Stomatal Behaviour and Water Relations of ChilledPhaseolus vulgarisL. andPisum sativumL.

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Abstract
Leaf diffusion resistance interpreted as stomatal resistance, leaf water potential (Ψw), solute potential (Ψs) and leaf turgor potential (Ψp) of the chilling sensitive species Phaseolus vulgaris were determined during chilling at 4 °C in the light. Both chill-hardened and non-hardened plants were used. For comparison, the chilling resistant species Pisum sativum was also used. The results for chilled P. sativum were similar to those obtained for chill-hardened P. vulgaris plants receiving a chilling treatment. In both cases a reduction in stomatal aperture and the maintenance of a positive leaf turgor were the responses to chilling. Leaves of chilled but non-hardened P. vulgaris plants were found to maintain open stomata throughout the chilling treatment despite a severe wilt developing after 7 h at 4 °C. This was in contrast to the chill-resistant P. sativum. which showed a rapid closing and subsequent re-opening of the stomata to a new reduced aperture. During the first 12 h of chilling Ψwof P. vulgaris leaves changed from −0.47 MPa to −1.24 MPa. On more prolonged chilling Ψw tended to return to pre-chilling values. In addition. Ψp decreased from 0.42 MPa to zero after only 9 h of chilling, and remained at this value for the remainder of the chilling period, Ψs, changed rapidly from −0.89 MPa to −1.35 MPa in the first 7.5 h, and after 9 h. Ψw and Ψs, were equal, i.e. zero Ψp. In contrast, the chilling resistant plant P. sativum maintained a positive Ψp throughout the chilling period, and there was little difference between values of Ψw, and Ψs in control and chilled leaves.