The effect of plant density on the micro climate of sunflower (helianthus annus l) crop in a medium potential, semi-humid area in Kenya
The relationship between soi I factors (soil moisture, soil temperature and soil nutrients), plant factors (stomatal conductance, leaf temperature, leaf area index (LAI ) and photosynthetic rate) and environmental factors (diffuse and global irradiance) was studied in a sunflower field. The sunflower was planted at four density plots with plot 1 having a higher than normal and plots 3 and 4 having ~l lower than normal plant densities. Plot 2 had normal plant density and acted ~lS control. Measurements of global and diffuse irradiance, photosynthetic rate. photosynthetically active radiation (PAR), stomatal conductance and leaf temperature were made in the upper and lower strata of sunflower canopy. Soil moisture was measured weekly at 30cm, 40cm, 60cm and 1OOcm while soil temperature was measured once a week at 5cm, 10cm and 20cm depths. Measurements were done at three stages of crop growth. These were the vegetative. reproductive and maturity stages. The experimental site was located at the University of Nairobi, Kabete Field Station, The field station is about 1800 metres A.S.L. and about 10 km north of Nairobi. It is located at at 1(l lS'N, 3611 44'E. The station experiences two rainy seasons. with the main rainy season being March-May (long rains) and October- November (short rains). The land was prepared for planting and subdivided into four equal plots of 9m by om The plots were 1m apart up the slope and 2m apart across the slope (cf. fig . .:;).The land was planted with sunflower seeds (Hybrid 8998) on 1-4-97. No fertiliser was applied to the plots. Weeding was done twice during the experimental season; on 24-4-97 and on 26-5-97. l.eaitemperature, stomatal conductance, PAR and photosynthetic rate measurements were done using an Infrared Gas Analyser (lRGA). These were done 1'01' three Jays during each of the three phenological stages of sunflower growt Daily duration period of measurements was from 800-1700 hours local time. The global and diffuse irradiance were measured at the same time as the physiological parameters using Kipp solarimeters. Results showed that leaf temperature increased from morning hours to afternoon hours before decreasing slowly as sunset approached. Photosynthetic rate and PAR were highest in late morn ing and early afternoon hours. Stomatal conductance was highest in early morning and late afternoon hours. The average soil temperature was higher at 5cm depth than at lOcm and 20cm depths. However. in early morning hours. soil temperature at 5cm was lower than at the other t\\0 depths. Global and diffuse irradiance in the upper strata of canopy were independent of plant density. In the lower strata of canopy, the global and diffuse irradiances decreased with increasing plant density. The irradiances were also greater during the vegetative stage than during the reproductive and maturity stages. Results of yield assessment (above ground dry matter) and economic yield sho« eel that plot 1 gave the highest yield for total above ground dry matter per plot followed by the control (plot :2). Plots 3 and 4 recorded the lowest values both on the above ground dry matter and economic yield per plot. The plant physiological parameters did not reveal much about the effects of planting density, as evidenced by the results of the analysis of variance. However, the annual patterns presented for the various parameters shed some light on the differences as manifested by the plant density. These differences were mostly pronounced on irradiance (global and diffuse) below the sunflower canopy. The irradiance decreased with increasing plant density. Soil temperature at all depths also decreased with increasing plant density.