Stipules are the principal photosynthetic organs in the papilionoid species Lathyrus aphaca

Abstract

Occurrence of stipules of different shapes, sizes and varying configurations, has been reported in species of many angiosperm families. However, the functions of stipules in the biology of plants that carry them remain to be understood. There is perception that stipules that have the green coloration of chlorophyll are photosynthetically active. Here, total photosynthesis in the stipules and leaves of Lathyrus aphaca was investigated, together with the nature of heteroblasty and relative morphology and anatomy of stipules and leaves in this species. At each of the hundreds of nodes in the extensively branched post-flowering stage plants of L. aphaca, more than 95 % of photosynthesis occurred in the foliaceous stipules. The leaves in the form of simple tendrils at the nodes accounted for <5 % of photosynthesis. The stipules of L. aphaca were about as effective in photosynthesis as the foliaceous stipules of af and AF pea (Pisum sativum) plants. The L. aphaca stipules possessed all the cells/tissues that characterized photosynthetic leaves, stomata in epidermis, palisade and spongy mesophyll parenchyma, and vascular bundles. The tendrilled leaves were devoid of palisade tissue. The embryonic nodes bore stipule pairs of small size and compound leaves comprising of a pair of leaflets. Several nodes, above the embryonic nodes, bore fully developed stipule pairs but vestigial leaves (tendrils). It was concluded that stipules are the principal photosynthetic organs in L. aphaca.