Growth, development and photosynthesis of the snapdragon (Antirrhinum majus L.) leaf canopy during different seasons
Production and plant growth through the winter solstice and that through the summer equinox were compared. Cultivars of both groups were able to produce a flowering shoot in either the summer or the winter tests. However, the prevailing light environment predominately affected the commercial quality of the crops. Crops of all cultivars grown during the winter acclimated similarly to low-light. All flowering shoots under this conditions required a longer period to reach commercial maturity (20 to 30 DAS). An extended greenhouse cultivation was reflected in longer stems and flowering racemes. The stems produced in the winter were also heavier than those produced during the summer particularly in the GIV cultivars that produced a larger leaf canopy consisting of many lateral branches. The extensive growth of laterals parallels a poor quality of the GIV cultivars. It appears that acceptable commercial production of GI cvs during the winter can be attributed to the early flowering of these cultivars, the development of smaller flowering sinks and the ability of the axillary leaves to maintain photosynthesis at low-light levels (i.e., low LCP, high PE and similar PnMax compare to the GIV cultivars during high light episodes (Table 12). Importantly the LCP for axillaries of GI cultivars was 43% lower than that of the GIV cultivars (18 versus 32 [mu]mol·m-2·s -1). The PE of GI cultivars was about 26% higher than that of GIV cultivars (2.4 versus 1.9 [mu]mol CO2·m-2 ·mol PAR-1 * 10-2). Both GI and GIV cultivars are able to maintain high photosynthetic and export rates on bright winter and summer days. These results are noteworthy since unlike shade plants that also have low LCPs, the snapdragon leaf did not appear to be subject to photo-oxidative damage when exposed to a high-light condition, point, high photosynthetic efficiency (PE) but similar photosynthesis at high light to that observed in the GIV cultivars. Whole plant net carbon exchange (NCER) measurements confirm as a net result, that at a canopy level GI cultivars had lower LCP reflecting the physiology of the axillaries. This cultivars grew more rapidly than did the GI cultivars under low light conditions. It should be pointed out that the GIV cultivars accumulated biomass under the low-light winter conditions but much of this growth was due to the development of a large of lateral branches making the GIV cultivars unsuitable commercially. In summary as a model system for low-light production of winter grown crops, productivity, photosynthesis and partitioning in the GI and GIV cultivars indicate that the selection of plants may need to be targeted to crops with low LCPs and rapidly developing flowers. To our knowledge, these studies set an interesting precedent since represent the first report that chronicles how a specific difference in leaf photosynthesis bred into a crop might lead to a commercial advantage when growing that plant under low-light conditions as would occur in a greenhouse during the winter months.