Effect Of Deficit Irrigation On Growth, Yield, Fruit Quality And Capsaicinoid Synthesis Of Hot Pepper Grown In Soilless Culture

Sand is considered an important substrate types used in soilless culture, and also has a low cost and ready availability. However, it has poor physical and chemical properties. Waste material from major agricultural crops such as cotton are an abundantly available renewable resource, it could be used to make compost to be used as an amendment to improve the properties of sand. Available water for agriculture is becoming scare and there is an urgent necessity to adopt effective strategies of irrigation management. Deficit irrigation is considered as water-saving irrigation strategy. Hot pepper (Capsicum annuum) fruits are a rich source of antioxidant compounds such as vitamin C; also it’s considered the only source of capsaicinoid compounds. Capsaicin and dihydrocapsaicin are the most abundant capsaicinoid in fruit of hot pepper.This study was carried out to obtain the optimum ratio of sand to cotton stalk compost for a good growing media in soilless culture and using this ratio as a growing media to study the influence of deficit irrigation, on physiological, biochemical parameters, vegetative growth, fruit parameters, yield, content of vitamin C, capsaicinoid contents and synthesis of fruit during different time of fruit ages, as well as expression of csyl gene, which responsible for coding capsiacinoid synthase in fruit. Four irrigation treatments; T1(100%) of water holding capacity of growing media (control); T2(85%); T3(70%) and T4(55%) of water holding capacity which were considered as deficit irrigation treatments. All irrigation treatments were given to the plants at the first day of transplanting and continued during the whole growing season. The main results summarized as follows:1-Addition of different proportions of cotton stalk compost to sand soil improved certain physical and chemical properties of sandy soil. The optimum ratio was1:1v/v sand to cotton stalk compost. The physical properties of this mixture were a BD of1.04g/cm3, an AS of14.34%, a WHC of40.11%and a TP of54.45%. The chemical properties of this mixture were a pH of7.86, an EC of1.18mS/cm, a TOC of84.42g/kg, a TN of8.41g/kg, an OM of145.55g/kg and a C/N ratio of10.05. Thus, this mixture is a good growing media candidate for soilless culture.2-Increasing deficit irrigation has decreased chlorophyll content and net photosynthetic rate. The highest reduction in chlorophyll content was30.98%and34.00%which obtained from T4on30and45days after transplanting respectively. While the lowest reduction was13.67%and13.08which obtained from T2on30and45days after transplanting respectively, as compared with the control. The maximum reduction in net photosynthetic rate was29.85%and40.39%which obtained from T4on30and45days after transplanting respectively, while the lowest reduction was14.15%and14.26%which obtained from T2on30and45days after transplanting respectively, as compared with the control.3-Increasing deficit irrigation led to increase in the activities of antioxidant enzymes and MDA content. The maximum activities of SOD and CAT enzymes were97.03%and88.99%, respectively which obtained from T4on45days after transplanting. Meanwhile the lowest activities of these enzymes were38.56%and48.49%, respectively which obtained from T2on45days after transplanting as compared with control. The maximum content of MDA was126.40%obtained from T4, while the lowest content of MDA was33.29%obtained from T2on60days after transplanting, relative with the control. The period from30to45days after transplanting had the highest level of chlorophyll content and net photosynthetic rate as well as the maximum activity of antioxidant enzymes with lower values of MDA content. Thus, this period is considered critical period of irrigation this cultivar under our condition.4-Deficit irrigation had a negative effect on plant growth and yield. Increasing irrigation deficiency exhibited a reduction in vegetative growth, fruit parameters and yield and a non significant increase in irrigation water use efficiency and a corresponding reduction in the amount of irrigation water.5-Increasing deficit irrigation led to decrease in vitamin C and capsaicinoids (capsaicin and dihydrocapsaicin) contents in fruit at various time of fruit ages. Water holding capacity of100and85%, resulted in the highest content of vitamin C (91.43and87.38mg/100g FW) respectively, which obtained on30days after flowering and there was no significant differences in vitamin C content between these treatments on30days after flowering. The maximum values of capsaicin and dihydrocapsaicin contents (1.92and0.39mg/g DW) respectively, was obtained from T2on30days after flowering.6-A positive correlation between concentration of capsaicinoids and the activity of enzymes involves in synthesis and this effect was correlated with fruit age. A corresponding reduction in peroxidase activity in fruit was observed. The highest activities of enzymes involves in capsaicinoid synthesis obtained from85%of water holding capacity on30days after flowering with lower activity of peroxidase enzyme. Also the changes in expression level of csyl gene were correlated with the changes in the activity of capsaicin synthase enzyme. The greatest transcription level of csyl gene was obtained from85%of water holding capacity on30days after flowering. These results provide new information on the accumulation and synthesis of capsaicinoid in fruit of hot pepper in response to deficit water.Finally, we concluded that ’Battle’ hot pepper is sensitive to deficit irrigation. Supplying this cultivar with water at85%of water holding capacity could be a feasible irrigation technique for high value of vitamin C production and high contents of capsaicinoid as well as saving large amount of water which outweighs the decrease in total fresh yield of fruit especially in areas suffering from water shortage.