings conferred higher foliar K and lower B, Ca and Mg. The results demonstrate that avocado rootstocks affect the nutritional status of the tree, by a differential mineral transport, which is indicated by the scion leaf ionome.

The olive growing sector is transitioning from traditional to intensive irrigated cultivation, dictating a need to reconsider orchard management practices including fertilization. Potassium (K) is an essential nutrient, typically found in high concentrations in plants. Orchard K fertilization requirements are commonly derived from the disparity between assumed tree requirements and extractable soil K. The long-term impact of insufficient fertilization on K available in the soil, growth, and yield of irrigated field-grown olive trees was evaluated over six consecutive seasons. Withholding of K fertilization led to lower exchangeable and soluble K concentrations in the soil and significantly impaired yield. The reduction in yield was attributed to reduced flowering and fruit set, resulting in a lower fruit number. Tree vegetative growth and flowering quality traits were not affected. In addition, trees not receiving K appeared to be more susceptible to alternate bearing. Following two seasons of omitting K fertilization, leaf K concentration did not decrease below the conventionally accepted sufficiency threshold for olive (0.8%). In spite of this, the trees produced significantly lower yields. Our results suggest that long-term insufficient K fertilization results in reduced soil available K and consequently impairs tree productivity. The results imply that the sufficiency threshold for K in diagnostic leaves should be reconsidered for intensive orchards. Moreover, the current method for K deficiency detection using leaf K concentration may be inadequate for intensive orchards. Integration of other parameters, such as fruit K content, leaf Na, and changes in soil exchangeable K content or sorption energy, may promote a more reliable analysis of orchard K nutritional status.

The global avocado industry is growing, and farmers are seeking to expand their plantations. However, many lands suitable for avocado planting were previously cultivated with hosts of the soil-borne fungal pathogen Verticillium dahliae, which is the causal agent of Verticillium wilt (VW). VW can seriously impair avocado orchards, and therefore, planting on infested soil is not recommended. The use of different rootstock types allows avocado cultivation in various regions with diverse biotic and abiotic constraints. Hence, we tested whether genetic variance among rootstocks may also be used to manage avocado VW. Six hundred trees, mostly Hass and some Ettinger, grafted on 23 selected rootstocks were evaluated for five years in a highly V. dahliae-inoculated plot for VW symptoms, fungal infection, and productivity. The selected rootstocks displayed a significant variation related to VW tolerance, and productive avocado rootstocks with potential VW tolerance were identified. Moreover, the rootstock productivity appears to correlate negatively to the susceptibility level. In conclusion, planting susceptible rootstocks (e.g., VC66, VC152, and VC26) in infested soil increases the likelihood of massive tree loss and low productivity. Whereas, tolerant rootstocks (e.g., VC804 and Dusa) may restrict VW and enable avocado cultivation on infested soils.

Phosphorus (P) availability significantly impacts olive tree reproductive development and consequential fruit production. However, the importance of P fertilization in olive cultivation is not clear, and P application is usually recommended only after P deficiency is identified. In order to determine the long-term impacts of continuous P fertilization in intensive irrigated olive cultivation, the growth and production of trees in an intensive orchard with or without P fertilization were evaluated over six consecutive seasons. Withholding of P resulted in significant reduction in soil P quantity and availability. Under lower P availability, long-term fruit production was significantly impaired due to reduced flowering and fruit set. In addition, trees under conditions of low P were characterized by higher alternate bearing fluctuations. Olive tree vegetative growth was hardly affected by P fertilizer level. The impairment of tree productivity was evident in spite of the fact that leaf P content in the treatment without P fertilization did not decrease below commonly reported and accepted thresholds for P deficiency. This implies that the leaf P content sufficiency threshold for intensive olive orchards should be reconsidered. The results demonstrate the negative impact of insufficient P fertilization and signify the need for routine P fertilization in intensive olive cultivation.

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Avocado (Persea americana Mill.) is an economically important tree crop that exhibits a high rate of immature fruit abscission (IFA), reducing yield. As the seed coat derived from recently abscised fruitlets displays a senescence phenotype, we hypothesized that the seed coat plays a critical role in initiating IFA. Here, we show that fruitlets fated to abscise undergo growth arrest before shrinking and detaching from the tree. Comparative RNAseq analysis indicates that growth arrest is associated with a transcriptome reprogramming that is first initiated in the seed coat then transmitted to pericarp and embryo. Gene expression and hormone profiling results indicate that fruitlet growth arrest is associated with a decline in auxin activity and an increase in abscisic acid, the ethylene precursor, 1-aminocyclopropane-1-carboxylic acid, and the bioactive jasmonate, jasmonoyl-isoleucine, in the seed coat. At a late stage of growth arrest, transcriptomic signatures further suggest that a dormancy-like program of development is induced in the seed and a senescence phenotype is activated in the seed coat. Together, our data indicate that avocado IFA is initiated by hormone-driven transcriptome reprogramming that functions to transition the seed coat to a senescence program of development, which induces growth arrest, seed dormancy and ultimately, fruitlet abscission.