Growth, yield and physiological response of carrot (Daucus carota L.) to different fertilizer levels and bio-stimulants

English: Concerns regarding environmental pollution, emanating from agricultural effluent due to abusive use, have led to a paradigm shift in production technology. On the other hand, subsistence farmers in developing countries cannot afford fertilizer at the current prizes. Farmers, therefore, have to reconcile agronomic, economic, agricultural product quality and environmental aspects of crop. In view of these concerns this study evaluated the response of carrot (Daucus carota L. cv. Karina) to varying fertilizer levels, applied singly and in combination with two commercial bio-stimulants, ComCat® and Kelpak®. Both biostimulants are plant extracts, containing natural active compounds, and are applied exogenously to manipulate crop growth and yield. No reports on the use of either of the two products on carrot production could be found in literature. During both seasons of greenhouse studies, the incremental increase of NPK fertilizer contributed to increased vegetative growth and root fresh mass, though not significantly at all times. The application of bio-stimulants had an erratic effect on carrot growth under greenhouse conditions. However, both bio-stimulants in combination with the standard fertilizer level enhanced root fresh mass at harvest (Chapter 3). Under field conditions, especially during the 2007 growing season, foliar application of both bio-stimulants enhanced yield. Although the ensuing increases were not significant, a higher increase of 7 ton ha-1 was attained due to application of a combination of ComCat® with the standard fertilizer. A lower increase of 4 ton ha-1 was achieved with combinations of ComCat® and Kelpak® with the half-standard fertilizer level (Chapter 4). Growth and yield response of tap roots due to application of different fertilizer levels separately and in combination with bio-stimulants was verified through determination of selected physiological activities (Chapter 5). Sucrose content tended to increase, in concert with yield, as the fertilizer levels were incrementally elevated. Conversely, glucose and fructose content decreased in tap roots as maturity was attained, probably due to less sucrose being hydrolyzed at this development stage. ComCat® had a strong enhancing effect on sucrose translocation as evidenced by the positive relationship between sucrose content, radio-active translocation to the roots and final carrot root yield. The latter applied in both instances where the standard and half the standard fertilizer levels were applied in combination with ComCat®. This effect could be related to ComCat® enabling improved sucrose transport across membranes. The effect of Kelpak® on sucrose accumulation and translocation was less evident but, in combination with half of the standard fertilizer, its application led to a slight increase in yield. The accumulation of -carotene, a terpene associated with taste and quality of carrots, was not influenced by fertilizer application. The level, however, increased where ComCat® was combined with the half-standard fertilizer level. To further comprehend the manipulative effects of fertilizer and bio-stimulants on physiological processes influencing growth and yield of carrots, root respiration as well as activities of glycolytic and oxidative pentose phosphate pathway regulatory enzymes was determined. In all cases the activities were only measured at 30% plant development and 24 hours after second bio-stimulant application. Both ComCat® and Kelpak® increased the respiration rate as well as the activity of glycolytic and oxidative pentose phosphate pathway key enzymes during early carrot development under 25% and zero fertilization. The latter was probably due to nutrient stress during early development. However, as carrots matured nutrient acquisition and energy needs were probably reduced as indicated by the respiration rate remaining at a steady state during later development stages and at harvest across all fertilizer levels. This correlated positively with increased sugar levels at maturity and the final yield. In conclusion, the collective interpretation of growth and physiological data acquired during this study at different fertilizer levels, especially when considered in relationship with final yield data at harvest, does not supply a sufficient rationale to recommend either the use of sub-optimal levels on their own or in combination with the two bio-stimulants included in this study for the cultivation of carrots. However, it is recommended that this study be repeated under irrigation conditions but with plots far enough apart to prevent sideways movement of fertilizer and possible contamination of plots.