Internal Factors

Beyond the external environment, several factors intrinsic to the plant itself also set a practical ceiling on how fast it can photosynthesise. Photosynthetic pigments are an essential factor, but the plant needs only a comparatively small quantity of them to sustain photosynthesis at all - even so, pigment deficiency, as seen in variegated or chlorotic leaves, does proportionally reduce the achievable rate. The protoplasmic factor refers to the general condition of the cell's protoplasm: photosynthesis depends on a properly hydrated, metabolically functional protoplasm, including all of the specific enzymes the photosynthetic pathway relies on, so dehydration or enzyme inactivation directly limits the rate the cell can sustain. The accumulation of carbohydrates matters because photosynthesis's own end products - sugars and starch - can become rate-limiting if they are not efficiently translocated away from the leaf; when translocation lags behind production, the resulting build-up of carbohydrate feeds back to slow further photosynthesis. Leaf anatomy affects the achievable rate through several structural features acting together: the thickness of the cuticle and epidermis, the density and distribution of stomata across the leaf surface, whether or not the leaf has Kranz anatomy (section 13.14), and the …