Genotypic Difference And Mechanisms Of Resistance To Fe Deficiency In Pak-choi (Brassica Campestris Ssp. Chinensis (L.) Makino)

Pak- choi ( Brassica campestrisssp. chinensis (L. ) Makino) is very important and necessary in the vegetable supplement in south China, and gradually planted in north China recently. It becomes a kind of world - wide vegetables growing extensively in Europe, America and Japan etc. The most serious problem during the pak —choi cultivation is diseases and insect pests, and it's difficult to grow well in hot summer. Soilless culture of pak—choi is one of effective way to overcome these problems, with the limited factor of Fe-chlorosis. Meanwhile, Fe—chlorosis appears when pak—choi grows in north China. EDTA—Fe is the most expensive element of soilless nutrition input. And Fe also is the nutrient element which causes nutrient inbalance easily. It's effective to work out these problems through the breeding of Fe—deficiency resistant genotypes which can be used in the production. Selection of Fe—deficiency resistant genotypes and the study of mechanisms were carried out in this experiment. Research will enrich the knowledge of Fe-deficiency resistance dicots and serve as scientific guide for pak —choi soilless culture.1. Continuous two times selections were carried out in 40 varieties pak —choi which are from different ecoarea under low Fe Hoagland I soilless nutrient solution. The results showed that Fe—deficiency resistant ability varied significantly among different pak —choi genotypes. Using Fe—chlorosis index, chlorophyll content and growth weight etc as selection parameters, DTBHT and SYB are two precious Fe—stress resistant varieties. While HXW, LBYC and LBY are very sensitive ones.Fe—content differed significatly among various genotypes under normal Fe supply. Fe content of MET is more than 3 times higher than BYSYM under 3 mg/'L Fe3+ nutriton, the former is 295 μg/gDW, the later is 90μg/gDW. The results is useful in supply ing valuable genotyes and new methods to improve the popular Fe — nutrition through breeding and soilless culture to produce vegetables of high Fe nutrition.2. Two pak —choi genotypes were cultured in complete nutrition solution at different Fe levels of 1. 0, 5. 0, 15. 0, 50.0mg/L to study the effects of Fe contents on seedling growth and plant Fe, Mn, Zn, Cu contents, Fe requirements of the two genotypes varied greatly. DTBHT was more tolerate to low Fe level. The optimal Fe contents for dry weight accumulation was 5. 0mg/L and 15. 0mg/L respectively. Fe efficiency of DTBHT is greatly higher than HXW under low Fe levels. High concentration of Fe (50mg/L) greatly inhibited growth of the two genotypes, and induced slightly poisonous injury, Mn, Cu, Zn contents of roots and older leaves decreased when Fe concentration was increased to 50mg/L. The response of the two genotypes varied in this respect, especially, Zn content of DTBHT was lower than that of HXW. It's proved that Fe efficiency of DTBHT is higher than that of HXW. And the selection parameters are reliable. Breeding and production of high Fe content vegetables should begin with the genotypes of high Fe content and increase the Fe content of nutrition later.3. Response of 4 different Fe stress sensitive genotypes to Fe stress. The results showed that pH of these 4 genotypes growth medium increased after Fe stress, but was lower than Fe—suply treatment. Acidity increas caused by pak—choi root system activity was weak when the NO3- used as the only nitrogen resource. Peak period of root cytoplasm membrane Fe3+ reduction capicity appears on the sixth day after Fe — dificiency treatment, that of DTBHT and SYB, which were tolerant to low Fe were much higher than that of HXW and LBYC, which were sensitive to low Fe. Chlorophyll of upper most expanded leaves of these 4 pak — choi genotypes decreased significantly under Fe stress, the effect being smaller in Fe—stress resistant genotypes than in sensitive ones. Fe content genotypes than of upper most expanded leaves of DTBHT is much higher than that of LBYC, So, DTBHT has higher Fe efficiency.4. Diural change of Fe3+ reduction capacity under different light regimes treatment was carried out on different Fe — stress sensitive pak — choi genotypes. The results showed that Fe3+ reduction capacity in the root of Fe — difiency resistant genotypes changes diurnally since the sixth day after Fe stress treatment under normal light regime (14h light/10h dark) Fe3+ reduction capacity decreased in the dark and increased under light, and reached the highest Fe3+ reduction capacity 4—6h after lighted. In contrast, reduction capacity of Fe—deficiency sensitive genotype changed irrgularly. There was little difference of root Fe3+ reduction capacity amony different genotypes in Fe supply treatmen under normal light regimes. Diurnal changes of Fe3+ reduction capacity of Fe — dificiency resistant genotypes can be broken through continous light. It's the first time that such a law was proved in pak -choi Stevens also found the similar law in soybean in 1994. So, it's possible the Fe absorption of plant not changes diurally. It's inportant intheory and practice in the management in soilless nutrition and the selection of Fe—deficiency resistant genotypes vising reduction capacity as parameter.5. Effects of different Fe—supply on seedling Fe, Mn, Cu and Zn contents of Fe di-ficiency—sensitive genotypes including DYBHT and LBYC- Fe—stress significantly lowered leaf Fe content, the effect being greater in new leaves than in older ones. Fe efficiency of DTBHT was higher than LBYC in Fe—stress and Fe—supply treatments. While root Fe content of DTBHT was greatly higher than LBYC in the case of Fe—supply, it become similar in Fe—stress treatment. From the results it can be concluded that roots of DTBHT have higher Fe absorbance and transportation efficiency than LBYC under Fe stress condition. Under Fe stress, Mn, Cu, Zn contents of leaves increased greatly and Mil contents of roots decreased in the two genotypes tested DTBHT showed higher Mn enriching abiolity than LBYC So, the difference of Fe using efficiency of pak—choi under Fe stress was determined by its genetic background.6. Two Fe — deficiency resistant genotypes and two ssensitive ones were selected fristly, and the genotypic difference of Fe nutrition was proved in this experiment. These are the base of the further theoretical research and high Fe—content genotypies. Fe3+ reduction capaticy of different Fe stress sensitive genotypes changed diurally, it's more obvious in Fe deficiency resistant genotypes. The results filled the gaps in fields of such research. Root system of pak—choi absorbed Fe, Cu, Mn, Zn competively or coorperative-ly- The research enriched the knowledge of Fe deficiency resistance dicots and found a new way to overcome Fe chlorosis and reduce the cost of soilless nutrition.