The Research On The Photosynthetic Physioecological Characteristics Of Cassava

Cassava is one of the three potato categories, mainly cultivated in tropical and sub-tropical regions. Because during the course of growth and development of cassava often have droughts, typhoons and other natural disasters, so will have a greater impact on cassava production by environmental factors. Moreover, cassava is still a lack of systematic cultivation of theories and techniques to compare with the crops of rice and wheat, in particular the basic research of photosynthesis, stress physiology is less. Photosynthesis is the basis of material production, photosynthetic characteristics of cassava and its relationship to the environment not only for the rich theories of cassava cultivation and stress physiological and important for exploring the potential production capabilities of cassava, and while provide theoretical basis and technical parameters for the varieties layout, adaptability cultivation and high yield and stable. Therefore, this study used the cassava which cultivated under different moisture and light as a test materials, and investigated photosynthetic characteristics of cassava and its response to environmental factors from the ecophysiology point of view. The main results were as follows:1. Through the analysis of many environmental factors, we found that soil relativ water content (SRWC), photosynthetically active radiation (PAR) and air humidty (RH)had a great influence on stomatal conductance. RH with constant strong positive correlation with Gs was the principal environmental factors working quite independently. Although SRWC and PAR had significant positive correlation with Gs, yet their impact magnitude would vary with the change of SRWC. This was because when the SRWC was low, the SRWC became the dominant factor affecting Gs, while the SRWC was high, the PAR was the dominant factor. The relationship between the Gs and SRWC, PAR, and RH could be expressed as exponential exponential model. And this model has high accuracy, not only could be described Gs quantitatively by three environmental factors of SRWC, PAR and RH, but also the b2 of the model's parameter indicated that the critical value of drought stress, and it provided the data basis to go on investigating the characteristics and mechanism of drought tolerance among cassava varieties. Using the constructed model, we found that the lower limit threshold of appropriate SRWC for cassava variety SC8 was 51.9%.2. The results showed that the maximum photosynthetic rate (Pmax), apparent quantum yield (AQY), light saturation point (LSP), carboxylation efficiency (CE) and CO2 saturation point (CSP), had evidently threshold responsed to the variations of soil moisture. The main parameters of photosynthesis with a little change under the range from 55% SRWC to 75%SRWC, but with significant increase or decrease trends under the range from 35%SRWC to55%SRWC. According to the impact of SRWC on photosynthetic efficiency, we preliminarily viewed that the drought tolerance cassava varieties of SC10, SC5 and SC8 was better than SC9 and SC205, mainly due to the small decline of Pmax and AQY relatively; the small decline of stomatal conductance relatively, while the small limition of non-stomatal factor on photosynthesis relatively; chlorophylla(Chla), chlorophyllb(Chlb), chlorophyll(a+b)(Chl(a+b)), carotenoid(Car), Chla/b and Car/Chl were relatively sensitive to the changes of soil moisture and rose more steeply under drought conditions. The ratio of phosphoenolpyruvate carboxylase and ribulose-1,5-bisphosphate carboxylase increased with the decreasing SRWC, that was the C4 pathway to increase expression.3. With the increase of shading level, Pmax, LCP and LSP decreased, while the AQY was different in varieties. The trend was generally decreased of Pmax-co2 and CE in all varieties and increased of CO2 compensation point(CCP), CO2 saturation point(CSP) in SC9, SC8, SC205; the trend was down-up-down of the CCP, CSP in SC10, and down-up in SC5. By comparing the photosynthetic efficiency of shading55% and 85%, we preliminarily viewed that the shade tolerance of SC10, SC5 and SC8 was better than SC9 and SC205, mainly due to their stomatal conductance showed a smaller decline, and the increase large of AQY. The LSP, AQY, intercellular CO2 concentration (Ci), LCP were closed to Pmax in all the factors that influence the Pmax. Therefore, the LSP, AQY, Ci, LCP were preliminarily considered as evaluation indicators for shade tolerance of cassava.4. The study used the expression of relationship among net photosynthetic rate (Pn), Gs, atmosphere CO2 concentration (Ca)and Ci, that is Ci=Ca - Pn/Gs to analyse the main factor affecting on photosynthesis of cassava under different moisture and light conditions. We found that the main affect factor on photosynthesis of cassava closely related to the light intensity. In a certain light intensity range, photosynthesis was mainly affected by stomatal limit rather than PAR. Out of this range, photosynthesis was obviously affected by PAR, and the critical turning point of PAR was observed with the change from stomatal limit to non-stomatal limit. In this experiments, the critical turning point of PAR was 600μmol·m-2·s-1. And its action degree closely related to the soil moisture, also there was difference in varieties. Under different light conditions, the main factors affect the photosynthesis of cassava also shift the PAR threshold, that was, when the light intensity in a range of value, photosynthesis was mainly affected by stomatal limit, when the light intensity out of the range, photosynthetic materials (CO2) to achieve dynamic balance of supply and demand, photosynthesis was affected by stomatal and non-stomatal factors were small, and the threshold level was different due to varieties and their differences in shading. 5. The induction of drought on PEPCase activity was different among varieties. The ratio of PEPCase and RuBPCase of SC10, SC8 and SC5 increased, while the SC9 and SC205's decreased under drought conditions. Although the ratio of PEPCase and RuBPCase from initial increase to decrease when the drought exceeded a certain degree. The variation range of the ratio of chlorophylla and chlorophyllb was 2.812～3.347 under different soil moisture. Under different soil moisture and light conditions the variation range of CO2 compensation point was 57.5～92.2μmol·mol-1,69.5～104.6μmol·mol-1, respectively. The photosynthetic rate was increased more than 10% by short time increase CO2 concentration. These changes of cassava were similar to the C3 plants.