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Several structural changes in cotton (Gossypium hirsutum L.) leaves attendant on development under conditions of water deficit were examined. Cell size was less and cell wall thickness greater in the leaves of stressed plants than in leaves of well‐watered plants. A short review of the literature suggested that the lesser cell size is a fairly general observation and that it may contribute to plant resistance to moisture stress. A simple model is developed to investigate the influence of the reduction of cell size on cellular water relations. The predictions which can be drawn from simulations with this model are that smaller cells should maintain turgor to lower values of water potential than larger cells. Rather large changes in cell water relations are predicted for small changes in cell size. These effects are related principally to the changing proportion of cell water which resides in the cell wall and is external to the plasmalemma and the osmotic adjustment system. This prediction is in agreement with several observerations on the behavior of stress‐hardened plants and supports the hypothesis that plants or tissues with the smaller cell size will be more tolerant of low water potential.

supplies of water and nutrients, and in occurrence of pests and disease, flexibility in morphogenesis and accli-The structure, control, and efficiency of photosynthetic and respiramation of the physiological systems is a key requirement tory systems are examined. Genetic control is complex and highly for achieving high and stable performance. conserved. While many features are still unresolved, basic efficiencyWith limited space, we focus on photosynthesis and seems little altered by domestication and breeding of crops. Rubisco, the carboxylase-oxygenase enzyme central to photosynthesis and pho-on respiration related to synthesis and maintenance. torespiration, remains a weak point but may be amenable to improve-Whether the biological efficiency of these processes has, ment. However, the actual radiation-use efficiency of crops is generally or might be, improved through breeding are imporless than the potential with present rubisco kinetics, leaving considertant questions. able room for improvement without change in rubisco. Good opportunities for progress lie in definition of optimal canopies of leaves having Photosynthetic Production suitable acclimation and photoprotection. The efficiency of the respiratory system also seems unaffected by plant breeding. Precise evalua-Whether a canopy (amount of leaf area, LAI, and its tion of the roles and efficiencies of the glycolytic pathway and the manner of display) is optimal for photosynthesis in a tricarboxylic acid cycle in production is difficult because, in addition particular environment is reciprocally linked with develto being sources of energy carriers and reductant, those systems also opment and properties of individual leaves, including supply carbon skeletons for biosyntheses. How those systems are their longevity. According to a leaf's position in the controlled and balanced for such diversions is largely unknown. Thecanopy, variations occur in the components of its photoalternative oxidase found in mitochondria may be involved in that synthetic system, its acclimation to changing conditions, balance but its true role(s) is also unknown. Distinguishing two compoand its protection from excess photon flux density nents of respiration, one related to maintenance and the other to (PFD).