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High light intensity increases external B requirements for leaf growth of sunflower (Helianthus annuus L. cv. Hysun 25) in boron-buffered (B) solution culture

Huang, L., Gherardi, M., Bell, R.W. and Dell, B. (2002) High light intensity increases external B requirements for leaf growth of sunflower (Helianthus annuus L. cv. Hysun 25) in boron-buffered (B) solution culture. In: Goldbach, H.E., Rerkasem, B., Wimmer, M.A., Brown, P.H., Thellier, M. and Bell, R.W., (eds.) Boron in Plant and Animal Nutrition. Kluwer Academic Publishers, Dordrecht, The Netherlands, pp. 213-225.

Abstract

In previous glasshouse studies and field observations, severe B deficiency symptoms in leaves were associated with periods of high light intensity (Cakmak et al., 1995; Noppakoonwong et al., 1993; Warington, 1933). Tanaka (1966) reported that B deficiency symptoms increased with increasing light intensity in Lenma paucicostata. Shading (35o/o full sunlight) treatment in the glasshouse decreased the critical B concentration for leaf blade elongation of black gram from 15 mg B kg-1 dry matter (at 75 % full sunlight) to about 10 mg B kg-1 dry matter (Noppakoonwong et al., 1993). However, the above studies did not identify whether high light intensity increases plant external or internal B requirements, or both.

High light intensity may increase the external B requirement by increasing B demand to meet the requirement of stimulated growth and by altering transpiration and B distribution patterns in leaves. Boron uptake by plants is mainly a transpiration-driven process (Hu and Brown, 1997) and its deposition and accumulation in an individual leaf is correlated with the transpiration intensity in the leaf concerned (Oertli, 1994). Increasing light intensity stimulates leaf photosynthesis rate and growth, resulting in a greater B sink strength in the actively growing leaves. By contrast, the existing older/mature leaves may transpire more under high light intensity than low light and therefore, divert a significant proportion of the B transported into the shoot away from the actively growing leaves and shoot tips. With an adequate B supply to the roots, the amount of B transported into the shoot may be sufficient to meet the expanding B sink in the rapidly growing leaves and shoot tips, and hence maintain adequate B concentrations in the leaves for sustaining their high growth rate even under high light. However, the opposite may occur when plants are supplied with marginal/low levels ofB in the rooting medium under high light conditions.

The previous studies described above used conventional solution culture, with B being supplied in the form of boric acid. As plants grow faster in response to increasing light intensity, the B removal rate from the nutrient solution increases, resulting in a declining external B supply in the nutrient solution. In this conventional solution culture system, test plants may have experienced declining or fluctuating B supply levels in the nutrient solution during the experiment and B concentrations may not reflect the current B status in the leaves but rather the amount of B accumulated over a period of time. This complicates the interpretation of B requirements in the growing leaves, especially in high-B demanding species such as sunflower when cultured at marginal or deficient B supply. In the present study, the levels of B supply in the nutrient solution were maintained by means of a B-buffered solution culture (Huang et al., 1999), to separate effects of increasing light intensity on the external B requirement from the internal B requirement.

Publication Type: Book Chapter
Murdoch Affiliation: School of Environmental Science
Publisher: Kluwer Academic Publishers
Copyright: © Kluwer Academic
URI: http://researchrepository.murdoch.edu.au/id/eprint/14783
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