Manganese Nutrition of Cotton Under Two Boron Levels. II. Critical Mn Level1

Reference guideline values relating Mn concentration in plant tissue to growth is necessary for the assessment of the Mn nutritional status in cotton. Many of the presently available critical Mn levels were related to the presence or absence of deficiency symptoms. Refinement appeared necessary particularly in relation to the inter‐action of another nutrient. Therefore, a Mn ✕ B interaction study was conducted in nutrient solution under greenhouse conditions to determine the influence of low and adequate B levels on critical Mn levels of Upland cotton (Gossypium hirsutumL.) with 11 manganese rates from 0 to 4000 μg Mn/1 at two B levels of 35 and 500 μg B/1. The objectives were to determine 1) the most sensitive tissue relating Mn content to growth, 2) the critical Mn level in relation to growth and 3) the interaction of B on the critical Mn levels. Critical Mn levels of blade 1 through 5 ranged from 10 through 15 ppm Mn on a dry weight basis. Although Mn content in .blade 1 was the most sensitive in relating nutrient content to growth, the recently matured leaf (blade 3) was selected as the most practical tissue on the basis of sampling ease to use for evaluating the Mn status of cotton. The critical Mn level was 10 ppm Mn in blade 3. The critical Mn levels in blades 1 through 4 were independent of the B levels; however, low B increased the critical Mn level in the older blade (Blade 5). The stem and petiole tissue contained a small fraction of the Mn in blade tissue at high Mn levels. This reduced the desirability of selecting any of the vascular tissues for estimating the Mn nutrient status of cotton plants.