Physiological and morphological responses to boron deficient chinese cabbage

This study aimed to investigate the effects of low boron (B) supply on plant growth, gas exchange, leaf B concentration and distribution during the development of B deficiency, and B symptom development in the Chinese cabbage (Brassica campestris L. ‘Chun-gwang’). Low B supply (deficiency) led to a significant (p <0.05) decrease in total leaf number, maximum leaf width and length by around 40, 46 and 59%, respectively, when compared with those of B-sufficient (0.5 mg B·L^-1) cabbage at 32 days after transplanting (DAT). Measurement of gas exchange in the leaves revealed that the photosynthesis rate, stomatal conductance and transpiration rate within B-deficient leaves significantly decreased, whereas leaf to air vapor pressure deficit (VpdL) and leaf temperature increased. Microscopic investigations revealed that the majority of stomatal pores in the epidermal layer of the B-deficient leaves were closed. A 56% decrease in total root length and a 35% increase in average root diameter was also observed in B-deficient cabbage. The increase in the average root diameter of B-deficient cabbage was associated with a decrease in fine root length within a range of 0 and 0.2 mm in diameter, as well as an increase in the root length of 0.9 mm in diameter. No visual symptoms of B deficiency were detected in the old leaf tissue of B-deficient cabbage at 26 DAT, of which the B concentration was 2-fold higher (16.8 ± 2.9 μg·g^-1), although symptoms developed on other leaves (8.9 ± 0.8 μg·g^-1). Standardized measurements of brown streaks (necrosis), and leaf area affected by corking symptoms versus total leaf area indicated that symptoms developed continuously, while leaf growth ceased.