Interactive effects of mechanical impacts, temperature and humidity on water relations and tissue elasticity of stored carrots

In roots of the biennial carrots (Daucus carota L.) an environmentally cont rolled (winter-) dormancy can be easily induced by dark, cold and humid con ditions. This makes it a suitable crop for long-term storage. However, in p ractice carrots are not always kept in optimum conditions, but are often ex posed to drought stress during postharvest handling. This negatively affect s root water status and texture. Mechanical impacts during mechanised harve st and postharvest handling enhance the stress effects. Plants respond to w ater deficits by several distinct metabolical adaptation mechanisms. By mea ns of a scholander-type pressure bomb and a material testing machine we stu died the interactive effects of mechanical impacts, temperature and air hum idity on water status and tissue elasticity in stored carrots. Pressure vol ume analysis was used to characterize the interrelationship between these t wo parameters. Even short-term storage (3 d) at room temperature(ca 18 degr eesC) in high (ca 90% rH) and very high (ca 98% rH) humidity led to a prono unced reduction of both water potential and pressure potential in hand-harv ested carrots (control). Controlled artificial mechanical stress (24 falls from a 30 cm height onto a belt) significantly enhances these responses alt hough relative water losses were only slightly increased. Carrots adapt to drought stress by osmotic adjustment, i.e. a net increase in osmotically ac tive organic solutes. Cold (5 degreesC) high humidity (98% rH) storage (15 d) diminished the reduction of water potential, pressure potential and wate r content but did not influence osmotic adjustment. At room temperature a d ecline in both the apparent compressive (E, determined from force-deformati on curves), and in the volumetric (epsilon, obtained from pressure volume a nalysis) modulus of elasticity always accompanied the reduction of water st atus. However, during cold storage, E significantly increased in both contr ol and mechanically stressed roots, indicating that carrots are able to adj ust elastically. Elastic adjustment is an effective metabolically controlle d mechanism, which, in addition to osmotic adjustment, helps to maintain a positive turgor despite slow water losses. Mechanical stress, however, enha nces the decline in water status and reduces the ability to metabolically a dapt to water deficits thus reducing the storability of carrots.