Effects of Plant Density, Seeding and Harvest Time on the Growth of two Kenaf (Hibiscus Cannabinus L.) Varieties

Kenaf is an annual C3 multipurpose crop for the fibre and energy industry, whose growth has been widely investigated in the tropics, but not at relatively-high latitudes. This work aimed at evaluating the effects of two genotypes (Tainung 2 and Everglades 41), two plant densities (20 and 40 plants m-2), two seeding times (S1 and S2) and two harvest times (H1 and H2) on growth and its relations with climatic factors over three years (2003-2005) in Northern Italy (c. 45° N). Fitting curves for whole-plant dry biomass (DB) and dry stems (DS) on heat sums always gave a reliable description of the growth pattern along the season, explaining over 90% of the total variation. In general, the best-fitting models were the sigmoid and the exponential one for DB and DS, respectively. Among the four studied factors, only seeding time originated consistent growth differences among years, whereas the two varieties showed an equivalent behaviour, as well as the two densities. Furthermore, the thinner density allowed savings in the cost of seed at no prejudice for yield potential. S1 in general showed higher asymptotic yields than S2 in 2003 and 2004, while S2 consistently grew faster than S1 in all the three years. RUE showed a generally low value (e.g., 1.35 g MJ-1 for DB in S1), indicating a moisture constraint on crop growth, especially in the first year. As for the correlations, three traits, plant height, base stem diameter and fresh biomass, resulted significantly associated to DB and DS, with correlation coefficients (r) ranging from 0.65 to 0.90; a higher degree of association with DB and DS was achieved by the multiple linear regressions of the same three traits (adj. R2 of about 0.85). A high dependence of DB and especially of DS on associated heat and rain (adj. R2 0.76 and 0.86, respectively) was also observed in the variable environmental conditions among the three years, which attributes a non-negligible power of prediction to the two environmental parameters. These results offer the opportunity of estimating growth through the measure of non-destructive crop variables of easier assessment, and encourages their adoption, in association with that of simple climatic factors (temperatures, precipitation) as tools of potential support to facilitate biomass assessments in commercial fields.