Stomatal Movement and CO_2 Exchange Rate of Sweet Potato Plant (Ipomoea batatas Lam.) in Relation to Water Environments : A Comparison between Native and Improved Varieties

Abstract

We studied the stomatal response to internal and external water environments and clarified its varietal difference by comparing the gas exchange rate between variety Koganesengan (var. Keg) and variety Tsurunasigenji (var. Tsu). The former is an improved variety with high yield and the latter is a native one with low yield. Photosynthesis of leaf was regulated through gas exchange by stomatal aperture degree (SDA), and a high SAD was prerequisite to increase photosynthesis. SAD of var. Kog was high and stable in both wet and dry conditions, while SAD of var. Tsu was low and unstable. After the cut-leaf treatment, var. Kog maintained a higher CO2 exchange rate (CER) and transpiration rate (TR) during a longer time than var. Tsu. The rigidity of leaf of var. Kog was rapidly lost after this treatment, but a relatively high CER could be maintained. Relative pressure of guard cell to subsidiary cell was suggested to have an important role in stomatal movement; a higher relative pressure was predicted in var. Keg. Also the relative volume of a subsidiary cell to a guard cell in var. Kog was 50% larger than that of var. Tsu. The pretreatment of high humidity air (nearly 100% relative humidity) was effective in increasing TR of var. Tsu. As a whole the gas exchange response of var. Tsu to vapor pressure deficit (VPD) was unclear, but that of var. Kog was much clear; VPD of 2 KPa was a turning point in the gas exchange response. The feature of stomatal movement in the plants grown in a field was recognized as an important criterion for selection of the genotypes with high photosynthesis.