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 profiles contributes to a late-season K deficiency that limits cotton (Gossypium hirsutum L.) yields on affected soils. Split-root solution culture 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. For two cultivars examined in solution culture, 74% of new root development occurred in the half-pot providing 90% of the total NO3(-)N supply. Only 60% of cultivar root development occurred in the half-pot providing 90% of the PO4-P. No compensatory root growth was observed in response to localized K enrichment. in the split-pot system, the proportion of total root surface area developing in a half-pot was highly correlated 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 distribution. Mean soil NO3-N supply to the whole root system determined shoot 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 sensitive to K deficiency than 'Acala SJ-2', had significantly larger root diameter in all nutrient-supply environments. Under conditions of K stress,'Acala GC510' had increased root branching and allocated greater dry matter to roots relative to shoots than 'Acala SJ-2'. The results demonstrate that K acquisition by cotton is strongly influenced by the quantity and distribution of NO3-N in the root zone through its effects on root proliferation, and that distinct cultivar differences associated with crop performance on low K soils can be detected in short-term, solution culture growth systems.