Effects of support angles on the morphology and growth in the herbaceous climber Trichosanthes kirilowii

Climbing plants differ from non-climbing and self-supporting plants in many growth characteristics, which mainly result from biomechanical constraints. External supports, whose status determines the strategies of climbing plants to search sunlight and other external supports, are important resources for climbing plants. The morphology and growth of climbing plants may be affected by external supports to a large extent. However, the climbing plants are often shaded by their own shoots when they grow on the supports, and changing support angles might induce different shading effects. We called this phenomenon as self-shading. It has reported in many literatures that plants would adapt themselves to different light conditions through altering biomass allocation and growth behaviors. It is therefore hypothesized that self-shading would influence growth and morphology of climbing plants through changing morphology and biomass allocation. We tested this hypothesis in a garden experiment, carried out in the Ecological Garden of Southwest China Normal University with a perennial herbaceous climbing species Trichosanthes kirilowii. On 17 May 1999, the seedlings of Trichosanthes kirilowii with the same size were planted in 40 figuline containers filled with cultivated soil, one seedling per pot. Seedlings were directly offered external support made of iron line(Φ4mm) at the beginning of the experiment. All plants were divided into 4 groups, and each group was offered external supports with given angles (i.e. 0°, 20°, 40°and 80°). The direction of supports was northward. Each plant was supplied with the same quantity of water and nutrient during the experiment. The following parameters were measured at intervals of 14 days from June 6 to August 20: stem length; stem diameter; internode length; leaf number; shoot number; single leaf area; petiole diameter and petiole length. At the end of the experiment, all the plants' above-ground parts were harvested and measured the same parameters above mentioned. The plant was separated into main stem, shoot, lamina, petiole and tendril, and cleaned by tap water. All plant parts were oven-dried for 72h at 84℃ and then weighted separately. The morphological plasticity in different developmental stages of Trichosanthes kirilowii responded to self-shading in varying ways. The plants showed more sensitively plastic responses to self-shading in the early than in the late developmental stages. The degree of self-shading increased with the enlarging of the support angles. The specific stem length and specific petiole length increased with the enlargement of the support angles, and the values of the two parameters at 0°, 20°, 40°and 80°levels were 155.83cm·g -1, 147.34 cm·g -1, 168.37 cm·g -1, 176.92 cm·g -1 and 368.98 cm·g -1, 415.63 cm·g -1, 496.53 cm·g -1, 588.33 cm·g -1, respectively. These results indicate that the plants show stronger responses in morphological plasticity under strong self-shading conditions than under weak self-shading ones. Specific stem length, specific leaf area and biomass allocated to lamina and petioles had no significant differences among four climbing angles. The plants adapted to different support angles mainly through changing the number of shoots, shoot morphology and shoot biomass allocation. These results suggest that the self-shading only had limited effects on plant morphology and plant biomass allocation. The plants climbing on the supports with large angle had stronger branching intensity, more shoots and more shoot biomass allocation than those climbing on the supports with small angle. The shoot number, branching ratios and shoot biomass allocation at 0°, 20°, 40°and 80°levels were 5.60, 7.40, 9.60, 10.60; 2.40 shoot·m -1, 5.40 shoot·m -1, 7.50 shoot·m -1, 5.42 shoot·m -1 and 14.13%, 21.65%, 23.35%, 23.56%, respectively. The plants on small angle-leaned supports occupied the rich-light habitats through increasing stem biomass allocation, while those on large angle-leaned supports sought for rich-light habitats th