| Water and nutrients, especially nitrogen, are two kind factors of plant growth, which are :losely related to each other. One hand, shortage of water and fertilizer resources occurs iniversally. On the other hand, problems of resource waste and environmental deterioration issociated with low efficiency of water and fertilizer utilization frequently happen. The situation becomes more serious in some places. Therefore, how to regulate the interaction between plants and the environment and use the potential of plant adaptation to the environment, in order to realize high efficiency of water and fertilizer while improve environmental quality, is top problems which Plant Physiology, Agricultural Soil and Water Engineering, Soil Science and Plant Nutrition commonly focus on. In recent few years, much effort has been spent on studies of responses of plant to water deficit and the water-saving effect of partial rootzone irrigation. Nevertheless, the localization of water supply in rootzone must lead to heterogeneity of nutrient supply. But very few works has been conducted to understand how the uptake and use of water and nutrients are affected when part of the root system is supplied with nutrients and water during their growing season. The objective of this paper is to study the combined responses of root growth and development, the absorption and use of water and nutrients to localized irrigation and fertilization in rootzone and identify the mechanism influencing these responses. This is not only helpful to realize high combined efficiency of water and fertilizer, but also benefit to enrich the study of plant adaptation to localized supply of resources and its regulation. In this dissertation, three experiments were conducted on maize grown in split-root containers. The one was to study the effects of the method of irrigation in rootzones on root growth and root hair development, absorption and xylem transport of water and nutrients from different rootzones. The experiment lasted 40 days that was divided into four periods, i.e., I, II, III, IV. Maize was irrigated in both halves of the pot (control, C) or water was continuously supplied to the one-half of the pot and the other half kept dry (fixed partial rootzone irrigation treatment, F), or water was alternatively supplied to two halves of the pot at the interval of 10 days(alternative partial rootzone irrigation treatment, A). In order to study the effects of localized supplies of water and nitrogen fertilizer on crop growth, absorption and use of water and nitrogen from different rootzones, another split-root experiment was conducted. Maize was irrigated using the methods of C and A, the same as above. Urea was applied to both halves of the pot (CN) or was continually fertilized to the one-half of the pot and the other half kept no N fertilizer (fixed partial rootzone fertilization treatment, FN), or alternatively applied to two halves of the pot (alternative partial rootzone fertilization treatment, AN), all with four levels of fertilization, i.e. zero, low, medium and high. Also, the effects of alternate water and (or) nitrogen supply to partial root zone were studied on maize seedlings grown in Hoagland nutrient solutions and with their roots divided equally into two containers. Half of the root system was applied with osmotic stress (W) using PEG 6000 added in the solution at -0.2 MPa and/or nitrogen stress (N) with the N-free Hoagland solution. After 6 days, four of such treatments, C (control), W, N and WN, were shifted to the other halves of the root systems. The results are shown as follows: 1. After the dynamic of root dry weight, length, area and root hair development of root system from each half rootzone was investigated for three irrigation methods, it was found that alternative partial rootzone irrigation was helpful for root system to grow equivalently in different rootzones and for root hair to develop, and could stimulate the compensatory effect of root growth in irrigated rootzone again and again. Also, the time point of compensatory effect was obtained. Under the control(C), the root area, length and dry weight from two half rootzones and the average increase rates were similar all the time. Under F-irrigation, those attributes of irrigated zone were markedly higher than those of non-irrigated zone. Under A-irrigation, the increase rates for two rootzones changed alternately, the root area, length and dry weight for two rootzone tended to equivalent with treatment time extend. We may conclude that root system could equivalently grow in different rootzone under alternative partial rootzone irrigation. Compared to those of each half rootzone of the control, the increase rates of root area, length and dry weight of root system from irrigated zone of A and F treatments enhanced significantly during the first five days of I period. No difference occurs during the later five days of I period. During the first five days of II period? the increase rates from irrigated zone of A treatment enhanced substantially, and the increase rates from irrigated zone of F were similar with the control, suggesting that A-irrigation can stimulate the compensatory effect of root growth in irrigated rootzone again and again, but the compensatory effect disappeared when the interval of alternate irrigation extended. When soil water content is in 65-95 percent of field water