THE ECOPHYSIOLOGY OF VERONICA-CHAMAEDRYS, V-MONTANA AND V-OFFICINALIS .2. THE INTERACTION OF IRRADIANCE AND WATER REGIME

1. The growth responses of Veronica chamaedrys, V. montana and V. officinalis to five levels of irradiance in conjunction with dry, normal and waterlogged soil were investigated using growth analysis. 2. There were no significant differences between species in either relative growth rate (RGR) or unit leaf rate (ULR) and both rates fell with lowered irradiance. Reductions caused by waterlogging were greater than those caused by drought. Water regime had more influence on RGR than did irradiance. 3. Low irradiance caused increases in leaf area ratio (LAR), specific leaf area (SLA) and leaf weight ratio (LWR), although increases in LAR and SLA were suppressed by either soil water deficit or saturation. For both V. chamaedrys and V. montana, waterlogging had the more suppressive effect. Mean SLA in V. officinalis, calculated across the five irradiances, was the same between normally watered plants and those subject to drought. Drought had no effect on LWR in either V. chamaedrys or V. montana. whereas it was increased by waterlogging. In contrast, LWR in V. officinalis remained unaffected by waterlogging but was decreased by drought. 4. Stem and petiole data were combined. The compound weight ratio increased with decreasing irradiance and also with increasing soil water content, although the effect of water treatment was dominant over that of irradiance. 5. Regression analysis demonstrated a positive, linear relationship between root weight ratio (RWR) and irradiance. Root weight ratio also increased with decreasing soil water content and was, in fact, influenced primarily by water treatment. Of the three species, V. officinalis had the highest RWR and showed the greatest reduction in RWR under low irradiance. Overall, waterlogged V. chamaedrys suffered the largest reduction in RWR. 6. Given its low RWR and high LAR and SLA, V. montana would be unable to optimize water use in environments in which low soil water potential was concurrent with high irradiance. In fact, V. montana suffered leaf chlorosis under these conditions. V. officinalis possesses a high RWR but low LAR and SLA and was tolerant of both waterlogging and drought, irrespective of irradiance. Of the three species, V. chamaedrys was least tolerant of waterlogging. At low irradiances the shade responses of V. montana would enable it to maintain a more-favourable carbohydrate balance than V. officinalis, irrespective of water regime.