RESUMO
Climacteric fruits display an increase in respiration and ethylene production during the onset of ripening, while such changes are minimal in non-climacteric fruits. Ethylene is a primary regulator of ripening in climacteric fruits. The ripening behavior and role of ethylene in blueberry (Vaccinium sp.) ripening is controversial. This work aimed to clarify the fruit ripening behavior and the associated role of ethylene in blueberry. Southern highbush (Vaccinium corymbosum hybrids) and rabbiteye (Vaccinium ashei) blueberry displayed an increase in the rate of respiration and ethylene evolution, both reaching a maxima around the Pink and Ripe stages of fruit development, consistent with climacteric fruit ripening behavior. Increase in ethylene evolution was associated with increases in transcript abundance of its biosynthesis genes, AMINOCYCLOPROPANE CARBOXYLATE (ACC) SYNTHASE1 (ACS1) and ACC OXIDASE2 (ACO2), implicating them in developmental ethylene production during ripening. Blueberry fruit did not display autocatalytic system 2 ethylene during ripening as ACS transcript abundance and ACC concentration were not enhanced upon treatment with an ethylene-releasing compound (ethephon). However, ACO transcript abundance was enhanced in response to ethephon, suggesting that ACO was not rate-limiting. Transcript abundance of multiple genes associated with ethylene signal transduction was upregulated concomitant with developmental increase in ethylene evolution, and in response to exogenous ethylene. As these changes require ethylene signal transduction, fruit ripening in blueberry appears to involve functional ethylene signaling. Together, these data indicate that blueberry fruit display atypical climacteric ripening, characterized by a respiratory climacteric, developmentally regulated but non-autocatalytic increase in ethylene evolution, and functional ethylene signaling.
RESUMO
Blueberry (Vaccinium sp.) is thought to display a preference for the ammonium (NH4+) form over the nitrate (NO3-) form of inorganic nitrogen (N). This N-source preference has been associated with a generally low capacity to assimilate the NO3- form of N, especially within the shoot tissues. Nitrate assimilation is mediated by nitrate reductase (NR), a rate limiting enzyme that converts NO3- to nitrite (NO2-). We investigated potential limitations of NO3- assimilation in two blueberry species, rabbiteye (Vaccinium ashei) and southern highbush (Vaccinium corymbosum) by supplying NO3- to the roots, leaf surface, or through the cut stem. Both species displayed relatively low but similar root uptake rates for both forms of inorganic N. Nitrate uptake through the roots transiently increased NR activity by up to 3.3-fold and root NR gene expression by up to 4-fold. However, supplying NO3- to the roots did not increase its transport in the xylem, nor did it increase NR activity in the leaves, indicating that the acquired N was largely assimilated or stored within the roots. Foliar application of NO3- increased leaf NR activity by up to 3.5-fold, but did not alter NO3- metabolism-related gene expression, suggesting that blueberries are capable of post translational regulation of NR activity in the shoots. Additionally, supplying NO3- to the cut ends of stems resulted in around a 5-fold increase in NR activity, a 10-fold increase in NR transcript accumulation, and up to a 195-fold increase in transcript accumulation of NITRITE REDUCTASE (NiR1) which codes for the enzyme catalyzing the conversion of NO2- to NH4+. These data indicate that blueberry shoots are capable of assimilating NO3- when it is directly supplied to these tissues. Together, these data suggest that limitations in the uptake and translocation of NO3- to the shoots may limit overall NO3- assimilation capacity in blueberry.