Cotton Root-Shoot Water Relationships: Seeding Response to Root Zone Temperature and Growth Medium

Judith M. Bradow and John W. Radin


 
ABSTRACT

The effects of root-zone temperature and growth medium on cotton root and shoot growth and water relationships were examined in seedlings grown for 14 days, either hydroponically or in fritted clay, sand, or commercial soil-less medium, at root-zone temperatures of 20 or 30 C. The effects of shifting from one root-zone temperature to the other on Day 10 were also monitored. Root and shoot elongation, fresh and dry weight accumulation, and relative water contents were determined, as well as cotyledon expansion and transpiration rates and leaf water potentials.

Significant differences in plants that were grown hydroponically and those grown in solid rooting media were expected, but the enhanced growth of cotton roots in nutrient solution, compared to any of the solid media used, was greater than expected. the much larger root systems of hydroponically-grown plants indicate that comparison between results obtained with solution-grown and potting mix- or sand-grown plants must be made with extreme care, particularly in nutrient uptake studies where root surface areas and/or volumes are significant factors.

Of the solid root-zone media tested, fritted clay appeared to introduce the fewest confounding effects, but clay-grown roots were not directly comparable to solution-grown roots were not directly comparable to more apparent in the roots than the shoots when standard growth parameters, i.e., weight, height, relative water considered. In general, growth was most rapid in hydroponic culture and slowest in sand.

Root-zone temperature effects, but not those of root-zone medium, were apparent in the cotyledon expansion rates. Temperature shift in either direction on Day 10 damped out the root-zone temperature effects on cotyledon growth.

Cotton responses to both root-zone medium and temperature were independent of air and leaf temperatures, as was al relative humidity, indicating that those using environmental chambers in temperature-dependent studies can NOT rely of chamber (air) temperature as the treatment parameter.

Root-zone temperature effects on transpiration rates were obscured by variations in PPFD < 900 µmolm-2s-1 were analyzed separately from those obtained at PPFD above that limit, significant effects of both root-zone temperature and medium were detected. When clay was the root-zone medium, the temperature effects were consistently present at PPFD > 900 µmolm-2s-1 only. Conversely, root-zone temperature effects were stronger at the lower PPFD levels when the plants were grown in nutrient solution. Leaf water potential was also affected by PPFD. The discrepancies in transpiration rates and leaf water potentials that are caused by variations in PPFD can NOT by avoided through the use of environmental chambers PPFD levels are well into the "cloudy" range defined here. Extreme caution must be used when water deficiency studies involve plants grown densely under the low light levels available in environment chambers if the results obtained in the artificial environmental are to be at all relevant to quantitative and qualitative growth of plants in the field.

The results presented also suggest that extended period of dark, rainy weather may have significant effect upon plant growth beyond those usually associated with soil-waterlogging or flooding.



Reprinted from 1992 Proceedings Beltwide Cotton Conferences pg. 1088
©National Cotton Council, Memphis TN

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Document last modified Sunday, Dec 6 1998