Gas exchange, nitrate assimilation and dry-matter accumulation of Theobroma cacao seedlings submitted to different irradiances and nitrogen levels

Abstract

SummaryThe experiment was conducted at CEPEC/CEPLAC (Ilheéus, BA, Brazil) using open-pollinated Theobroma cacao seedlings whose female progenitor was SCA 6. The seedlings were submitted to four irradiance levels (10, 25, 50 and, 100% of photosynthetically active radiation, PAR) obtained through artificial shading. At 115.d after planting, the orthotropic axes were cut off at 50.cm height and nitrogen treatments were applied at the rates of 0, 3.5, 7.0 and 14.mM. After axillary bud break, the growth of only one ¯ush per plant as maintained on the orthotropic axis. At the F2 stage, mature leaves of pre-flush 1 (PF-1) were used to measure gas exchanges, nitrate reductase activity (NR), N-NO3– and chlorophyll concentrations. In general, increases in net photosynthetic rates (Pn) were found with increase in N doses from zero to 7.0.mM at all irradiance levels. From this dosage, Pn decreased at the 10 and 25% PAR levels. The plants to which no N was added after cut off showed significantly higher Pn values at 10 and 25% PAR and lower leaf and air temperatures in relation to 50 and, 100% levels. The opposite was under 14.mM N where the highest Pn values were obtained at 100 and 50% PAR levels. The highest value of internal CO2 concentration (Ci) was obtained at 0.mM N, independently of irradiance level. Values of Ci and stomatal conductance (gs), at 50% PAR, were higher than the other measured values in all nitrogen doses. Conditions of maximum irradiance did not favour higher gs, contributing to the reduction of transpiration rates (TR). However, seedlings at 0.mM of N showed higher TR than the others. There were increments in NR activity with the increase in nitrogen, except at 100% PAR. This increase in activity was observed only from 0 to 3.5.mM N. The increase in N doses proportionally increased the stem N-NO3– concentrations, mainly at the 10% PAR level. On the other hand, N-NO3– concentration of PF-1 and pre-flush 2 (PF-2) leaves were higher than the concentration of the remaining leaves after cut off, at the different PAR levels and N doses evaluated. Leaves of PF-1 and PF-2 were responsible for the higher dry mass acccumulation in relation to stems, with the increase of irradiance levels and N doses, except at 10% PAR. At this irradiance level, the accumulation of leaf and stem dry mass increased from zero to 7.mM, decreasing afterwards.