Why some fruits are green when they are ripe: carbon balance in fleshy fruits.

Fruits that are green upon ripening ("green-ripe") tend to be dispersed by a limited range of frugivores, whereas those that are brightly colored ("bright-ripe") are dispersed by a wide range of birds and mammals. Because green fruits are probably less conspicuous than other colors of fruits, their pigmentation cannot be attributed to the attraction of seed dispersers. Instead, we hypothesize that a major benefit of green pigmentation is the ability to photosynthesize when ripe. Photosynthesis by fruits may lower their costs of production, and could result in fruits with greater nutrient reward. We present data on physical, chemical, and photosynthetic characteristics of ripe fleshy fruit of variable colors for 28 plant species at the La Selva Biological Station, Costa Rica. In addition to color and morphological characteristics of pulp and seeds, we report soluble solids content (refractive index), and photosynthetic/respiratory carbon-dioxide balance of ripe fruits. Carbon balance was much more dependent upon ambient light levels in green-ripe fruits than in bright-ripe fruit. In particular, data from light response curves indicated that green-ripe fruits may go into positive carbon balance at high light levels (above 300 μmol/m2/s). Rather than finding a positive relationship between soluble solids content and green fruit, as we predicted based on photosynthetic capacity, our data indicate that greater respiration rates of green-ripe fruits may result in carbon losses at low light levels. Our results were consistent with previously described morphological differences between the two color classes, with green-ripe fruits displaying significantly greater wet pulp mass, wet seed mass, and total fruit mass. Our data suggest that photosynthesis due to the retention of chlorophyll in ripe fleshy fruit may offset respiratory costs for plants with large or otherwise costly fruit, but this advantage should be evident only under high-light conditions.