capacity, the alternate interval of 5days is favorable to the compensation.After 40days of treatment, for root system from non-irrigated zone of F treatment, vestigial root hair occurred on the part near to the starting point of root, its proportion was 6.08%-36.17%, but on other part root hair was erect and very flourishing. For irrigated zone of F treatment, blet spots occurred on root system and the degree of denseness of root hair is lower than that on dense section of non-irrigated zone, but the proportion of the section of vestigial root hair was 4.82%-27.24%. For the control, root appearance and root hair development is similar with the irrigated zone of F treatment. For A treatment, no blet spots occurred on root system from two rootzone, and root hair was long and very flourishing on the part near to the starting point of root. The proportion of the section of vestigial root hair was 2.28%-17.68% and 2.88%-19.65% for two rootzone, respectively. These results showed that being dry or wet in long time was unfavorable to root hair development and root growth, and the cycle of drying-rewetting may improve root hair development and root growth. 2. It was revealed that high hydraulic conductivity in soil-root system in irrigated zone could efficiently compensate for localized water supply. The characteristic of hydraulic conductivity in soil-root system from all rootzones was investigated and the linear correlation equations were established between the hydraulic conductivity and soil water content. It was made clear that the importance of different rootzones in water uptake was depended on the method of irrigation in rootzone. Compared with one half rootzone of the control, hydraulic conductivity in soil-root system for irrigated zone of F and A treatments increased significantly at 5th and 10th day of every period. The total hydraulic conductivity increased by 56.83%-l04.40%, hydraulic conductivity per root area increased by 38.66%-129.07%, and hydraulic conductivity per root length increased by 47.34%-106.67%. For three irrigation methods, the amount of crop water consumption was significantly correlated with the hydraulic conductivity from the whole rootzone or irrigated rootzone and the determine coefficients were more than 95%. For non-irrigated rootzone of A or F treatments, the hydraulic conductivity was not correlated with crop water consumption. Under the control, the water transport in whole maize was depended on soil water content in all rootzones. Under F treatment, the water transport in whole maize was depended on soil water content in irrigated rootzone while non-irrigated zone decreased the hydraulic conductivity in soil-root system of maize. Under A treatment, the water transport in whole maize was determined by soil water content in irrigated rootzone. The hydraulic conductivity of non-irrigated zone of A treatment was markedly higher than that of F treatment. These results showed that alternative partial rootzone irrigation could use all root system resources to satisfy water requirement of plant.3. It was discovered that under alternative partial rootzone irrigation, th< compensation occur in the assimilation and transport of mineral nitrogen by root system in irrigated rootzone, and the importance of two half root systems was more close tc each other than that of F treatment. There also existed obvious compensation for xylem transport of H2PO4\ K+, Ca2+, Mg2+, Cl. The transport rate of amino acid nitrogen(AA-N) by the root system from irrigated zone oi A and F treatments increased significantly compared to one half that of the control. The transport rate of NHj-Nn NO3-N by the root system from irrigated zone of A treatments was markedly higher than that of one half root system of the control on the 5th day, but was near to the control on the 10th day of every period. For H2PO4\ K+, Ca2+, Mg2+, Cl", the results was different. The transport rate of these nutrients by the root system from irrigated zone of A treatments was markedly higher than that of the control both on the 5th and 10th day of every period, suggesting that the compensatory effect of nitrogen transport lasted in shorter period. For F treatment, the proportion of xylem transport of NFU-N-. N03-N> AA-N by the root system from irrigated zone was large and higher than that of A treatment or the control. For A treatment, the proportion of xylem transport of NFLrN> NO3-NN AA-N by the root system from non-irrigated zone was more than 40%. The proportion of irrigated zone was about 60%. In conclusion, root system from non-irrigated rootzone can absorb and assimilate some mineral nitrogen and then transport AA-N from root to shoot under localized irrigation. As to alternative partial rootzone irrigation, the function of two half-root systems was relatively close to each other. 