Role of apoplastic ascorbate and hydrogen peroxide in the control of cell growth in pine hypocotyls.

The growth cessation of plant axis has been related with the formation of diphenyl bridges among the pectic components of the cell wall caused by the action of apoplastic peroxidases using hydrogen peroxide as electron acceptor. The formation of diphenyl bridges is prevented by the presence of ascorbate in the apoplastic fluid which acts as a hydrogen peroxide scavenger. The current work focuses on the role of the apoplastic ascorbate and hydrogen peroxide in the cell growth. The addition of hydrogen peroxide caused an inhibition of the auxin-induced growth as well as a significant decrease in the cell wall creep induced by acid-pH solutions. The hydrogen peroxide content in apoplastic fluid increased with the hypocotyl age and along the hypocotyl axis of 10-day-old pine seedlings, as the growth capacity decreased. On the other hand, the ascorbate content in the apoplastic fluid decreased with the hypocotyl age and along the hypocotyl axis of 10-day-old seedlings. A very significant correlation between the hydrogen peroxide apoplastic level and the growth rate as well as between the ascorbate/hydrogen peroxide molar ratio and the growth rate of hypocotyls have been found suggesting that the redox state is the main factor controlling the cell wall stiffening mechanism and thus growth in pine hypocotyls.