| Hydroponically cultured maize plants are needed in the study of the functions of maizeroots for the acquisition of intact root system, or in the accurate control of the physical andchemical environment of the rhizosphere, especially in the study of saline and osmoticstresses to maize. However, in our practice, the application of the two most extensively usednutrient solutions for the hydroponic culture of plants, the Hoagland and Long ashtonsolutions, could not produce healthy maize seedlings, because yellow strips appearedextensively in the young leaves in just one week of cultivation. The prolonged cultivationresulted in the death or even the drying out of the leaves below. The study presented here wastrying to find out the relationship between the changes in the rhozosphere microenvironmentand the ion absorption in maize roots, through the analyses on the differences of therhozosphere microenvironment between soil and hydrocultrue, on the effect of root exudates,on the alteration of the pH and magnesium concentration, ect.The results showed that the young maize root grown in agar medium containing a mixtureof pH indicator could alter the colour of the pH indicator from the original alkalinicblue-green to acidic orange, indicating that the maize root was able to secret acidic substancesto change the pH of the rhozosphere microenvironment.In the experiments of changing the pH of the Hoagland culture medium to simulate therhozosphere microenvironment, it was found that root length, fresh and dry weight,chlorophyll content all increased with the decline of pH (acidation) and reached a maximumat pH4. Beyond this threshold, a further increase in the acidity the culture medium to pH3resulted in a decrease of the parameters mentioned above, and a serious acid injury andgrowth retardation was seen when the acidity of the growth medium was reduced to a value ofpH2.The results from the experiments of a single factor Mg-enrichment showed that when theconcentration of the magnesium sulphate was raised from2to16m mol/L，healthy maizeseedlings can be obtained as well, and the seedlings still maintain healthy when the mediumcontained a magnesium sulphate concentration of16m mol/L.The experiments with changes in both factors, pH decline and Mg-enrichment showed thatthe root length was the longest in the growth medium with pH4.5+8mmol/L Mg2+, the plant height and fresh weight of the shoot was the highest in the growth medium with pH4.0+32mmol/L Mg2+. In terms of the chlorophyll content in maize leaves, the plants grown in themedium with with pH4.0plus16and32mmol/L Mg2+showed the highest value. In spite ofthe better performance of the plants grown in mediums containing very high concentrations ofmagnesium, we still cautiously recommend the application of the modified Hoagland solutionof pH4+8mmol MgSO4as the optimal solution for hydroculture, taking into consideration ofthe long term growth and the overall ion balance.To sum up, this study demonstrated the importance of the roles that the maize root exudateand rhozosphere microenvironment played in plant growth and nutrient absorption; showedthat the maize root needs a acidic environment, at least in the rhozosphere; screened out areceipt of modified Hoagland solution optimal for maize growth. The study also that either byenhancing the acidity of the nutrient solution to pH4, or by raising the Mg2+concentrationgreatly (up to8times of the original Hoagland solution), or by a simultaneous increase in theacidity and Mg2+concentration in the hydroculture solution, the growth of hydroponicallycultured maize seedlings can be greatly improved to yield healthy and fast growing plants.Whether the improvement was because the two factors, or the combination of the two factorschanged the features of ion absorption or the features of selectivity in similar ways the rootsystem, or because the regulation substances or signals from the root to the shoot was changedby the two factors, is still unknown and further researches are still needed to give answers tothe questions. |
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