Structural and functional damage caused by boron deficiency in sunflower leaves

Boron deficiency induced a dramatic inhibition in sunflower plant growth, s hown by a reduction in dry mass of roots and shoots of plants grown for 10 d in nutrient solution supplied with 0.02 mu M B. This low B supply facilit ated the appearance of brown purple pigmentation on the plant leaves over t he entire growth period. Compared to B-sufficient (BS) leaves, leakage from B-deficient (BD) leaves was 20 fold higher for potassium, 38 fold for sucr ose, and 6 fold for phenolic compounds. High level of membrane peroxidation was detected by measuring peroxidase activities as well as peroxidative pr oducts in ED sunflower plants. Soluble and bound peroxidase activities meas ured in ED thylakoid membranes were accelerated two fold compared to those detected in BS-membranes. No detectable change in soluble peroxidase activi ty in roots whereas a 4 fold stimulation in bound peroxidase activity was d etected. Thylakoid membranes subjected to low B supply showed enhancement i n lipoxygenase activity and malondialdehyde (MDA) content in parallel with 40 and 30 % decrease of linoleic and linolenic acid contents (related to to tal unsaturated fatty acids), A slower rate of Hill reaction activity (40 % ) and a suppressed flow of electron transfer of the whole chain (30%) were detected in ED thylakoid membranes, This reduction was accompanied with a d ecline in the activity of photosystem 2 shown by a diminished rate of oxyge n evolution (42 %) coupled with a quenching (27.5 %) in chlorophyll a fluor escence emission spectra at 685 nm (F-685) Thus B is an important element f or membrane maintenance, protection, and function by minimizing or limiting production of free oxygen radicals in thylakoid membranes of sunflower lea ves.