Studies on Matter Production of Rape Plant : V. Carbon dioxide budget and efficiency of solar energy utilization in rape plant populations

Abstract

The carbon budget and efficiencies of solar energy utilization for gross and net production of rape plant populations have been calculated from data on diurnal courses of CO₂ exchange and dry matter production. This was done both for individual days at different growth stages and for the whole growth period. The results arc summarized as follows: 1. The daily gross production (Pg), derived from data on diurnal courses of. CO₂ exchange, increased gradually during early stages of growth. At the beginning of bolting (early March), Pg began to increase steeply attaining to a maximum at the beginning of ripening (early May) and then decreased rapidly. The largest value of Pg was 61.53 g CO₂/m²/day (Table 1, Fig. 1). 2. The daily net production (Pn) varied with growth stage in a manner similar to Pg althought the maximum occurred a little earlier than that for Pg. The largest value obtained was 41.14g CO₂/m²day (Table 1, Fig. 1). 3. The Pn/Pg ratio, as well as Pg and Pn, varied largely as a result of changes in daily solar radiation (S). That is, the ratio increased with increasing S approaching a certain level asymptotically. 4. The level of the Pn/Pg ratio, calculated for fine day at each growth stage, remained around 70% during the period from the earliest growth stage to the beginning of ripening and then decreased rapidly. The ΣPn/ΣPg ratio (ΣPn and ΣPg denote total net and gross production for the whole growth period) was fairly constant and fell in the range of 53-56% in the three season's experiments. 5. The efficiencies of solar energy utilization for Pg(Eg) and for Pn(En), based on total solar radiation incident of the field, were high on cloudy days and low on clear days. As for the developmental changes in these efficiencies, it was observed that maxima appeared in the period from the middle of flowering to the beginning of ripening. The averages for days during this period, which included various weather conditions, were 0.85% and 0.47% respectively (Table 4). 6. The proportion of seed to total net production (ΣPn) on a calorie basis was 36%. Oil content of seed on a calorie basis was 62%. Thus, efficiencies of solar energy utilization for seed yield and oil production, based on total solar radiation incident on the field during the whole growth period, were estimated as 0.162% and 0.101% respectively (Table 4). 7. The results of analysis of the yield production process of rape were compared with those from other crops. The authers concluded that it is particularly important in the case of rape plant to promote development of the leaf canopy at early growth stages to increase the interception of solar radiation.