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

[摘要] English: Concerns regarding environmental pollution, emanating from agricultural effluent due toabusive 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 qualityand environmental aspects of crop. In view of these concerns this study evaluated theresponse of carrot (Daucus carota L. cv. Karina) to varying fertilizer levels, applied singlyand in combination with two commercial bio-stimulants, ComCat® and Kelpak®. Both biostimulantsare plant extracts, containing natural active compounds, and are appliedexogenously to manipulate crop growth and yield. No reports on the use of either of thetwo products on carrot production could be found in literature. During both seasons ofgreenhouse studies, the incremental increase of NPK fertilizer contributed to increasedvegetative growth and root fresh mass, though not significantly at all times. Theapplication of bio-stimulants had an erratic effect on carrot growth under greenhouseconditions. However, both bio-stimulants in combination with the standard fertilizer levelenhanced root fresh mass at harvest (Chapter 3).Under field conditions, especially during the 2007 growing season, foliarapplication of both bio-stimulants enhanced yield. Although the ensuing increases werenot significant, a higher increase of 7 ton ha-1 was attained due to application of acombination of ComCat® with the standard fertilizer. A lower increase of 4 ton ha-1 wasachieved 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 fertilizerlevels separately and in combination with bio-stimulants was verified throughdetermination of selected physiological activities (Chapter 5). Sucrose content tended toincrease, 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® hada strong enhancing effect on sucrose translocation as evidenced by the positiverelationship between sucrose content, radio-active translocation to the roots and finalcarrot 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 berelated to ComCat® enabling improved sucrose transport across membranes. The effectof Kelpak® on sucrose accumulation and translocation was less evident but, incombination with half of the standard fertilizer, its application led to a slight increase inyield. The accumulation of-carotene, a terpene associated with taste and quality ofcarrots, was not influenced by fertilizer application. The level, however, increased whereComCat® was combined with the half-standard fertilizer level.To further comprehend the manipulative effects of fertilizer and bio-stimulants onphysiological processes influencing growth and yield of carrots, root respiration as well asactivities of glycolytic and oxidative pentose phosphate pathway regulatory enzymes wasdetermined. In all cases the activities were only measured at 30% plant development and24 hours after second bio-stimulant application. Both ComCat® and Kelpak® increasedthe respiration rate as well as the activity of glycolytic and oxidative pentose phosphatepathway key enzymes during early carrot development under 25% and zero fertilization.The latter was probably due to nutrient stress during early development. However, ascarrots matured nutrient acquisition and energy needs were probably reduced asindicated by the respiration rate remaining at a steady state during later developmentstages and at harvest across all fertilizer levels. This correlated positively with increasedsugar levels at maturity and the final yield.In conclusion, the collective interpretation of growth and physiological dataacquired during this study at different fertilizer levels, especially when considered inrelationship with final yield data at harvest, does not supply a sufficient rationale torecommend either the use of sub-optimal levels on their own or in combination with thetwo bio-stimulants included in this study for the cultivation of carrots. However, it isrecommended that this study be repeated under irrigation conditions but with plots farenough apart to prevent sideways movement of fertilizer and possible contamination ofplots.

[发布机构] University of the Free State