Residual transpiration rate, epicuticular wax load and leaf colour of pea plants in drought conditions. Influence on harvest index and canopy temperature

Abstract

Residual transpiration rates, epicuticular wax loads and leaf colours of 20 cultivars of pea (commercial varieties and bred, improved lines) were studied. The residual transpiration rate varied between genotypes from 0.77 to 1.82 mg m −2 s −1, while wax content varied between 0.19 and 0.41 g m −2. Leaf colour varied on the Munsell colour saturation scale between 4 and 8. No significant correlation was found between epicuticular wax load and residual transpiration rate. This suggests that all the studied varieties have more than enough wax to be able to adequately control the loss of water from the cuticle. The differences between cultivars in residual transpiration rates are therefore due to other factors. No significant changes in residual transpiration rate were seen in any of the genotypes studied when subjected to drought, with the exception of variety 53. However, in the majority of varieties, the epicuticular wax load increased significantly when plants were subjected to this treatment. All three studied parameters affected the performance of pea plants under drought. A positive correlation ( P<0.001) was found between residual transpiration rate and harvest index: cultivars with greater water loss via the epidermis showed greater harvest indices. It is possible that a high harvest index is a consequence of a reduction in heat stress (a significant correlation ( P<0.001) was found between residual transpiration rate and canopy temperature). Epicuticular wax load also correlated significantly ( P<0.001) with harvest index. Cultivars with greater wax loads gave higher rain-fed harvest indices. Wax-rich varieties developed significantly lower canopy temperatures ( P<0.001). This suggests that incident radiation is more efficiently reflected with increasing wax load, which leads to the alleviation of heat stress. Varieties with less saturated leaf colours gave higher rain-fed harvest indices. A more pallid green shade probably indicates a lower chlorophyll content and a lower quantity of antenna complexes at each reaction centre of photosystem II. In drought this would permit lower absorption of radiation, which might lessen heat stress and reduce the risk of over-excitation of photosystem II reaction centres.