| Root tuber yield of sweetpotato (Ipomoea batatas (L.) Lam) is closely related totransport and distribution of photosynthates, which is affected by loading capacity of source(leaves), transport capacity of flow (stems) and unloading capacity of sink (root tubers). Thisstudy has proved that the photosynthates unloading in root tuber is the key process affectingphotosynthates transport and distribution of sweetpotato. Now, it is a mystery as to whatunloading pathway for photosynthates the root tuber has and what factors determine thedifferences of unloading capacity of root tubers. In addition, the regulatory effects ofpotassium application on unloading capacity are undefined. In the present study, threecultivars (Hong Xiangjiao, Sushu NO.8and Beijing553) with different characteristics ofsource and sink were chosen, and four potassium application times were used: K0, nopotassium was applied; J-K24, all potassium applied as basal fertilizer; F-K24, all potassiumapplied as top cover fertilizer; G-K24, all potassium applied as top-growth peak fertilizer. Thepathway of photosynthates phloem unloading of root tubers was studied by methods offluorescent tags and ultrastructure observation. The main factors which caused unloadingcapacity differences of sweetpotato root tubers were studied by means of physiologicalmethod. In addition, the effects of potassium application on unloading capacity of root tuberswere studied. The main results are as follows:1. Difference of root tuber yield among varieties and effect of potassium application on roottuber yield.Compared with low-yield variety, high-yield varieties got higher root tuber yield andgreater economic coefficient, and the similar biomass, which proved that distribution structureof photosynthates had main effects on tuber yield. Potassium application increased root tuberyield and economic coefficient, but had little effect on biomass. Treatment with potassiumapplied as basal fertilizer got greater increase.2. The key process that affected the transport and distribution of photosynhtates of sweetpotato Root tubers of low-yield varieties took more time to become the distribution center ofphotosynthates than high-yield varieties; compared with high-yield varieties, sucrose contentof stem base of low-yield varieties was significantly higher, and sucrose content from stem tobase had a bigger amplification; during early bulking stage, the diameter of stem base wassimilar among varieties, but low-yield varieties got significant bigger diameter of stem basethan high-yield varieties at late bulking stage. This showed photosynthate unloading in roottubers was not smooth of low-yield varieties which caused retention of photosynthates instems, especially in the stem base. To sum up, photosynthates unloading in root tubers wasthe main process that affecting the transport and distribution of photosynhtates.3. The main unloading pathway of photosynthates in root tubers during the whole bulkingstages.CF is one of the perfect symplasmic unloading tracers, which was unloaded into theroots of sweetpotato by microjection for study. The result indicated that CF unloaded in roottubers was confined in the SE/CC complexs, without leak to the parenchymal cell, whichproved its apoplasmic unloading pathway at early bulking stage (about35days afterplanting). Based on the ultrastructure observation, small intercellular space between SE/CCcomplexes or parenchymal cells was found. One sieve element was always companied by twocompanion cells; CC contained electron dense protoplasm and vacuole fragmentation;numerous plasmodesmata were found between sieve element and companion cells (CCs),phloem parenchyma cells and SE/CC complexs. Things about ultrastructure observationproved its symplastic unloading pathway at middle bulking stage. Meanwhile, the highactivity of sucrose synthase (SS), low activity of insoluble acid invertase (IAI) and highcontent of starch of root tubers at this stage proved symplastic unloading pathway played animportant role at this stage as well. The photosynthates unloading pathway went through atransformation process from apoplast to symplasm in the bulking stage.4. Physiology causes of difference of unloading capacity of photosynthates in root tubers andeffect of potassium on unloading capacity of photosynthates.The result of correlation analysis showed that the activities of SS, ADP-glucosepyrophosphorylase (ADPGase) were significant positive correlated with the starch content ofroot tubers, activities of soluble starch synthase (SSS) and granule-bound starch synthase(GBSS) were positively correlated with the starch content of root tubers; the activities of SSand ADP-glucose pyrophosphorylase (ADPGase) were significant positive correlated withstarch yield, the activities of UDPGase GBSS and IAI were positively correlated with starch yield; the activities of SS and ADPGase were significant positive correlated with dry matteryield of root tubers; GBSS and IAI activity was positively correlated with dry matter yield. Inother words, the capacity of sucrose hydrolysis of cytoplasm, transformation capacity ofstarch synthesis, and the capacity of sucrose hydrolysis in apoplast space played main roles inphotosynthates unloading capacity of root tubers. High-yield varieties had higher activities ofSS and IAI than that of low-yield varieties during the whole growth stages and late bulkingstage, respectively. At middle bulking stage, low-yield variety got higher activities ofUDPGase, SSS and GBSS and lower activities of SS. In addition, it got higher starch contentbut lower starch yield. Both illustrated that activities of key enzymes of starch synthesis couldpromote starch synthesis, but the increase of starch accumulation depended on activities ofSS. Took the significantly positive correlation between activity of SS and dry matter yield ofroot tubers into account as well, the result showed that activity of SS was the most importantfactors influencing the unloading capacity of root tubers.Potassium applied as basal fertilizer improved SS activity of root tubers, and the largerincrease appeared at early bulking stages, and the increase was88.74%,29.23%and158.60%,respectively, and big increase also appeared at late bulking stage. Potassium application asbasal fertilizer also increased IAI activity, and the larger increase appeared at late bulkingstages, and the increase was29.84%,77.41%and18.87%, respectively. It also increasedUDPGase activity in root tubers, and the larger increase appeared at early and late bulkingstages. It also improved ADPGase and GBSS activities, both varieties got larger increase atearly and middle bulking stages. The average increase of ADPGase was14.05%,18.66%and20.37%, and the average increase of GBSS was14.48%,9.49%and5.52%. Therefore,potassium application as basal fertilizer enhanced unloading capacity of root tubers by meansof improving activities of SS、UDPGase、ADPGase and GBSS at early and middle bulkingstages, and by means of enhancing activities of SS、IAI and UDPGase at late bulking stage.Potassium applied as basal fertilizer mainly effect on activities of SS、ADPGase、GBSS andIAI for high-yield varieties, and mainly on activities of SS、UDPGase、ADPGase and IAIfor ow-yield varieties.5. The regulatory effect of potassium applied as basal fertilizer on transport capacity ofphotosynthate from source to sinkPotassium applied as basal fertilizer enlarged the cross sectional area of photosynthatestransport corridor and improved osmotic pressure of photosynthates transportation along the stem, both contributed to photosynthates unloading from stem base which was good forformation of strong sink in time. During middle and late bulking stages, potassium applicationimproved photoaynthates unloading by means of increasing sucrose content and the ratio ofsucrose which was used for transportation, adding the number of transport corridors,shortening the transport distances, raising the efficiency of transportation, and improvingosmotic pressure of photosynthates transportation along the stem. More photosynthatesunloaded from stem base to root tubers, and more root tubers bulked powerfully whichresulted in high fresh yield. |
PDF Full Text Request