The Ecophysiology of Veronica Chamaedrys, V. Montana and V. Officinalis. I. Light Quality and Light Quantity

1. In view of the contrasting ecologies of the closely related species Veronica chamaedrys, V. montana and V. officinalis, the morphological effects of changes in light quality and quantity were investigated using the technique of growth analysis. By definition, the term `weight ratio' refers to the quotient of the dry weight of a particular organ to the total dry weight of the plant. 2. Three light climates were investigated: unshaded; muslin shaded (c. 37% full daylight, with no change in light quality); canopy shaded, using Ricinus zanzibariensis, also about 37% full daylight, but with lowered red:far-red (R:FR) ratio. 3. There were no species differences in relative growth rate (the efficiency of growth) or unit leaf rate (the assimilatory efficiency of the leaves). Although both rates fell with photon flux density (PFD), only relative growth rate decreased when R:FR ratio was also lowered. 4. Leaf area ratio (total leaf area as a quotient of total dry weight per plant) and specific leaf area (the relative thinness of the leaves) increased with reduced PFD, although the magnitude was species specific, with V. officinalis being least susceptible to change. Low R:FR ratio partially inhibited the increase of leaf area ratio in V. montana and both specific leaf area and leaf area ratio in V. chamaedrys, whereas it had no influence on V. officinalis. 5. Whilst leaf weight ratio was stable in V. officinalis, it was significantly smaller under low PFD in both V. chamaedrys and V. montana. Leaf weight ratio was further reduced by low R:FR ratio in V. montana. 6. Stem weight ratio and petiole weight ratio were increased by both low PFD and low R:FR ratio. Stem weight ratio was ranked in the order V. chamaedrys > V. montana > V. officinalis. V. montana allocated most biomass to petiole. In full light V. chamaedrys and V. officinalis had similar petiole weight ratios but V. officinalis exhibited a greater increase when shaded. 7. Root weight ratio fell with PFD. Sensitivity to R:FR ratio was manifest only in V. officinalis, which consistently had the greatest root weight ratio of the three species whereas V. montana had the lowest. 8. The ability to increase stem weight ratio in response to shading, whilst simultaneously reducing leaf weight ratio and root weight ratio, is suggested as maximizing the carbohydrate economy of the plant. On this premise, V. officinalis is least well adapted to long-term shading, whilst V. chamaedrys and V. montana are equally well s