Effects of Boron defiency on morphology, gas exchange, and Glusinolate composition of hydroponically grown oilseed rape (Brassica napus var. oleifera)

Oilseed rape (Brassica napus var. oleifera) was grown in a continuously recirculating hydroponic system designed to accurately and consistently deliver micronutrient concentrations. Perlite was used as the root support medium. Boron (B) treatments ranging from adequate or normal (0.60 mg*L^-1 added B) to severely deficient (0.01 mg*L^-1 added B) were applied as modified Hoaglands solutions in three experiments. The effects of B deficiency on plant morphology, seed yield, gas exchange, and seed glucosinolate content were analyzed. Tissue B concentrations were determined by inductively coupled plasma atomic emission spectrometry. Severely B deficient plants were malformed and did not produce seed. Leaf tissue B content was the best indicator of B availability and uptake. Leaves of normal plants contained from 35 to 80 μ*g^-1, whereas normal appearing but seedless plants averaged 18 μ*g^-1 B. Histological analysis of stem tissue confirmed that deterioration of vascular tissue and pith cells was directly related to severity of B deficiency. Sporadic cell wall lignification was associated with cellular breakdown in the pith. The best morphological indicators of B deficiency were number of axillary branches and main inflorescence height. An analysis of photosynthesis (Pn) and stomatal conductance (SC) values indicated a minimum leaf tissue B content of 21 μ*g^-1 was required for normal functioning of leaf Pn and SC. The stage of plant development was more important than leaf maturity in determining the effect of B on gas exchange capacity. Glucosinolate content was compared by gas chromatography and colorimetry procedures. Gas chromatography proved to be the more accurate and informative method. Seed glucosinolate levels remained stable at leaf tissue B values above 21 μg*g^-1. Boron deficient plants produced lower quality seed with glucosinolate concentrations significantly above the acceptable canola quality standard of 30 μmol per gram of defatted meal. Indole glucosinolates appeared to be less influenced by B availability than aliphatic glucosinolates. The individual glucosinolate composition of seeds was unaffected by B treatment. Tissue B uptake was consistent with treatment B availability throughout all experiments.