Comparative Study of Diurnal Rate of Photosynthesis at Various Levels of Carbon Dioxide Concentration for Different Crops

Abstract

Crops capture solar energy and transform it into chemical energy by process of photosynthesis. Light energy is transformed into chemical energy, which, in turn is used for reduction of carbon dioxide to carbohydrates. With an extra supply of carbon dioxide both saturation light intensity and efficiency of light utilisation increases, which ultimately increases dry matter production. Crops with C4 pathway show less response to increased carbon dioxide as compared to crops with C3 pathway. Photosynthetic rate also fluctuates with diurnal variation of temperature. With this background an actual field experiment was conducted at adjacent plots of CAgMO (lat. 18° 32′, long. 73° 51′), Pune, with four crops viz., sunflower, soybean, mustard and jowar. All the crops were sown on 28 October 2003, in order to have better comparative study among the crops. Weekly observations were taken at 3 levels of carbon dioxide concentration viz., normal, 550 and 650 ppm by using LI 6400 portable photosynthatic system from 08 December 2003 to 19 January 2004 (i.e., from 42nd to 84th days after sowing or DAS). At each of the weekly intervals, observations were taken at 2 hourly intervals viz. 0800, 1,000, 1,200, 1,400 and 1,600 h IST to measure the rate of photosynthesis from which daily mean values were computed. Statistical analysis was carried out in Randomized Block Design (RBD) to find out critical differences in daily mean rate of photosynthesis at different levels of carbon dioxide concentration and various hours of observation, to identify statistically significant variation. The objective of the study was to quantify and compare mean rate of photosynthesis at various levels of carbon dioxide and at various hours of observation among different crops. The paper discussed about how the rate of photosynthesis varied with levels of carbon dioxide and different hours of observation. Among the four crops studied during the study period, the rate of photosynthesis was the highest for sunflower crop and the lowest for soybean crop. For all the crops studied, the rate of photosynthesis increased with levels of carbon dioxide at the earlier weekly observations. For sunflower crop, the rate of photosynthesis varied significantly between normal and 650 ppm CO2 level, though the difference between 550 and 650 ppm levels were not significant up to 4th weekly observations (i.e., up to 63 DAS). Low critical difference values were observed at later weekly observations (i.e., after 70 DAS). For soybean, significant variation in rate of photosynthesis was observed up to 4th weekly observations (i.e., up to 63 DAS). However, in case of mustard, the rate of photosynthesis at various levels of CO2 did not differ significantly at earlier weekly observations (i.e., up to 63 DAS), though low critical difference values were observed at later weekly observations (i.e., after 70 DAS). In case of jowar crop, rate of photosynthesis increased up to 3rd weekly observations (i.e., up to 56 DAS) and the critical difference values at normal and 550 ppm CO2 level were almost significant. Similarly, the diurnal rate of photosynthesis showed that the values increased up to 1,200 or 1,400 h IST for the crops studied. The rate of photosynthesis mostly differs significantly between morning/evening observations and 1,200 or 1,400 h IST observations up to 4th weekly observations (i.e., up to 63 DAS), though critical difference values were less in later observations (i.e., after 77 DAS). In the present context of climate change scenario and its possible impact on crops, the study will be useful in predicting net assimilation.