Studies On Heat Injury Mechanism And Cultivation Techniques Of Pak-Choi And Lettuce In Summer

Pak-choi (Brassica campestris L. ssp. chinensis (L) Mak ) is a diurnal likable leaf vegetable in southern China while letturce ( Lactuca sativa L. ) is also a necessary ingredient of western-style food for the open cities, harbors, hotels and resteraunts in the recent 10 years. Both of them are important vegetables in south of China and balanced providence are required through out the year. But they are cold-tolerant and half cold-tolerant plants , the optimal growth temperature is 15 — 20℃ , and the growth inhibition emerges under high temperature in summer. In some points, we can say off - season vegetables in summer in southern vegetable practice are short of the cool leaf, stem and root vegetables. So how to insure the supply of Pak-choi and lettuce is a difficult problem demanding prompt solution. Since we can not adopt some high technological methods such as gene egineering, "plant work", cool-chain system to solve the problem at present in China, these cool vegetables are provided by anti-high-temperature methods during the period of time. For example, at the end of 1980s, the shading-net covering techniques of vegetables that our research group took part in and our successful lettuce regenerated seedling by soilless culture in summer gave a new simple and efficient method to Pak-choi and lettuce providence during the summer.In general, the study of basic theory on overcoming the problem of vegetables in summer is far behind the practice. So Pak-choi and lettuce, the two cool vegetables which had seldom studied were selected in our experiments. Starting from the high temperature stress physiology, the two plants were cultured: with and without shading-net covering in clear summer, in NFT in greenhouse as well as in the artificial high temperature ecological champer, to ascertain heat injury of the two vegetables and its physiological mechanism. At the same time, to verify the efficency and physiological reasons to relieve heat injury of several anti-high-temperature methods such asshading-net covering and regenerated seedling cultivation through the summer, and it provides the scientific basis for the establishment of anti-high temperature cultivated system of cool vegetables in southern China-The experiment began in 1989 and ended in 1993, lasting 4 summers. The result indicated : the sensitivity of the roots of the two vegetables to heat injury was more obvious than that of the shoots. The growth of the root and its physiology function were disordered initially under high temperature stress. Followed by the obvious growth inhibition of the shoot: the plant dwarfed, the number and area of the leaf decreased, crude fibre increased, the content of water went down, the process of senescence accelerated, and the product quality degraded. Besides, stem elongation and the loss of commodity value were shown in lettuce.Further studies on the physiology machanism of the injury by high temperature stress proved: high temperature stress induced the growth inhibition of the root with decreased absorption capacity of water and nutrients, which led to the growth and transpiration of the shoot weakened and the leaf surface temperature went up. Moreover, the synthesis al^lty °f CTKs such as iPA, as well as GAs was severely inhibited with obvious decreasing of these plant hormones both in the root and in the shoot. But the content of ABA increased and the ratio of GAs/ABA went down; the content of polyamines, especially the content of putresine had a relative reduction. The metabolism of activated oxygen enhanced, which broke the balance of the generating and clearing system of free radical; and the amount of MDA, the product of the peroxidation of membrane lipid increased, and the integration of membrane structure damaged, bringing about the decrease of the process of cell compartmentalization, disorder of metabolism function, disintegration of chloroplast, decrease of the activity of RuBp carboxylase and the net assimilation rate.In addition, the experiments indicated; the essence of shading-net covering cultivation techniques of Pak-choi and regenerated seedling lettuce in summer is that they improved the growth condition and physiology function of ?he root, which efficiently relieve the high temperature injury of the shoot. Compared to the cultivation without covering, the yield of Pak-choi with shading-net covering increased by 122 — 435%, while the exterior commityquality got an obvious improvement. The relatively active root absorption and synthesis contributed to the improvement of the root condition that relieved the high temperature injury of the shoot , to the increasing content of iPA and GAs in the shoot, to the low ABA level and the improvement of biosynthesis of polyamines, and to the decreasing leaf temperature through transpiration; Moreover, it helped to retard the dissolution of chlorophyll and the destroy of chloroplast by high temperature in the shoot. The inhibition of membrane lipid peroxidation and the relative integration of memebrane structure help the synthesis of nucleic acids and proteins, the process of cell division as well as other physiological metabolism processes.Studies on the character of shading net showed; the efficency of shading net is that it reduces high luminosity of 10, 000 lux to its 2/3 — 1/3 in clear summer. The decrease of leaf temperature and the temperature of 20cm around the ground, especially the sharp decreased gound surface temperature, improved the root growth condition. The biosynthesis of iPA is vigorious in the root and the contents of iPA, GAs and the ratio of GAs/ABA increased remarkably; The metabolism of activated oxygen is low, the regenerating and clearing system of free radical were relatively balanced, which weakened the destroy of free radical to membrane, slowed the membrane lipid peroxidation and protected the integration of membrane structure. The reduction of high luminosity limited photo-inhibitant photosynthesis, which led to ligh activity of RuBp carbocylase and net assimilation rate; The mechanism that the regenerated lettuce seedling retards heat injury lied in the anter strong root with acitve absorption and synthesis capacity, which provided the water needed by the shoot for transpiration as well as plenty of physiological active substances such as iPA. Stroma resistence declined, transpiration enhanced, the leaf temperature went down, the high activity of those enzymes for the clearing system of free radical in the shoot such as SOD and POD, relieved the membrane damage by free radical from the activated oxygen metabolism, slowed membrane lipid peroxidation and kept relatively intact membrane structure. On the other side, the increase of ABA level and decrease of GAs enhanced the stress-resistance.The physiological basis of the formation of regenerated seedling of lettuce were also studied. The result showed the formation of regenerated seedling isrelated to the variety heredity. The formation ability of butter lettuce is stronger than that of loose leaf lettuce. The formation of regenerated seedling was also related to the substances in stubble root, especially the content of proteins. It has been found that the endogenous plant hormone level and its changes had an important effect on the formation of regenerated seedlings. Those hormones such as iPA, GAs, which prompt the division and growth of cells, had positive effects on the formation. While the other hormones which inhibit the cell growth obstracted the formation process. The content of the two kinds of plant hormones and their consistency determined the regenerating ability of the variety.Studies on the two summer cultivation techniques suggested : the key of the simple, efficient shading net covering of cool vegetable off-season culture is to select a correspondent net type with appropriate colour and shading percentage according to the climate, the plant growth condition and the aim of covering. For example, it will get a good effect for selecting the black net with 70% shading percentage when maxium air temperature surpases 35°C in clear summer. The shading net must be taken off from Pak-choi 5 days before harvest, otherwise, the negetive effects such as the content of vitamin C and proteins decrease and nitrate accumulation appeared. The key points of lettuce regenerated seedling cultivation techniques in summer; the first, select the varieties such as shanghai Baituo, which had strong regenerating ability and hard to earlier bolting, in order to make use of the huge anter root; the second, the methods such as remaining 2 pieces of leaves on anter root, selecting 1 or 2 strong buds and improving N-nutrient a week before harvest were also applied to obtain better quality products.