A study of non-pathogenic factors affecting the marketable yield of carrots (Daucus carota L.)

Production of carrot taproots for domestic arid export fresh commodity markets is a rapidly growing industry in Tasmania (Australia). While the total biomass of taproots within a crop can be high, the marketable yield is much lower. Major exporters of carrots have identified the three most important non-pathogenic causes for low marketable yields to be: (1) large variation in taproot sizes, (2) misshapen taproots and (3) taproots with longitudinal fractures (splits). During this study all three issues were investigated within the context of a commercial carrot production system on clay loam soils in a cool temperate environment. A survey of 12 commercial carrot crops revealed the current level of variation of individual plant and taproot sizes. This was recorded as a coefficient of variation (CV). Field studies examining the sources of variation showed that seed quality influenced both the CV and mean weight of plants soon after emergence. However, the effect of seed quality on variation in weight of mature taproots was negligible. An increase in the CV of taproot sizes was recorded with increasing stand density. The row position in multi row planting arrangements on beds and the uniformity of within-row spacing had only minor effects,on the CV of mature taproot sizes under current commercial planting arrangements. The longitudinal straightness of taproots was expressed early in the development of carrots, most likely during the downward growth of the primary taproot. Increasing the uniformity of within-row spacing of plants was found to decrease the proportion of misshapen roots. The length of taproots was inversely related to the stand density and soil bulk density. Mature carrot taproots were found to be most susceptible to splitting at dawn, decreasing to midday before increasing again later in the afternoon. The diurnal fluctuation of taproot splitting susceptibility was consistent with the growth pattern of the taproot, which was characterised by lateral expansion during the night time period with small shrinkages during the daylight hours. The degree of shrinkage during the day and the rate of night time expansion were affected by climatic variables. Diurnal changes in water relations, and residual strain were consistent with this patten of growth. A tangential gradient in osmotic potential was also consistent with the circumferential stress that developed in taproots. From the understanding of the lateral growth of taproots gained in this study; the hypothesis that partial defoliation the day prior to harvest would reduce taproot splitting susceptibility during harvest the following dawn was successfully tested. Taproots that spontaneously split during growth were found to do so largely in the weeks prior to harvest. Partial defoliation of leaves up to a week prior to commercial harvest also reduced growth splitting and harvest splitting. The strategic use of partial defoliation to reduce the incidence of taproot splitting is discussed. Commercial operations trialing the defoliation treatment supported the results presented.