Relationships Between B Deficiency Symptoms in Sunflowers and the B and Ca/B Status of Plant Tissues1

Abstract

AbstractSunflowers (Helianthus annuus L.) have an apparently high boron requirement but little information exists on the quantity of B necessary in plant tissues, particularly in a field‐grown crop. ‘Smena’ sunflowers grown in a field trial on an Avalon medium sandy loam (an Ultisol) showed symptoms of B deficiency in the absence of B fertilization, the most severe symptom being a malformation of the capitulum at anthesis which led to areas of no seed set. Since this symptom was apparently related to the reduced seed yield of the B deficient plants, the aim of this study was to establish relationships between the percentage deformed heads and the B concentrations and Ca/B ratios in three plant tissues, viz (i) month‐old seedlings, (ii) the topmost, mature leaf at flowering, and (iii) the seed. The mathematical relationship of best fit was established as that which minimized the residual sum of squares and maximized the correlation coefficient. In general, the B concentration proved superior to the Ca/B ratio as a measure of the status of B nutrition of the plant. Critical B concentrations, below which B deficiency symptoms increased markedly in severity and corresponding to 5% deformed heads, were calculated as 61 ppm in seedlings, 57 ppm in the leaf, and 17 ppm in the seed. It was estimated that on this soil type 1 kg borax/hit would, on average, increase the B concentration by 2 ppm, 3 ppm, and 0.2 ppm in these three tissues, respectively. The seedlings and the topmost, mature leaf at flowering provided reliable estimates of the B nutritional status of sunflowers in this study but the use of seed as an index tissue overestimated the B fertilizer requirements.