NITROGEN-FERTILIZATION AND SEX EXPRESSION AFFECT SIZE VARIABILITY OF FIBER HEMP (CANNABIS-SATIVA L)

Mechanical harvesting and industrial processing of fibre hemp (Cannabi s sativa L.) require uniformity of stem length and weight. In 1991 and 1992 we carried out field experiments to investigate the effects of s oil nitrogen level (80 and 200 kg ha(-1) N) and row width (12.5, 25 an d 50 cm) on the variability of weight and height in hemp plants. The c rops were sampled 5 times between early June and early September. Row width did not affect size variability. At final harvest coefficients o f variation (CV) of both weight and height were about 1.5 times higher at 200 than at 80 kg ha(-1)N. Distributions of dry weight were positi vely skewed at all sampling dates except the first, with skewness larg er at 200 than at 80 kg ha(-1) N. Distributions of height were negativ ely skewed at all sampling dates except the first at 80 kg ha(-1) N. A t 200 kg ha(-1) N they changed from negative skewness during the first part of the growing season to negative kurtosis in the second part of the growing season. More suppressed plants were present at 200 than a t 80 kgha(-1)N. Contrary to most published results, we did not find a reduction of CV of weight nor of CV of height at the onset of self-thi nning. Suppressed hemp plants can survive relatively well in the low-l ight environment under the canopy Sexual dimorphism contributed to var iability of height and weight, but the effects were smaller than those of nitrogen fertilization. The ratio of female to male plants was hig her at 200 than at 80 kg ha(-1) N, as a result of a shift in sex-ratio within the population of suppressed plants. As suppressed plants were much more slender than dominating plants, self-thinning eliminated th e most slender individuals in a hemp crop. However, the presence of ma ny more heavy individuals of low slenderness at 200 than at 80 kg ha(- 1) N was probably the major cause of the difference in slenderness bet ween 200 and 80 kg ha(-1) N.