The Biologische Bundesanstalt, Bundessortenamt und Chemische Industrie (BBCH) scale was developed to standardize the coding of phenological stages of mono- and dicotyledonous plant species (Lancashire et al., 1991). This scale can be used for defining phenotypic characters that are highly heritable and expressed in all environments; for determining the timing of cultural practices such as pesticide and fertilizer applications; and for planning the crop harvesting schedule. The BBCH scale is a unified system that aptly describes the phenological stages of most crops and weeds, although some specific stages such as those related to vegetable harvesting are not precisely delineated (Lancashire et al., 1991).
In the BBCH system, for leafy vegetables that form heads, a scale of 00 to 09 is used to describe the germination phase; a scale of 10 to 19 is then used for the leaf development phase (up to 9 true-leaf stage), and the scale values jump to 41 to 49 for the development of harvestable vegetative parts, which corresponds to the head formation phase. In the field, we often observe counts above 10 leaves before principal phenological phase 4 [i.e., the beginning of head formation (code 41)] is reached. The head formation stages are described based on percentage of expected head size reached, which can be highly variable and therefore difficult to evaluate with precision. Furthermore, growers are using not only head size, but also head firmness, to define commercial maturity.
After heading initiation, crisphead lettuce leaves continue to grow, overlapping onto each other to form a head of increasing density and size. At harvest time, maturity is evaluated on the basis of density and size (Garrett et al., 1969; Goddard et al., 1972). Crisphead lettuce must reach a desirable density to meet market requirements and to minimize handling damage. Although head density is more relevant than firmness in determining optimal maturity, estimating firmness by hand compression on a 1 to 5 scale (Kader et al., 1973) is easier and faster to perform in the field than calculating head density from measured weight and volume. Furthermore, evaluating head firmness by hand compression gives a simultaneous indication of head size. However, the assessment of firmness rating requires a well-trained evaluator. An alternative method consists of calculating head density from head weight and volume. Volume can be estimated from diameter, assuming the crisphead lettuce head is a sphere. However, a crisphead lettuce head is not uniform and more than one diameter may be necessary for an accurate estimation of volume. An equation that takes into account the departure of the shape from a sphere (Currence et al., 1944) has been successfully used for fruit such as muskmelon (Cucumis melo) (Jenni et al., 1997) and pepper (Capsicum annuum) (Ngouajio et al., 2003). Currence's equation uses two different values of a K component depending on whether the shape is more elongated or more flat, a variation found in crisphead lettuce heads.
The relation between actual head density and head firmness determined by hand compression has never been compared. Garrett et al. (1969) studied the relationship between head density and firmness based on the change in head height caused by a load increment. Schofield et al. (2000) compared firmness measured by a force-deformation method and by the hand compression method, but did not relate them to head density. In this article, we examine the relationship between head density measured by water displacement with head firmness determined by hand compression, as well as head density calculated from head weight and volume based on the polar and equatorial diameters. We also evaluate the precision of each of these approaches.
The objectives of this study were to adapt the BBCH scale for crisphead lettuce by incorporating a potential leaf count up to 19 and by providing a standardized description of the head formation phase based on head firmness; to select a nondestructive method for head volume measurements that uses the sphere equation, Currence's equation, and one to three head diameters; and to relate head density measurements made by water displacement with those derived by the hand compression method as well as with head density derived from the aforementioned equations for volume. This methodology will be easy to use in the field and will support the development of a bioclimatic model to predict maturity for planning seeding and harvesting schedules.
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