Varying levels of incident solar irradiance and microclimatic variations on banana (Musa spp) growth and productivity.

Abstract

A field experiment was conducted at Inselele, KwaZulu-Natal South Coast, South Africa, in 1999/2000, to assess the influence of shading as related to varying levels of incident solar irradiance as well as microclimatic variations on banana (Musa spp) growth, phenology trends, morphology and productivity. The trial was established in August 1999 on a ratoon plantation. The experimental site, 0.655 ha in extent, comprised of three replications with four treatments having varying levels of incident solar irradiance levels of 100 %, 70 %, 40 % and 20 % under a planting density of 1666 plants ha. The irradiance levels were derived from black shade cloth, erected 1 m above the banana canopy level. Banana plantations have vegetation that does not completely cover the underlying ground surface. For such a canopy, there are basically two distinct and interacting surface components, the overstorey/canopy and the understorey/soil. Independent investigations and measurements of the solar energy fluxes for each of these two components forms a vital step to comprehend the factors that control the overhead energy fluxes in the plantation. In this study, evaluation of flux components in the understorey of the canopy using plastic microlysimeters was conducted. Considering normal variations in field measurements, the agreement and consistency among the different measurements with previous findings was adequate. Microlysimeter measurements of daytime soil evaporation were generally less than 1 mm, with an average of 0.45 mm. The study did not include flux measurement at the two level approach (Bowen ratio and Eddy correlation methods) which could have been compared with the single level approach and microlysimeter results. Two methods of deriving/measurement of energy fluxes were used and the differences between them are discussed. A reassessment of the microlysimeter technique is suggested. For the understorey, the sum of sensible and latent heat fluxes derived from spreadsheet computation was equal to the available energy. Mean soil temperatures at a depth of 20 to 60 mm ranged between 13 and 16 degrees C. This study illustrated that energy flux measurement and interpretation in within the experimental area. Good agreement was found in the patterns of wind speed profile measurements, with the 1.5 m profile depicting a mean difference of 52 % compared to the 3 m profile between day of year (DOY) 230 and 248 inclusive. This suggested that wind speed attenuation was strongly correlated to increment in height within the plantation due to canopy roughness. Differential canopy temperatures (measured with infrared thermometry) were more sensitive to the vapour pressure deficit than to wind speed. The most dense shade affected banana productivity indicated by a bunch weight of 22.69 and 33.65 kg under the 20 % and 100% irradiance treatments respectively. The bunch mass reduction was 32 %. Flowering dates were delayed by 8 days, 13 days and 21 days with incident irradiances of 70 %, 40 % and 20 % of the unshaded control (lOO % irradiance), respectively. The phenological responses in this study appear to be a result ofa contribution or interaction of both seasonal responses and shading treatments and this is further evidenced by the high levels of correlation (98.4%) reported between these two variables. Shading resulted in diminished leaf emergence rates (LER), pseudostem circumference and pseudostem height. However, just before flowering, no significant differences were observed in the pseudostem circumferences. There was a progressive increment in pseudostem height for all the treatments, with the 20 % irradiance treatment depicting the least heights registered compared to the rest of the treatments. Evident seasonal differences were registered in the LER and emergence to harvest (EH) interval. Comparison of LER and mean air temperature trends revealed a similar curve pattern and depicted a highly positive correlation of90.4%. The highest LER of3.8 was recorded in the month of January at a peak mean monthly air temperature of24.3°C while the lowest LER of 1.2 was registered in July which had the lowest mean air temperature of 14.7 cc. The EH intervals measured between September and December flowering varied from 125 days to 112 days respectively before harvest yet the April to May flowering had an EH duration of186 to 195 days respectively. The strong seasonal influence on phenological responses is further confirmed by these EH trends. Reductions in LAI observed with time were principally due to leaf senescence.