THE IMPORTANCE OF SAMPLING IMMATURE LEAVES FOR THE DIAGNOSIS OF BORONDEFICIENCY IN OILSEED RAPE (BRASSICA-NAPUS CV EUREKA)

Plant analysis can diagnose boron (B) deficiency when the standards us ed have been properly developed by establishing that a close relations hip exists between B concentration in a plant part and its physiologic al function. The purpose of the present study was to demonstrate the i mportance of choosing the growing immature leaves for B deficiency dia gnosis and for establishing critical B concentrations for the diagnosi s of B deficiency in oilseed rape (Brassica napus). In Experiment 1, t he plants were subject to seven levels of B supply using programmed nu trient addition, for the estimation of critical B concentrations in pl ant parts for shoot growth. In Experiment 2, the plants were treated w ith two levels of B supply in solution: 10 (+B) and 0 (-B) mu M B, for the estimation of functional B requirements for leaf elongation. The results showed that critical B concentrations varied amongst the plant parts sampled and decreased with leaf age. As B taken up by roots is largely phloem-immobile, B concentrations in mature leaves are physiol ogically irrelevant to plant B status at the time of sampling, giving rise to a significant over- or underestimation of the B requirement fo r plant growth. By contrast, a growing, immature leaf, in this case th e youngest open leaf(YOL), was the most reliable plant part for B defi ciency diagnosis. Critical B concentrations developed from both method s were comparable - i.e. 10-14 mg B kg(-1) dry matter in the YOL at ve getative growth stages up to stem elongation.