Transpiration Response of Cotton to Vapor Pressure Deficit and Its Relationship With Stomatal Traits

Abstract

Many studies have demonstrated that the cotton in warm environments is vulnerable to water-limitations thus reducing the yield. A number of plant traits have been recommended to ameliorate the effects of water deficits on plant growth and yield. Limitation on maximum transpiration rate (TR) under high vapor pressure deficit (VPD), usually occurs during midday, is often considered as a water conservation trait. The genotypes with this trait are desirable in high VPD environments where water deficits commonly develop in the later part of the growing season. Our objective of the study was to find the genotypic variation for the trait limited TR under high VPD and also to study leaf temperature, water potential, photosynthesis, and stomatal conductance responses. Also, our objective was also to study the structural changes in the stomatal traits when exposed to long term high VPD conditions and involvement in such responses. In the present study, 17 cotton genotypes were studied for their (TR) response to various VPD environments under well irrigated conditions. Out of 17, eight genotypes limited TR after approximately 2 kPa VPD and rest of them increased their TR with increased VPD. Five selected genotypes with different TR response to increasing VPD were further studied for gas exchange and stomatal properties. All genotypes, irrespective of exhibiting limited TR at high VPD, reduced stomatal conductance, photosynthesis and water potential at high VPD of 3.3 kPa. The genotypes with limited TR modified their stomatal traits mostly on the adaxial surface with frequent and small stomata under high VPD. The genotypes with limited TR also exhibited an increase in epidermal cell expansion and stomatal index at contrasting VPD gradients to effectively balance the liquid and vapor phase conductance to limit TR at high VPD.

Keywords: cotton; gas exchange; stomata; transpiration; vapor pressure deficit; water potential.

FIGURE 1

Transpiration rate (TR) of cotton genotype to vapor pressure deficit (VPD). TR (mg H₂O m^-2 s^-1) response to increasing VPD (kPa) of the six cotton genotypes. (A) OL220 (B) DP555 BG RR and (C) LKT 57 represents genotypes with two segmental regression. The genotypes increased TR until VPD breakpoint (BP) X₀ (kPa) and then reduced TR. The values of X₀ indicated in each panel from (A–C). Panels from (D–F) represents genotypes 06-46-153P, CS 50 and Siokra L23, respectively, with linear TR response to increasing VPD.

FIGURE 2

Gas exchange properties of the cotton genotypes with differences in their TR response to VPD. Stomatal conductance (μmol m^-2 s^-1), transpiration (mmol m^-2 s^-1) and photosynthesis (μmol m^-2 s^-1) of the genotypes with limited TR to high VPD (A) OL220 and (B) LKT 57. These two genotypes were well represented by two segmental linear regression to increase in VPD for their stomatal conductance, transpiration, and photosynthesis with BP at around 2.7 kPa. The genotypes in panels (C) 06-46-153 P, (D) CS 50, and (E) Siokra L23 showed the linear response for their gs, E, and A to the increase in VPD.

FIGURE 3

Water potential (ψ) and leaf to air temperature differences (LT-T°C) of the five cotton genotypes to different VPD levels. (A) Water potential (Mpa) and (B) leaf to air temperature differences (°C) of the genotypes with limited TR and without limited TR to different VPD levels. The first two bars without pattern represent the genotypes OL220 and LKT 57 with limited TR. The other three bars with pattern represents the genotypes without limited TR. The genotypes were significantly different for water potential values (less than P < 0.05) at all VPD levels except at 3.3 kPa. The genotypes were significantly (less than P < 0.01) different for the leaf to air temperature differences at all VPD levels.

FIGURE 4

Stomatal ratio and stomatal index of the cotton genotypes with differences in their TR response to VPD on adaxial and abaxial leaf surfaces. (A) Stomatal ratio (%), (B) adaxial stomatal index (%), and (C) abaxial stomatal index (%) of the cotton genotypes grown under low (0.9 kPa) and high (3.3 kPa) VPD environmental conditions. The first two bars in all three panels represent the genotypes with limited TR and other three bars are the data of the genotypes without limited TR. All genotypes were significantly different from each for all parameters under low VPD at P < 0.05 and in high VPD at P < 0.01.