PROF. WAITHAKA KIMANI

The term 'plant tissue culture' is normally used as a 'blanket' term to cover the cultivation of all plant parts whether a single cell, a group of cells or an organ on an artificial medium. The in vitro cultures are now being used as tools for the study of various basic problems in experimental biology and agriculture. The two main applications of plant tissue culture in horticultural production are in propagation and sanitation. Tissue culture is being used effectively in asexual clonal propagation of various horticultural crops. Orchids, carnations, roses, gerbera, chrysanthemums, potatoes, strawberries, apples and coffee have all been successfully cloned in vitro and the list is increasing daily. Recovery of pathogen - free plants by meristem tip culture has become a common practice in producing virus-free stock material of vegetatively propagated plants. Other areas where tissue culture can contribute to horticulture include embryo culture and germplasm preservation and storage.

Dry matter production and partitioning and yields in clonal tea bushes planted at 3 different spacings (0.30 x 0.30 m; 0.61 x 0.61 m; and 1.22 x 1.22m) and brought into plucking at 2 different heights (0.25 m and 0.50 m) were investigated. After 36 months of growth the bushes at close spacing (0.30 x 0.30 m) produced 1.1 and 2 times dry matter than bushes grown at medium (0.61 x 0.61 m) and wide (1.22 x 1.22 m) spacings, respectively. The bushes at close spacing had less leaf percentage of total dry matter than those at medium and wide spacings. However, the bushes at wide spacing had less frame percentage of total dry matter than those at close spacing. There were no significant differences in the root percentages between bushes at the 3 spacings. The harvest index of the bushes grown at close spacing was 13% and 16% higher than those of the bushes at medium and wide spacings, respectively. Similarly, bushes at close spacing produced yields 2 and 3 times more than bushes at medium and wide spacings, respectively. The bushes at wide spacing had significantly higher leaf/frame ratio than those bushes at close spacing. Lowering the plucking surface height reduced total dry matter in all bushes grown at the 3 spacings. The results suggest that the harvest index and yields of tea bushes can be greatly increased by reducing the size of individual bushes through increasing plant density per unit area.

Tea bushes are manipulated so as to form strong woody frames with the largest possible plucking surface area. Studies on dry matter production and partitioning of tea bushes have shown that the harvest index is low in old bushes and high in young bushes. The low indexes in old bushes are partly due to large amounts of wood in the primary frame which provide a large cambial sink for photosynthates. Reducing the plant size by maintaining bushes at low heights greatly increased yields per unit area of plucking surface. Thus, maintaining large bushes does not necessarily mean high yields. The impression from dry matter production and partitioning studies is the need for a thorough reinvestigation and revision of the conventional methods of growing tea which encourage the development of unnecessary large amounts of woody tissues.