The role of the leaf on the dynamics of growth and rooting of leafy stem cuttings of rose

Key words : Rosa hybrida , cut-rose, propagation, cuttings, leaf, rooting, root initiation, root growth, axillary primary shoot, severance, photosynthesis, carbohydrates, reduced sink activity, planting material, quality

The present study aims at better understanding the relation between photosynthesis of the original leaf, carbohydrates, rooting and growth of single node leafy stem cuttings of rose. This knowledge can be used to improve success and efficiency in propagation and improving the uniformity of the planting material of cut roses derived form cuttings.

The effects of the original leaf area on the growth of cuttings of Rosa hybrida Madelon ^®were investigated during the first 10 weeks after severance. Total plant dry weight, and dry weight of the roots in particular, were proportionally related with the original leaf area of cuttings. When leaf area was modified, leaf area duration was linearly related to the rooting and growth of cuttings during the first 21 days of propagation. The presence of the leaf during the first week of propagation was critical for survival and its removal caused stem rot. This was caused by low carbohydrate concentrations.

Cuttings remained photosyntheticaly active after severance. Photosynthetic rates decreased immediately after severance, but recovered up to 70% of the rates measured on leaves on mother plants and remained constant during propagation. The PSII efficiency decreased during propagation with a simultaneous increase in its heterogeneity across the leaflets (patchiness) which may be attributed to decreased sink activity rather then to water stress. The root and shoot tissues accounted for about 70% of the increase in total fresh weight after 21 days of propagation, whereas the remaining 30% increase was due to dry weight accumulation in the leaf and stem. About 55% of the dry weight accumulated consisted of carbohydrates, in particular starch, which accumulated mainly in the first 14 days in leaves and stem tissues (pith and medullar rays). This accumulation may be explained by reduced meristematic sink activity following severance. In fact, the newly formed roots and primary shoot after 21 days of propagation only represented 10% of the total dry weight of cuttings.

Reduced light integrals and low CO ₂ concentrations resulted in reduced rooting and growth of cuttings and decreased carbohydrate levels. Number of roots, and particularly, dry weight of roots, were linearly related with total dry weight accumulation during the 21 days of propagation showing that photosynthetic activity of cuttings during propagation influences both root initiation and growth. The effects of low light, low CO ₂ concentration, and leaf area reduction on rooting and growth of cuttings were similar indicating that these effects could be explained to a great extent by photosynthesis. Growth in general depended on the length of the period cuttings were photosynthetically active during propagation. An exception was the growth of the axillary primary shoot, which was more negatively affected by reduced photosynthetic activity in the first 11 days of propagation. Root initiation was also more negatively affected by low photosynthetic activity in the first 11 days of propagation whereas root growth responded to the integral of photosynthesis. Cuttings were able to efficiently use reserves for growth. Optimal rooting and further growth of cuttings rely on the synthesis of new photosynthates because storage is limited in single node stem cuttings.