COTTON ROOT AND SHOOT RESPONSE TO LOCALIZED SUPPLY OF NITRATE, PHOSPHATE AND POTASSIUM - SPLIT-POT STUDIES WITH NUTRIENT SOLUTION AND VERMICULITIC SOIL

Vertical stratification of plant-available K in vermiculitic soil prof iles contributes to a late-season K deficiency that limits cotton (Gos sypium hirsutum L.) yields on affected soils. Split-root solution cult ure and split-pot soil experiments were conducted to determine whether root distribution and cultivar differences in root extension in these stratified profiles result from a compensatory response to localized enrichment with NO3-N, PO4-P, and/or K in the root zone. Compensatory root growth was greatest in response to localized NO3-N enrichment. Fo r two cultivars examined in solution culture, 74% of new root developm ent occurred in the half-pot providing 90% of the total NO3(-)N supply . Only 60% of cultivar root development occurred in the half-pot provi ding 90% of the PO4-P. No compensatory root growth was observed in res ponse to localized K enrichment. in the split-pot system, the proporti on of total root surface area developing in a half-pot was highly corr elated with localized soil NO3-N levels (r(2)=0.81), while increased K availability in one half of the root zone did not affect root distrib ution. Mean soil NO3-N supply to the whole root system determined shoo t N accumulation (r(2)=0.97). Shoot K accumulation was not related to soil K availability but was strongly correlated with mean root surface area density (r(2)=0.86). Cultivar 'Acala GC510', known to be less se nsitive to K deficiency than 'Acala SJ-2', had significantly larger ro ot diameter in all nutrient-supply environments. Under conditions of K stress,'Acala GC510' had increased root branching and allocated great er dry matter to roots relative to shoots than 'Acala SJ-2'. The resul ts demonstrate that K acquisition by cotton is strongly influenced by the quantity and distribution of NO3-N in the root zone through its ef fects on root proliferation, and that distinct cultivar differences as sociated with crop performance on low K soils can be detected in short -term, solution culture growth systems.