Principle Investigator: Esnart Yohane, Plant Breeder, Department of Agricultural Research Services
Project Objectives: Pigeonpea is an important crop in many parts of Malawi. It is often grown as an intercrop together with maize and can be an important source of protein or additional income for smallholder farmers, can protect the soil from erosion, and can increase biomass production (and improve soil properties) on farm fields. For these reasons, releasing improved varieties (more adapted to Malawi's agroecolgical conditions) can offer smallholder farmers many advantages.
Methods: Twelve locally crossed lines of medium duration pigeonpea were planted at Chitedze Agricultural Research Station. Crops were planted at the beginning of the rainy season in January 2015. On May 25-26 (at the beginning of pod-fill), the MultispeQ was used to measure relative chlorophyll content (SPAD), the efficiency of photosystem 2 (Phi 2), linear electron flux (LEF), photosynthetically active radiation (PAR), and non-photochemical quenching (NPQt). The PhotosynQ protocols used to measure these parameters were ‘Chlorophyll content (SPAD) I’ and ‘The One Protocol (Phi2, PSI, NPQ) II.’ In each plot, leaves from three different positions on the plant--upper, middle, and lower--were measured from 4 plants. At physiological maturity, pigeonpea was harvested and seed weight (per 100 seeds) and pigeonpea yield were calculated for each plot.
The study was a complete randomized block design with 3 replicates. Multiple linear regression analysis was used to determine the effects of light intensity, time of day (when the leaf was measured), leaf position on the plant, and pigeonpea variety on SPAD, Phi2, NPQt, and LEF and effects were considered significant at p ≤ 0.05. A mixed effects model, with light intensity, time of day, and leaf position as fixed effects and block has a random effect was used to generate unique coefficients for each plot. Simple regression was used to test for correlations between these unique coefficients and crop yield and seed size. This model was run to include all leaves from all three leaf positions and for each leaf position independently. Effects were considered significant at p ≤ 0.05
Results: Using multiple linear regression to account for the factors affecting photosynthesis (time of day and light intensity), pigeonpea variety did not have a significant effect on PhotosynQ data. The yield, seed weight, and PhotosynQ results are presented in Table 1. There do not appear to be any trends between crop yields and PhotosynQ data. One interesting trend is the apparent inverse relationship between Phi2 and NPQt. What this suggests is that has NPQt goes up (an increase in regulation of photosynthesis) the amount of light energy going into photosystem II is decreases.
There were very few significant correlations between crop yield, seed size, and PhotosynQ data (Table 2). Relative chlorophyll content (SPAD) is positively correlated with seed size when analyzing all leaves together and the top leaf only. The lack of correlations between yield and any PhotosynQ data may be due to the late date at which the PhotosynQ data was collected. In this study, the data was collected at the beginning of pod-fill, which most likely explains the relationship between SPAD and seed size. In another pigeonpea project (http://photosynq.org/projects/on-station-evaluation-of-long-duration-pigeonpea/results), collecting data at an earlier growth stage, after flowering but prior to pod-fill, resulted in numerous correlations between PhotosynQ data and crop yield and seed size.
There were a number of limitations in this study that impeded our ability to glean useful information from the PhotosynQ data. First, the number of plants measured in each plot (4), was not sufficient for robust statistical analysis. Secondly, the PhotosynQ data was taken too late in the growth cycle of the plants to offer insights into plant responses to the environment. In future projects, data should be collected at earlier growth stages, be collected at multiple growth stages, and more plants per plot should be measured.