Anatomical and Physiological Responses of Citrus Trees to Varying Boron Availability Are Dependent on Rootstock

Abstract

In Citrus, water, nutrient transport and thereby fruit production, are influenced among other factors, by the interaction between rootstock and boron (B) nutrition. This study aimed to investigate how B affects the anatomical structure of roots and leaves as well as leaf gas exchange in sweet orange trees grafted on two contrasting rootstocks in response to B supply. Plants grafted on Swingle citrumelo or Sunki mandarin were grown in a nutrient solution of varying B concentration (deficient, adequate, and excessive). Those grafted on Swingle were more tolerant to both B deficiency and toxicity than those on Sunki, as revealed by higher shoot and root growth. In addition, plants grafted on Sunki exhibited more severe anatomical and physiological damages under B deficiency, showing thickening of xylem cell walls and impairments in whole-plant leaf-specific hydraulic conductance and leaf CO2 assimilation. Our data revealed that trees grafted on Swingle sustain better growth under low B availablitlity in the root medium and still respond positively to increased B levels by combining higher B absorption and root growth as well as better organization of xylem vessels. Taken together, those traits improved water and B transport to the plant canopy. Under B toxicity, Swingle rootstock would also favor plant growth by reducing anatomical and ultrastructural damage to leaf tissue and improving water transport compared with plants grafted on Sunki. From a practical point of view, our results highlight that B management in citrus orchards shall take into account rootstock varieties, of which the Swingle rootstock was characterized by its performance on regulating anatomical and ultrastructural damages, improving water transport and limiting negative impacts of B stress conditions on plant growth.

Efficiency of foliar application of sparingly soluble sources of boron and zinc in citrus

“…v.78, n.1, e20180387, 2021 High doses of B applied as calcium borate and boric acid did not result in high concentrations of foliar B as has been found in other works with doses of B above 300 mg L -1 (Quaggio et al, 2010;Ullah et al, 2012;Khan et al, 2015). A low concentration of B in the leaves (40 mg kg -1 of B) was observed when the lowest dose in this study was applied and, compared to the maximum DM reached with 295 mg L -1 of B, reflected a decrease in DM (18 %), characterized as a nutritional deficiency by Mesquita et al (2016). At the highest dose of B tested, no significant nutrient toxicity was observed, represented by just a 5 % decrease in the DM of the aerial part.…”

“…This is likely explained by the medium concentration of B (97 mg kg -1 ) seen in the leaves. Excess B might reduce up to 50 % DM of citrus trees, associated with a decreased net assimilation rate of CO 2 and leaf transpiration when leaf B concentration > 300 mg kg -1 (Quaggio et al, 2003;Mesquita et al, 2016;Simón-Grao et al, 2018), a limit considered toxic to citrus seedlings based on Mattos Jr. et al (1995).…”

Foliar application of manganese increases sugarcane resistance to orange rust

“…For example, inoculated plants treated with Mn 0.5% or Mn 1% exhibited the absence of injuries to sclerenchyma fibres and an improved structural organization of xylem and phloem vessels compared to inoculated Mn 0% plants. Indeed, the structural damage observed in the mesophyll of these Mn 0% plants has been linked to hampered movement of water and solutes in leaf tissues and subsequent impairments in overall plant performance (Mesquita et al ., ; Zambrosi et al ., ).…”

“…To investigate the structure of the mesophyll, a subgroup of these fixed samples was immersed in liquid nitrogen and fragmented with a blade as described by Mesquita et al . (). Subsequently, all samples were dehydrated in an acetone series of increasing concentration (30%, 50%, 70%, 90% and 100% (three times)) and dried until the critical point.…”

“…Tissue samples (approximately 20 mm 2 ) from the middle third of the leaves were cut during the morning from three different plants per treatment, fixed in Karnovsky solution and postfixed for 1 h with 1% osmium tetroxide in 0.05 M cacodylate. To investigate the structure of the mesophyll, a subgroup of these fixed samples was immersed in liquid nitrogen and fragmented with a blade as described by Mesquita et al (2016). Subsequently, all samples were dehydrated in an acetone series of increasing concentration (30%, 50%, 70%, 90% and 100% (three times)) and dried until the critical point.…”