4. Application of '"N marked fertilizer has proved that alternative partial rootzone irrigation could stimulate the compensatory effect of nitrogen inflow in the root system in irrigated rootzone again and again while increased the nitrogen use efficiency. The method of irrigation in rootzone directly affects the source, allocation of N in plant and the end-result of N fertilizer. During the first five days of every period, 15N inflow of the root system in irrigated rootzone of A treatment enhanced significantly as compared with that of one half rootzone of the control, while 15N inflow of the root system in irrigated rootzone of F treatment increased largely only during the first period. In F treatment, nitrogen in plant was mainly absorbed from the irrigated zone, while for A, the amount of nitrogen from two zones equalized. The allocation of nitrogen from different rootzones was similar to the absorption. So, we may conclude that nitrogen in soil of whole rootzone could be evenly used under alternative partial rootzone irrigation. As for F, the use and lose efficiency of nitrogen fertilizer applied to irrigated zone increasedobviously as compared with the non-irrigated zone, while the residue efficiency was the contrary. With A treatment, the use and lose efficiency of nitrogen fertilizer applied to two rootzones was similar, which was higher than those for the non-irrigated zone of F treatment and lower than the irrigated zone. The contrary was true for the residue efficiency. As compared with the control, both F and A increased the nitrogen use efficiency of maize. 5. Responses of soil water content and water consumption from different rootzones to the method and rate of nitrogen fertilization were revealed For both C and A, while the level of nitrogen fertilizer rose, the amount of water consumption from different root zones simultaneously decreased for CN treatment, but for root zones without fertilization in FN or fertilized in later time for AN, water consumption almost had no change, and for root zones with fertilization in FN or fertilized in early time for AN, water consumption decreased markedly, causing that the difference between two root zones increased. The difference between two root zones was not statistically significant under low level of fertilization but significant under high level. Moreover, the difference was larger for FN. In addition, compared with C, A reduced the water consumption difference between two rootzones under localized supply of nitrogen fertilizer. Soil water content in different rootzones of all treatments was in agreement with the amount of water consumption. These results suggested that localized supply of nitrogen fertilizer lead to heterogeneity of soil water content and plant water consumption from different rootzone under higher rate of fertilization. 6. Responses of maize to the methods of localized supply of water and nitrogen were investigated. Localization of nitrogen fertilization could improve plant growth and the use efficiency of water and nitrogen. The appropriate rate of nitrogen fertilizer and method of fertilization was found for evenly irrigation and alternative partial rootzone irrigation. Under evenly irrigation, for AN or CN, maize dry matter accumulation, yield and the use efficiency of water and nitrogen got the highest at 4g nitrogen fertilizer per pot, ie. 4g pot"1, and for FN, the use efficiency of water and nitrogen also approached the highest at this rate while dry matter and yield got the highest at 2g pot"1. This suggested that the appropriate rate of nitrogen fertilizer rose for AN and CN. For the same nitrogen fertilizer rate of 4g pot"1, the yield, dry matter weight of shoot and root, and the use efficiency of water and nitrogen of AN treatment increased significantly as compared with those of CN and FN. In comparison with the control(CN), nitrogen content of root system decreased in AN and FN treatments. For AN, the nitrogen content of shoot decreased too. However, nitrogen content of seed unchanged.Under evenly irrigation, nitrogen content of half root system from fertilized rootzone was higher than that of non-fertilized zone with all rates of nitrogen fertilization for FN treatment, the increase proportion was up to 8.95%-14.13%. For AN treatment, nitrogen content of half root system from the rootzone fertilized in later was higher than that of fertilized at first, while the difference became smaller when rates of nitrogen fertilization increased. For evenly irrigation, medium level of nitrogen fertilizer(4g pot'1) using alternative partial rootzone fertilization is beneficial for yield and efficient use of water and nitrogen, while for alternative partial rootzone irrigation, low level of nitrogen fertilizer(2g pot"1) using fixed partial rootzone fertilization is favorable. 7. Using nutrition cultural experiment, it was proved that compensation effect of root growth and function occur in the process of alternate water and (or) nitrogen supply to partial root zone of maize, while stimulating root activity and root growth. Exudates from root system were explored under localization of water and nitrogen supply. For each treatment, the root activity of the non-stressed half root system increased a great deal as compared with that of the other half and the control. This occurred after 1, 3 and 5 days of treatment start. After each alternation, the rates of biomass accumulation of the all previously stressed half roots were significantly higher than those of the other half and of the control. Combined root dry weight increased markedly for all stress treatments. After 6 days from treatment, the amount of oxalic acid exudate from the non-stressed half root system of N treatment, increased markedly but not for the stressed half. Exudate of all half roots of W and WN decreased. This might be a kind of adaptation of plant root system to water deficit. |
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